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Unmanned Spaceflight.com _ Cassini's ongoing mission and raw images _ Enceladus Plume Search, Nov. 27

Posted by: jmknapp Nov 24 2005, 04:01 PM

Interesting item in the science plan kernel (S16) just released to the NAIF website:

OBSERVATION_ID: S1629

SEQUENCE: S16

OBSERVATION_TITLE: Plume Search

SCIENCE_OBJECTIVE: Hope to detect/observe plumes, whether from volcanic activity or geysers.

OBS_DESCRIPTION: Point and stare.

SUBSYSTEM: ISS

PRIMARY_POINTING: ISS_NAC to Enceladus (0.0,5.0,0.0 deg. offset)

REQUEST_ID: ISS_018EN_PLUMES001_PRIME

REQUEST_TITLE: ENCELADUS Geyser/Plume Search

REQ_DESCRIPTION: 1;ENCELADUS Geyser/Plume Search 1x1xNPp -- 3 different exposures

BEGIN_TIME: 2005 NOV 27 19:00:00 UTC

END_TIME: 2005 NOV 27 20:00:00 UTC

Posted by: jmknapp Nov 26 2005, 11:02 PM

Here's the view of Enceladus during the plume search tomorrow:



Both Saturn and the sun are on practically the opposite side of Enceladus from Cassini during the observation (sun phase angle 162 degrees; Saturn phase angle 164 degees).

The sub-Cassini point on Enceladus will be 0.8 degrees N, 171 degrees W--so the limb is approximately the 81W/99E meridians.

Posted by: ElkGroveDan Nov 27 2005, 12:15 AM

QUOTE (jmknapp @ Nov 26 2005, 11:02 PM)
Here's the view of Enceladus during the plume search tomorrow:
*
Looks like they need to use the serendipity filter.

Posted by: dilo Nov 27 2005, 08:04 AM

Thanks for all these informations, jmknapp (also in other threads).
Really hope search will be succesfull!

Posted by: jmknapp Nov 27 2005, 02:13 PM

QUOTE (dilo @ Nov 27 2005, 04:04 AM)
Really hope search will be succesfull!
*


Here's a comparison to theplume search they did on Feb. 17, 2005:



That was somewhat further away than today's observation (282,000 km vs. 183,000 km) and the solar phase angle was a bit less (154 degrees), also a bit more saturnshine (129 degrees). So today's observation is more favorable all around.

Here was the result of one long-exposure NAC image Feb. 17th:

http://cassinicam.com/enc_N00028218.jpg

Posted by: mgrodzki Nov 27 2005, 06:24 PM

that is a nice image… not color right? and i assume that blast there is just a light flare?

Posted by: dilo Nov 28 2005, 06:41 AM

QUOTE (mgrodzki @ Nov 27 2005, 06:24 PM)
that is a nice image… not color right? and i assume that blast there is just a light flare?
*

Only false color images from different exposures (no filters), like this one...

For previous discussions on these images, look at
http://www.unmannedspaceflight.com/index.php?showtopic=706&view=findpost&p=5657

Posted by: Decepticon Nov 28 2005, 01:10 PM

Images Up.



http://saturn1.jpl.nasa.gov/multimedia/images/raw/raw-images-list.cfm?browseLatest=0&cacheQ=0&storedQ=0

Posted by: SFJCody Nov 28 2005, 01:29 PM

http://saturn1.jpl.nasa.gov/multimedia/images/raw/casJPGFullS16/N00043446.jpg

One plume per stripe?

Posted by: alan Nov 28 2005, 01:31 PM

Possible plume? Doesn't look like a lens flare.


Posted by: ugordan Nov 28 2005, 01:38 PM

QUOTE (alan @ Nov 28 2005, 03:31 PM)
Possible plume? Doesn't look like a lens flare.


*

Amazing images!!!
IMHO, these prove, without a doubt that there's some very significant venting occuring on Enceladus. I could go on calling them genuine eruptions (Io, anyone? biggrin.gif) as well, the choice of rotating the spacecraft to discriminate from the possible scattered light problems was also very ingenious.

Most definitely one of the most significant Cassini results as of yet. Detecting outgassing is one thing, seeing it is completely different!

blink.gif

EDIT: Looks like Rhea won't be prime news even in this rev, actually devoted to its close flyby tongue.gif

Posted by: Bjorn Jonsson Nov 28 2005, 01:49 PM

This is the first time I see something like this that is not obviously an imaging artifact.

What's interesting is that the spacecraft has been rotated to make recognizing artifacts to due scattered light within the camera easier. I find it extremely interesting that the possible plumes appear roughly identical in all of the images despite the varying rotation. Also in the overexposed images a very large 'cloud' appears that does not appear next to the nightside limb (only against the bright limb) so this cannot be a diffuse ring in the background.

Still not totally convinced these are plumes but this looks very promising.

Posted by: tedstryk Nov 28 2005, 02:12 PM

I am at work, or I would do this myself, but someone ought to try matching them up to a map - if the bright area match tiger stripes, I will be fully convinced, and Enceladus will join the Io-Triton club of moons in my book.

Posted by: Decepticon Nov 28 2005, 02:14 PM

Is there enough pics for a animation?

Posted by: Bjorn Jonsson Nov 28 2005, 02:18 PM

QUOTE (tedstryk @ Nov 28 2005, 02:12 PM)
I am at work, or I would do this myself, but someone ought to try matching them up to a map - if the bright area match tiger stripes, I will be fully convinced, and Enceladus will join the Io-Triton club of moons in my book.
*

I'm at work too but the first thing I'll do at home is use the SPICE kernels to make a computer rendering with a lat/lon grid. Unfortunately my planetary renderer (written by me) is in a state of chaos at the moment because I'm adding some new features to it but hopefully I can make it work fairly easily for this special occasion. Murphy's law at work...

Posted by: jmknapp Nov 28 2005, 02:45 PM

Here's what I get using SPICE data and artificially illuminating the dark side:



The NAC image is rotated from that above (which is oriented with the north pole up), but the "plume" appears to be in about the center of the crescent, which would place it very near the south pole.

Another south pole hot spot?

Here's the corresponding raw image, rotated to about the same crescent orientation:



http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=54833

Posted by: Mariner9 Nov 28 2005, 03:10 PM

ohmygod!!!!!!!!!

I had no idea they were planning this. I checked the raw images this morning and I'm still pulling my jaw (metaphorically) off of the floor.

I'm rarely at a loss for words. This is no artifact, no lens trick, this is the real deal.

Posted by: Bill Harris Nov 28 2005, 03:21 PM

Whew.

I'm not as up on the entire archive of Enceladus images as I should be, but do we have a set of earlier images of the south polar region?

--Bill

Posted by: jmknapp Nov 28 2005, 03:27 PM

Too bad there isn't plume evidence on the dark limb though, rather than just where one might expect a "diamond ring" effect?

Posted by: ugordan Nov 28 2005, 03:30 PM

QUOTE (jmknapp @ Nov 28 2005, 04:45 PM)
Here's what I get using SPICE data and artificially illuminating the dark side:

The NAC image is rotated from that above (which is oriented with the north pole up), but the "plume" appears to be in about the center of the crescent, which would place it very near the south pole.

Another south pole hot spot?

Here's the corresponding raw image, rotated to about the same crescent orientation:
*

I suppose it would really be an overkill, but I figure a precise way to match the viewing geometry would be extracting the S/C velocity vector relative to Enceladus from the kernels and comparing it with one of the long star trails visible in the background. It would probably be too much work, but might be interesting to see if it's doable.

Anyhow, even this is accurate enough to conclusively say the plumes (plural!) are coming from the vicinity of the south pole.

In my mind, there's not a tiniest bit of doubt whether the feature we're seeing is real. The nail in the coffin to the nay-sayers would probably be a WAC shot which, I predict would show the very same plumes. wink.gif

Posted by: ugordan Nov 28 2005, 03:32 PM

QUOTE (jmknapp @ Nov 28 2005, 05:27 PM)
Too bad there isn't plume evidence on the dark limb though, rather than just where one might expect a "diamond ring" effect?
*

Well, if you consider the plume is not hot silicate lava but rather water wapor, I'd be in fact surprised to see it glow in the dark!

Posted by: JRehling Nov 28 2005, 03:35 PM

QUOTE (jmknapp @ Nov 28 2005, 07:27 AM)
Too bad there isn't plume evidence on the dark limb though, rather than just where one might expect a "diamond ring" effect?
*


Yeah, it's enough to make a jury consider acquitting... but I think this is a clincher for anyone who doesn't want to pop the champagne prematurely: Instead of one flare due to a bright limb, there are three distinct ones, with darkening inbetween. If we were merely seeing supersaturated brightness bleeding out, why wouldn't it bleed in the areas between the plumes, too? After all, they have more bright limb near them than the two outer plumes do.

This is a done deal -- Enceladus is venting. Now the interesting questions are: What is the shape and volume of the reservoir that is providing the fireworks? What is the access from subsurface to surface? And can we drop a submarine in there?

Posted by: jmknapp Nov 28 2005, 03:44 PM

QUOTE (ugordan @ Nov 28 2005, 11:32 AM)
Well, if you consider the plume is not hot silicate lava but rather water wapor, I'd be in fact surprised to see it glow in the dark!
*


But any vapor on the dark-limb side might only have to gain a little altitude before it was in sunlight?

Posted by: ugordan Nov 28 2005, 03:51 PM

QUOTE (jmknapp @ Nov 28 2005, 05:44 PM)
But any vapor on the dark-limb side might only have to gain a little altitude before it was in sunlight?
*

Why do you insist on the plumes originating from the dark side? There isn't much evidence to support this. In any case, if it were on the dark side, when it rose to get into sunlight, it would probably appear to be practically in front of the sunlit crescent, due to the very high phase angle.

In the light of the recent discovery, I think we'll need to re-analyze the Feb 17 high phase, outbound observations. There was one suspicious cloud-looking thing above the south pole also present back then, but it was attributed to scattered light/overexposure.

Posted by: tasp Nov 28 2005, 03:51 PM

What is the power source for this?

{Wild speculation alert}

There is a small asteroid, Toro, that was until about a 100 years ago, in a resonant orbit around the sun with Venus. Since then it has been in a resonance with earth.

IIRC, the transition period took several years.

Is there any feasability that Enceladus is doing something similar with other moons of Saturn? Could Enceladus have an orbit around Saturn that periodically swaps resonances with 2 other satellites? Perhaps the period 'overshoots' slowly one resonance and oscillates back towards another which it overshoots and repeats the process. We see the plumes as powered from the internal heating from the times the resonances are strong and flex the crust (like Io), but our observations are in a time period between and we don't calculate that Enceladus is currently in a resonance.

I don't have the math skills or theoretical background to evaluate this.

What other possibilities exist for heating Enceladus? Radionuclides seem most unlikely, residual heat from an impact would seem to be something that would dissipate very rapidly. Solar heating of an object with an albedo of 1 is ruled out.
Formation heat should have dissipated billions of years ago. Electrical effects of Saturn's magnetosphere are too weak (by orders of magnitutde).

What else is there to heat Enceladus?

We have visible plumes, what kind of dissipation are we looking at? Megawatts?

Great fun figuring this one out!

Posted by: Orlin Denkov Nov 28 2005, 03:55 PM

In the title of this thread isn't it Enceladus that should be written tongue.gif
edit: oop, already correct, sorry smile.gif

Posted by: volcanopele Nov 28 2005, 03:56 PM

^^ Fixed

Posted by: jmknapp Nov 28 2005, 03:59 PM

QUOTE (ugordan @ Nov 28 2005, 11:51 AM)
Why do you insist on the plumes originating from the dark side?
*


Not insisting, I just said that it's too bad there isn't evidence elsewhere too, rather than just in the center of the crescent.

QUOTE (ugordan @ Nov 28 2005, 11:51 AM)
In the light of the recent discovery, I think we'll need to re-analyze the Feb 17 high phase, outbound observations. There was one suspicious cloud-looking thing above the south pole also present back then, but it was attributed to scattered light/overexposure.
*


Sure enough, and lo and behold that suspicious cloud-looking thing was very near the south pole too as you say, and not in the center of the crescent!



In that image north is oriented up.

Dismissed at the time as an artifact? Well, well!

Posted by: The Messenger Nov 28 2005, 04:48 PM

The source of the heat is truly perplexing. I hope they will schedual some gravity runs of Enceladus in the extended mission. Enough to obtain a reasonable handle on mass distribution.

Posted by: tedstryk Nov 28 2005, 04:49 PM

QUOTE (jmknapp @ Nov 28 2005, 03:59 PM)
Not insisting, I just said that it's too bad there isn't evidence elsewhere too, rather than just in the center of the crescent.
Sure enough, and lo and behold that suspicious cloud-looking thing was very near the south pole too as you say, and not in the center of the crescent!



In that image north is oriented up.

Dismissed at the time as an artifact? Well, well!
*

I think that there was always suspicion, but they wanted to avoid the humiliation of announcing a great discovery and then having to retract it.

Posted by: jmknapp Nov 28 2005, 05:56 PM

Here's a map of the south polar region, based on Steve Alber's latest cylindrical projection (click for larger image):

http://cassinicam.com/espole.jpg

This is an orthonormal projection, so the limb is the equator and the south pole ("tiger scratch" feature) is at the center of the image.

Posted by: tasp Nov 28 2005, 06:29 PM

Maybe I am 'seeing' too much into your map, but I keep thinking of Miranda and race tracks. . . . .

Posted by: jmknapp Nov 28 2005, 06:41 PM

Found this image of the south polar region (approximate south pole marked with circle, click for larger image):

http://cassinicam.com/spnac.jpg

http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=45643

Posted by: canis_minor Nov 28 2005, 07:21 PM

The features seen in these images are located at the south pole. At the time the images were taken, the Sun was directly below Enceladus as seen from the spacecraft, but the spacecraft was roughly in the plane of the satellites. So the center of the bright limb is right about at the south pole.

Posted by: Bill Harris Nov 28 2005, 07:37 PM

The ridges and wrinkles in that region are a strong indication of tectonics and therefore heat flow, so it makes perfect sense that there will be venting and visible plumes.

Now to figure out why....

--Bill

Posted by: scalbers Nov 28 2005, 08:17 PM

QUOTE (tedstryk @ Nov 28 2005, 04:49 PM)
I think that there was always suspicion, but they wanted to avoid the humiliation of announcing a great discovery and then having to retract it.
*



I would certainly hope there was a healthy suspicion all along. In the previous forum discussion a few months ago, I expressed some of my own suspicion, even if it was tempered by some ensuing (and perhaps not entirely convincing?) replies.

On another note, I just made a south polar view in a perspective projection, available here:

http://laps.noaa.gov/albers/sos/saturn/enceladus/enceladus_rgb_cyl_www_P4.jpg

Posted by: The Messenger Nov 28 2005, 08:44 PM

QUOTE (jmknapp @ Nov 28 2005, 10:56 AM)
Here's a map of the south polar region, based on Steve Alber's latest cylindrical projection (click for larger image):

http://cassinicam.com/espole.jpg

This is an orthonormal projection, so the limb is the equator and the south pole ("tiger scratch" feature) is at the center of the image.
*

Stretch Marks?

Posted by: Bjorn Jonsson Nov 28 2005, 08:47 PM

I'm attaching a rendering I did showing the viewing geometry at 17:38 on November 27, 2005. Cassini took several images at roughly this time, for example these two:

http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=54839

http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=54840

I then rotated one of the Cassini images where the limb is overexposed to match the orientation of the rendered image, resized it to match the rendering size and pasted the plumes into the rendering.

This confirms that the source of the brightest plume (assuming we really are seeing plumes which seems very likely) is near the south pole. The fainter plumes appear farther from the pole. It should be noted that this might be an illusion, the brightest plume might be at the same distance or farther from the pole than the fainter ones if its source is well inside Enceladus' 'disc'.

 

Posted by: dilo Nov 28 2005, 09:03 PM

QUOTE (tedstryk @ Nov 28 2005, 04:49 PM)
I think that there was always suspicion, but they wanted to avoid the humiliation of announcing a great discovery and then having to retract it.
*


Yes, a STRONG suspicion (the same I had! wink.gif )...
Anyway, I tried to code with colors the huge dinamic range covered by 3 images taken with increasing exposure time (N00043435/36/37):

 

Posted by: volcanopele Nov 28 2005, 09:59 PM

Special release for the Enceladus plume:

http://ciclops.org/view_event.php?id=45

Spray Above Enceladus
http://ciclops.org/view.php?id=1652
this release shows the infamous images from January showing a brighting of the limb of Enceladus. Because this looked similar to artifacts seen at other moons, this was brushed off as an artifact as well. However, more indepth analysis of this sequence of images revealed that the" plume" was in fact real.

Fountains of Enceladus
http://ciclops.org/view.php?id=1688
In perhaps the fastest image release ever, this release shows the new jets found near in the south polar region of Enceladus.

Posted by: jmknapp Nov 28 2005, 10:01 PM

QUOTE (Bjorn Jonsson @ Nov 28 2005, 04:47 PM)
This confirms that the source of the brightest plume (assuming we really are seeing plumes which seems very likely) is near the south pole. The fainter plumes appear farther from the pole. It should be noted that this might be an illusion, the brightest plume might be at the same distance or farther from the pole than the fainter ones if its source is well inside Enceladus' 'disc'.
*


This might help pin things down, assuming I got it accurately. Here's the viewing geometry I think, from the vantage point of an observer 100,000 km directly above the south pole:


Posted by: mars loon Nov 28 2005, 10:31 PM

This news just released by CICLOPS website

"The Fountains of Enceledus"

http://ciclops.org/view_event.php?id=45

"In a wonderful start to the Holiday season, Cassini imaging scientists are delighted by recent sightings of fountain-like plumes towering above Saturn's moon Enceladus. A fine spray of small, icy particles, emanating from the warm, geologically unique province surrounding the south pole of Enceladus and believed now to supply the material comprising Saturn's E ring, was first observed in images taken back on Jan. 16, 2005. Images of a crescent Enceladus returned by Cassini this past weekend show multiple plumes in striking detail. Stay tuned for future announcements on the sightings of the Enceladus plumes".

check the link for more info and images

Posted by: Sunspot Nov 28 2005, 10:48 PM

What's also striking is how obvious the plumes are - even in the RAW unprocessed images posted online. ohmy.gif

Posted by: mars loon Nov 28 2005, 11:29 PM

QUOTE (Sunspot @ Nov 28 2005, 10:48 PM)
What's also striking is how obvious the plumes are - even in the RAW unprocessed images posted online.  ohmy.gif
*

Yes, this is a truly outstanding mission highlight !!!

Enceledus has been a mission star almost as bright as Titan. To see those plumes so clearly, and they have been on the hunt !!

NASA/ESA should consider an Enceledus lander to complement a Europa mission and follow-up for Titan.

Posted by: akuo Nov 28 2005, 11:29 PM

Looking at the raws I noticed that the whole background of the images is light, not just the plumes. This is especially evident on the overexposed images of encaladus:


Notice how the background space is lighter than the "dark side" of Encaladus. Does this mean that plume material is all around, maybe making up a donut of the material around Encaladus's orbit?

Posted by: David Nov 28 2005, 11:54 PM

QUOTE (ElkGroveDan @ Nov 27 2005, 12:15 AM)
Looks like they need to use the serendipity filter.
*


Hurrah for a remarkably effective use of the "serendipity filter"! biggrin.gif

Maybe they can name Enceladus' south pole "Serendip"? smile.gif

Posted by: EccentricAnomaly Nov 29 2005, 12:44 AM

The E-Ring is just such a donut of material around Enceladus' orbit, and is probably responsible for the light background as well. See here: http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=808 for a depiction of the E-ring.

QUOTE (akuo @ Nov 28 2005, 04:29 PM)
Looking at the raws I noticed that the whole background of the images is light, not just the plumes. This is especially evident on the overexposed images of encaladus:


Notice how the background space is lighter than the "dark side" of  Encaladus. Does this mean that plume material is all around, maybe making up a donut of the material around Encaladus's orbit?
*

Posted by: edstrick Nov 29 2005, 07:50 AM

Regarding the plume images: Feb 20, I posted:
http://www.unmannedspaceflight.com/index.php?showtopic=691&st=51

"Regarding the image<s> of Enceladus with a possible plume in the south polar region. There is a series of short exposure images of the crescent on the JPL RAW pictures pages and a longer exposure (posted on the CICLOPS web page as "862-1905-3", cleaned up and without JPG artifacts). An even longer exposure on the JPL RAW pages is N00028218. The images show Enceladus as a thin crescent, illuminated by sunlight from about a 4:30 clock angle and as a fatter crescent, illuminated by Saturn from the 9:00 clock angle.
The unilluminated side of the moon between the crescents is visible in silhouette against a lighter background. This background is *NOT* the ring-lit nightside of saturn, 1.) since the dayside of saturn is to the left and out of the image, and 2.) because all images show stars or nearly horizontal star-trails, all parallel, tilted slightly down to the right, and varying in length in proportion to the exposure. I have to conclude that we are probably seeing the diffuse E-Ring in forward scattering, with Enceladus between the spacecraft and the bulk of the E-Ring.
I'm attaching a composite image with the two images named above, and two spatial-bandpass-filtered enhancements of 862-1905-3. These have been processed to enhance fine detail in the plumelike feature close to the moon's limb, and details further away from the limb. None of the images, including the long exposure N00028218, show any trace of the feature against the darkside of the moon above the sunlit crescent, and structures in the plumelike feature converge on the bright limb just like cometary jets seen at comets Halley and Borelly and Wild. There seem to be maybe 3 "sources" for the main plume-like feature and a fainter single-source plume-like feature to the right.
Except for a faint diagonal line "behind" Enceladus, visible in the last picture, which I suspect is a camera artifact or something, all features in this image seem consistent with the plume-like feature being real, not light scattered by contamination in the camera (which is a problem with the NA camera) or a lens-flare. I would have to see the images of other moons with simlar faux-plume features referred to a couple days ago by a team member (earlier in this thread) and apply contrast stretching and enhancement to them (using clean versions, not RAW's from the JPL website) to convince me this feature is not real and is not active plumes from Enceladus. "

(See original post for the picture)

NOW..... VolcanoPele... tell me again this waas just the same old problem with scattered light in the camera....?

Posted by: ugordan Nov 29 2005, 08:07 AM

QUOTE (volcanopele @ Nov 28 2005, 11:59 PM)
Fountains of Enceladus
http://ciclops.org/view.php?id=1688
In perhaps the fastest image release ever, this release shows the new jets found near in the south polar region of Enceladus.
*

That really was fast! I would have expected at least a week's worth of sleep-overs before you guys would come up with something wink.gif

Posted by: BruceMoomaw Nov 29 2005, 08:23 AM

Crowing about one's foresight is bad manners, Ed. I should know; I do enough of it.

Posted by: edstrick Nov 29 2005, 08:39 AM

(grins a bit toothily at Bruce Moomaw, canary feathers in one corner of the mouth)

Agreed, I'm being a bit prickly, but I made specific points in my posting that were never addressed, and I felt the whole discussion including those points was rather summarily dismissed.

I think I asked somewhat later (maybe in another thread) for some other moons' high-phase images with non-pume features that were a good counter example as evidence that the Enceladus pics were just scattered light (so I could run comparison enhancements on them), but never got pointed toward any.

Posted by: Gsnorgathon Nov 29 2005, 10:52 AM

Good for you, Ed! Crow all you like! (Actually, from where I sit it doesn't sound much like crowing, mostly just "On date X I made statement Y and provided evidence Z." I suppose there's maybe a bit of neener-neener in there, but the fact-to-neener ratio seems plenty high.)

But to return to our regularly scheduled topic: How tightly is it possible to constrain the origin of those plumes? Among other things, I'm wondering if they're coming from multiple tiger stripes or just one.

Posted by: edstrick Nov 29 2005, 11:55 AM

I doubt there's enough parallax during the high-phase sequence to strongly constrain the plume's sources along the line of sight, but from postings in this thread, it's going to be pretty easy to see where lines of sight to plume bases cross the terminator and cross tiger stripes near the terminator. I very strongly suspect there are multiple vents along each of the tiger stripes, with most relatively weak. This seems to me to suggest that either there are relatively few extra-warm spots exposed within a stripe at one time, the hottest being discrete plume sources, or that a more active gysering source may be involved.

I can, however, imagine a vent with the hottest ice (with the highest vapor pressure) at the bottom, spewing upwards through a fissure or pipe, producing relatively collimated plumes like we seem to see in these pics.

Posted by: ugordan Nov 29 2005, 12:16 PM

Regarding the possibility of more intense Enceladus investigation during the (hopefully) extended mission, how long will it be until the south pole dips into years-long darkness?
I take it any extended mission will need focus on the south pole observations early on to ensure good coverage of the area while it's still receiving some sunlight. Might be interesting to repeatedly fly above the tiger stripes at a very low altitude and look for small changes in the fissures/try to locate the hotspots, at the same time sniffing out the plume materials to find out the composition of the heavier components.

Optional gravitational passes could (as well as CIRS nighttime temperature mapping), of course, be carried out later on when the focus shifts on Titan flybys.

Posted by: jmknapp Nov 29 2005, 02:18 PM

We're set for a Christmas rerun:



The above is oriented with north pointing straight up.

Posted by: ugordan Nov 29 2005, 02:33 PM

QUOTE (jmknapp @ Nov 29 2005, 03:18 PM)
The above is oriented with north pointing straight up.
*

That's practically the same viewing geometry, it should really provide clues to the temporal variability of the outgassing. The February images seem to show a slightly weaker outgassing effect, possibly due to less favorable viewing conditions. I wonder whether we'll see even more violent plumes than now or their virtual absence... It would be a nice Christmas present to be there just as there's another major eruption like the one that was supposedly responsible for a large increase in oxygen atoms in the rings, detected prior to Cassini's arrival...

Posted by: jmknapp Nov 29 2005, 03:40 PM

QUOTE (ugordan @ Nov 29 2005, 10:33 AM)
That's practically the same viewing geometry, it should really provide clues to the temporal variability of the outgassing.
*


I'm hoping that they take more images at different points during the flyby. The orientation of the crescent changes quite a lot during the flyby, & it's convincing to see that regardless of that orientation, the fountains remain at the purported location.

For the Nov. 27 flyby, images were taken from 144,000 - 174,000 km. Taking the images at the extremes of this range, and rotating them so north is up yields:



raw images:
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=54847
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=54829

So the fountains remain due south as the crescent rotates around. It might be interesting to see how their appearance changes at more points during the Dec. 25 flyby, different solar phase angles, etc.

Posted by: The Messenger Nov 29 2005, 05:18 PM

QUOTE (ugordan @ Nov 29 2005, 05:16 AM)
Optional gravitational passes could (as well as CIRS nighttime temperature mapping), of course, be carried out later on when the focus shifts on Titan flybys.
*

NO! No! Nooo!

There is no question that there is outgassing, no question about the source. If we want to know the reason there is a hot spot, we must establish more constraints, and the gravity data is an absolutely essential step. We know the energy is not solar - Enceladus is too reflective. We must learn how the mass of Enceladus is internally distributed in order to assess geological stress and other potential sources of heat.

By all means, more imaging passes should be scheduled into the extended mission, but no one knows when this mission will end, and we will never solve the puzzle without gravitational constraints.

Posted by: Omega Nov 29 2005, 05:53 PM

Regarding possible artifact--

QUOTE
Images of other moons, such as Tethys and Mimas, taken in the last 10 months from similar lighting and viewing geometries, and with identical camera parameters, were closely examined to demonstrate that the plume towering above Enceladus' south pole is real and not a camera artifact.

Posted by: volcanopele Nov 29 2005, 07:30 PM

QUOTE (edstrick @ Nov 29 2005, 12:50 AM)
NOW..... VolcanoPele... tell me again this waas just the same old problem with scattered light in the camera....?
*

For quite a while, scattered light in the camera was thought to be the cause of the plume appearance. this changed, as the caption for the January image mentioned, we took additional high phase images of the other satellites that showed that plume like features only appeared with certain twist angles of the camera, which did not match the January and February images, thus showing that the plume was real. This weekend's images sealed the deal.

We could have gone to press with the plumes story months ago. But it was important that we rule out artifact before hand. Just because it quacks like a duck and looks like duck doesn't rule out that it isn't a robotic duck. You have to look underneath and look for the on-off switch to seal the deal.




Some where in there was a good analogy. Don't know where it went.

Posted by: jmknapp Nov 29 2005, 08:08 PM

One thing that strikes me in the CICLOPS press release:

"...it is not clear if the plume emanating from the south pole arises because of water vapor escaping from warm ice that is exposed to the surface, or because at some depth beneath the surface, the temperatures are hot enough for water to become liquid which then, under pressure, escapes to the surface like a cold Yellowstone geyser." http://ciclops.org/view.php?id=1652

If it was water vapor escaping from exposed ice, wouldn't it be a diffuse cloud over the area, rather than concentrated in fountains or jets? And would the water molecules or other material have the escape velocity needed to join the e-ring? Are the linear features in images like that below then rays of sunlight shining through a diffuse cloud (like sunbeams through clouds or forest cover on Earth) rather than the representing the fountains themselves?



And yet they are using the term fountains, which would imply some sort of pressurized spray, no?

Posted by: scalbers Nov 29 2005, 08:44 PM

My guess is that the rays are actual streamers of material instead of sun rays. This somewhat reminds me of cometary jets. Are cometary jets presumed to be liquid or vapor generally? I suppose a geyser on earth can also emit either vapor (condensing into steam) or liquid water. Would a determination of ice particle size help constrain whether it was likely to be steam or water?

Posted by: Rob Pinnegar Nov 29 2005, 09:20 PM

QUOTE (scalbers @ Nov 29 2005, 02:44 PM)
My guess is that the rays are actual streamers of material instead of sun rays. This somewhat reminds me of cometary jets.

<Grin> So do we have to reclassify Enceladus as a comet now?

Posted by: Marz Nov 29 2005, 09:20 PM

QUOTE (scalbers @ Nov 29 2005, 02:44 PM)
Would a determination of ice particle size help constrain whether it was likely to be steam or water?
*


This is really exciting news! How much evidence does this point to a subsurface ocean, if only periodic? What are the odds of doing spectrometer study of the E ring to look for ammonia-based impurities... and perhaps trace amounts of amino acids? If there was a chance of organics in the plumes, how urgent would it be to send an "iceclipper" style mission to sample the plumes directly for signs of complex chemistry?

Posted by: JRehling Nov 29 2005, 09:21 PM

QUOTE (scalbers @ Nov 29 2005, 12:44 PM)
My guess is that the rays are actual streamers of material instead of sun rays. This somewhat reminds me of cometary jets. Are cometary jets presumed to be liquid or vapor generally? I suppose a geyser on earth can also emit either vapor (condensing into steam) or liquid water. Would a determination of ice particle size help constrain whether it was likely to be steam or water?
*


There is clearly some anisotropy going on here, and I think we can rule out the solar illumination and the camera as possible sources... It appears as though these fountains have a considerable non-vertical component in their initial emission, which means that this isn't just a mist rising lazily up from warm ice -- these are geysers.

We see more than three fountains (more like eight or nine, plus possibly more minor ones), so I agree that the tiger stripes are probably active in selective locations, and not everywhere along a stripe once.

As for the dynamics, we surely have some sort of pressure below. The crust may be moving, although in what way, I don't claim to know. Surely if the volume of emission falls below a certain rate at any location, rapid freezing will overwhelm the process and pinch a fault (locally) closed.

I think that each tiger stripe overlies a warm area about as wide as the area *between* the stripes, with the stripe being the place where emission can take place. This activity probably shifts over time, although it may be going on all the time *somewhere*. For example, maybe there are sixty or so places where fountains can spray out, but at any given time, only ten or so are actually spewing. And, yes, I pulled those numbers out of thin air.

I'm not sure what kind of mission would target Enceladus next. An orbiter might face a mechanical hazard from the plumes. The question is if a stable orbit could fly above the plumes. If the plumes are all highly localized, one solution would be an inclined orbit that misses the full blast of the spray. A lander is always an option, and seismology would be interesting. Another possible followup would be a Saturn orbiter -- perhaps one that observes both Enceladus and the rings without sacrificing too much in design to the dual needs? It seems less likely that a focused Enceladus+Titan mission could do much beyond what Cassini already provides. Other than quaint images of opportunity, it seems like a mission dedicated to one of those high-priority moons would not offer much in terms of performing science at the other. An incremental improvement on Cassini would fit the bill, but I think any Titan followup will be a quantum leap from Cassini to the next thing (eg, aerobot).

Posted by: tedstryk Nov 29 2005, 11:44 PM

This makes me wonder about Europa, where high phase coverage is extremely limited. Also, a volcano of equal force would have a smaller plume at Europa, since there would be much stronger gravity.

Posted by: jmknapp Nov 30 2005, 01:59 AM

QUOTE (ugordan @ Nov 29 2005, 08:16 AM)
Regarding the possibility of more intense Enceladus investigation during the (hopefully) extended mission, how long will it be until the south pole dips into years-long darkness?
*


Don't know, but it will still be in light for the next scheduled close flyby, over two years from now--March 2008! Currently that flyby is set to pass right over the south pole at a fairly close distance:



Note the field of view above is 15 degrees, and the NAC fov is 0.35 degree. At least they'll have plenty of time to figure out which areas to target.

Posted by: BruceMoomaw Nov 30 2005, 03:00 AM

QUOTE (jmknapp @ Nov 29 2005, 08:08 PM)
If it was water vapor escaping from exposed ice, wouldn't it be a diffuse cloud over the area, rather than concentrated in fountains or jets? And would the water molecules or other material have the escape velocity needed to join the e-ring? Are the linear features in images like that below then rays of sunlight shining through a diffuse cloud (like sunbeams through clouds or forest cover on Earth) rather than the representing the fountains themselves?

And yet they are using the term fountains, which would imply some sort of pressurized spray, no?
*


What they're referring to is the possibility that ice is rising to the surface in the hottest spots which -- although still cold enough to be solid -- is warm enough that water vapor sublimates off it like crazy, but only in those limited places, and then refreezes as a cloud of microscopic ice particles.

We are definitely looking at genuine, honest-to-God separate plumes here, just as we are with comet nuclei.

As for Europa plumes, don't forget that Galileo made at least one intensive search for them using a similar technique, and came up totally empty-handed. I think Europa is currently in the cold, thick-crust portion of its tidal-heating cycle, so that plumes of any significant size are rare or actually nonexistent. In another few tens of millions of years, it will be a different matter.

But as for Enceladus: while we now know that there are indeed geysers, we still have no idea just what's driving them -- and may not know for a long time. (We apparently don't even know yet how much ammonia is mixed with the water.) For a summary of the current debate, see

http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05%2Ffm05&maxhits=200&="P32A-04"

http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05%2Ffm05&maxhits=200&="P32A-05"

Posted by: tedstryk Nov 30 2005, 03:54 AM

Galileo came up cold, but Europa is much bigger, and the plumes would therfore be smaller. And so it could simply have looked in the wrong place, or at the wrong time. It is worth some surveying.

Posted by: The Messenger Nov 30 2005, 04:34 AM

QUOTE (BruceMoomaw @ Nov 29 2005, 08:00 PM)
...
But as for Enceladus: while we now know that there are indeed geysers, we still have no idea just what's driving them -- and may not know for a long time.  (We apparently don't even know yet how much ammonia is mixed with the water.)  For a summary of the current debate, see

http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05%2Ffm05&maxhits=200&="P32A-04"

http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05%2Ffm05&maxhits=200&="P32A-05"
*

Water plumes, visible from space, but no ammonia detected yet, anywhere in the atmosphere? I don't think it is there in ANY significant quantities.

Posted by: edstrick Nov 30 2005, 06:24 AM

Various comments and responses:
Looking at jmknapp's side-by-side posting of images from 144000 and 174000 km, there are quite strong differences in plume apperances. I really doubt that the plumes are "chugging"--varying rapidly with time over an hour or whatever, so I expect that the effect is parallax. Since multiple observation were taken over a moderate range of viewing azimuths, if the plumes were in fact time-invarient, it will be possible to do computed tomographic reconstructions of their 3-D structure. Not perfect, since it would be "limited angle tomography", but extremely useful.

Regarding flyby's optimized for gravity measurements, once the pole is in winter night, imagery will be relatively useless, unless the plumes continue to vent into sunlight well the shadowed surface. Thermal imaging would still be useful, but nighttime passes would be most useful to "divert" to gravity measurements. Gravity mapping from flyby's is best done with passes at different latitudes and different longitudes, so (besides searching for gravitational anomalies) they can measure the triaxial ellipsoid shape of the gravity field to compare with the triaxial ellipsoid shape of the surface, which tells you a *LOT* about concentration of mass toward the center: Core vs No-Core, etc. Magnetic data from such flybys also tells you about interactions with Saturn's mag field, which at Jupiter revealed electically conductive, presumably fluid, layers inside moons.

Regarding scattered light problems, while I'd seen fogging and some blotching beyond the limb of earlier moon images, I never saw anything that really looked plume-like, and never got pointed toward specific comparison images. What really pushed my buttons in the January images were that the features progressively increased in contrast, narrowed, and sharpened toward the limb, and there were no plumelike features at all on the terminator side of the overexposed and saturated crescent, just the usual trace of camera-fog.

Anyway, I'm not claiming credit.. that goes to the team, paricularly for a very nicely designed imaging sequence that covered all bases and seems to have provided far better information on the venting than can be extracted from the previous images. I'll be very interested in color results and phase angle dependent photometry. Does the December sequence go to higher phase angles?.. the graphic suggests it may..... the results might be even more spectacular if so.

jmknap: If the venting is from exposed ice, the hottest ice will "retreat" relative to colder ice. Everythign I see in the images of Enceladus suggests an extremely high thermal gradient below the surface. We can arm-wavingly-imagine venting pits retreating down into the surface, forming volcano-like or dry gyser-like pipes, with hot sublimating ice tens to hundreds of meters below the local surface.

Marz: The E-ring is very very faint, except at very high phase angles, where foreward scattering of sunlight by the fine dust-like ice grains makes it relatively bright. This scattering is probably mostly by diffraction or "Mie scattering" (which makes colorless and sometimes colored aureoles around the sun in our sky), and probably has only weak internally-scattered contributions that yield composition data. Certainly, there will be attempts as the mission proceds to get VIMS spectra of the plumes as close to the sources as possible. Cross fingers that they see anything beyond water ice.

I don't know the last decade's post-Voyager science on the E-ring, much less any of Cassini's beyond press release data. One model of the ring based on earthbased and Voyager photometry (and maybe polarimitry?) data stated that the ring had a narrow size range of particles and appeared to consist of more or less spherical water "droplets": flash frozen ice-sphere grains, rather than snow-like or crushed ice grains. This was one thing that suggested long before Cassini got there that Enceladus might actually be venting, rather than that we were seeing something like a torus of ejected ice from a recent impact or something.

Posted by: edstrick Nov 30 2005, 06:29 AM

And, Oh... from the Utter Loonacy Department.

Mimas's resemblence to the Death Star is not coindicence. The craters all over it show it was bombarded to inoperability and never used. Enceladus is obviously a live, powered up battle station hiding under a coating of ice, but the heat released from the reactors keeps messing up the carefully cratered camoflauged surface.

(and if you believe THAT.....I've got a cost effective operational space shuttle system to sell you...)

Posted by: Webscientist Nov 30 2005, 09:36 AM

Astonishing news,
I'm really surprised to see that a moon 10 times as small as Titan in diameter ( 500 km ) is able to generate so much energy from its interior. It should be a dead world, like our moon. We have now a strong evidence that a subsurface ocean is a possibility beneath this icy crust.But, how deep is this icy crust?
Someone has concluded that the temperatures in the south polar grooves from which the vapor is ejected might reach 300 k, that is 26.85°C or 80.33°F.

www.titanexploration.com

Posted by: Bill Harris Nov 30 2005, 10:42 AM

> from the Utter Loonacy Department...

Aye, Cap'n Ed, we canna fire the engines up anna quicker....

<VBSEG>

--Bill

Posted by: jmknapp Nov 30 2005, 01:24 PM

QUOTE (edstrick @ Nov 30 2005, 02:24 AM)
Various comments and responses:
I really doubt that the plumes are "chugging"--varying rapidly with time over an hour or whatever, so I expect that the effect is parallax.  Since multiple observation were taken over a moderate range of viewing azimuths, if the plumes were in fact time-invarient, it will be possible to do computed tomographic reconstructions of their 3-D structure.  Not perfect, since it would be "limited angle tomography", but extremely useful.
*


Another complication would be that the solar phase angle is changing throughout, but maybe you're right that something could be done in that vein. FWIW, here's the position of the limb at 144,000 and 174,000 km out:



So that shows the range of angles involved. There were 19 NAC frames taken beween these extremes, at varying exposures.

Interesting that at 144,000 km the limb was nearly parallel and on top of the center scratch, and at 174,000 it cut across all three somewhat.

Posted by: ugordan Nov 30 2005, 01:38 PM

QUOTE (jmknapp @ Nov 30 2005, 02:24 PM)
Interesting that at 144,000 km the limb was nearly parallel and on top of the center scratch
*

Of course you meant tiger stripe, cats and scratches reside on a totally different moon tongue.gif

EDIT: So far, have there been any estimates from these recent images on the amount of material released/second?

Posted by: jmknapp Nov 30 2005, 04:48 PM

QUOTE (ugordan @ Nov 30 2005, 09:38 AM)
Of course you meant tiger stripe, cats and scratches reside on a totally different moon  tongue.gif

EDIT: So far, have there been any estimates from these recent images on the amount of material released/second?
*


And here I thought it looked like a giant tiger had dragged his claws across the south pole, and deeply!

As for the material, I wonder how much it could really be, as nothing is visible looking down over the south pole (marked with a circle):

http://cassinicam.com/spnac.jpg

In the "forget what we told you yesterday as fact" department, compare this statement from the latest CICLOPS home page:

"A fine spray of small, icy particles, emanating from the warm, geologically unique province surrounding the south pole of Enceladus and believed now to supply the material comprising Saturn's E ring, was first observed in images taken back on Jan. 16, 2005." http://ciclops.org/index.php?flash=1

...to the statement made back in July by Linda Spilker, the Deputy Project Scientist for the Cassini-Huygens mission:

"The water vapor is very different from the E ring particles themselves. So we have this sort of cloud, patchy atmosphere over the south pole, and then the E ring particles seem to be coming uniformly off of Enceladus, probably through micrometeorite impact kicking up particles. So the vents are not the source of the E ring." http://www.planetary.org/news/2005/0730_Enceladus_South_Polar_Stripes_Spew.html

Posted by: BruceMoomaw Dec 1 2005, 12:33 AM

That last quote was actually a misinterpretation by Emily Lakdawalla of what Spilker told her, which is that there was some evidence from particle distribution that the E Ring particles were coming off Enceladus as a whole rather than from the vents. That particular theory very quickly became inoperative; it's the vents, all right.

What I can't yet discover is whether the stuff being spewed from them is a water/ammonia mixture (as would have seemed logical), or just plain water. I'm still trying to get clarification on this, but Cassini seems to be indicating that much more of the nitrogen of Saturn's moons is instead in the form of HCN than had been previously been believed.

Posted by: mars loon Dec 1 2005, 12:48 AM

QUOTE (BruceMoomaw @ Dec 1 2005, 12:33 AM)
What I can't yet discover is whether the stuff being spewed from them is a water/ammonia mixture (as would have seemed logical), or just plain water.  I'm still trying to get clarification on this, but Cassini seems to be indicating that much more of the nitrogen of Saturn's moons is instead in the form of HCN than had been previously ben believed.
*

That is the key question. Are ammonia, HCN, organics present?

Thats the scientific justification for NASA/ESA sending a lander/penetrator to Enceledus in a follow on mission to the Saturnian system

Posted by: The Messenger Dec 1 2005, 03:17 AM

QUOTE (edstrick @ Nov 29 2005, 11:24 PM)
What really pushed my buttons in the January images were that the features progressively increased in contrast, narrowed, and sharpened toward the limb, and there were no plumelike features at all on the terminator side of the overexposed and saturated crescent, just the usual trace of camera-fog.

Anyway, I'm not claiming credit.. that goes to the team, paricularly for a very nicely designed imaging sequence that covered all bases and seems to have provided far better information on the venting than can be extracted from the previous images. 

None-the-less, it was a gutzy call at a time when everyone seems to be stuck scratching their heads...including Bruce.

Posted by: ugordan Dec 1 2005, 08:39 AM

QUOTE (BruceMoomaw @ Dec 1 2005, 01:33 AM)
What I can't yet discover is whether the stuff being spewed from them is a water/ammonia mixture (as would have seemed logical), or just plain water.
*

I'm certainly not an expert on this, but haven't there been some mentions recently about ammonia being destroyed by UV sunlight practically immediately so that's one of the reasons it hasn't been detected yet? Personally, though, I'm having a hard time imagining such a rapid breakdown rate which would prevent any traces of NH3 to be seen. I could buy that for the Enceladus' surface spectra which obviously gets a lot of sunlight, but these plumes should conceivably escape from the interior fast enough to bring some ammonia high up before it breaks down. Which makes me wonder: would UVIS be capable of picking up a NH3 signature if a suitable stellar occultation pass could be set up right "through" the plumes? What about INMS?

Posted by: deglr6328 Dec 1 2005, 09:14 AM

There is a second derivative of two ammonia absorption lines which occurs in the near IR at 788nm and is quite strong. VIMS should have ample spectral resolution to pick it out. The line overlaps with a methane overtone at 790nm as seen here in this jupiter spectra but there should be relatively little methane around in this particular observation and if there is any it should be easily constraind by its other strong absorption lines. Also INMS is perfectly suited to detecting this sort of thing if a plume flythrough were to occur.

Posted by: edstrick Dec 1 2005, 11:28 AM

Ugordan: "....but haven't there been some mentions recently about ammonia being destroyed by UV sunlight practically immediately so that's one of the reasons it hasn't been detected yet? ..."

I've pointed that out, because NH3 is split by the far more abundant long-wavelength UV than H20, split by much lower intensity short-UV. (In addition, NH3 in an atmosphere like primordial Titan doesn't form the equivalent of an Ozone layer that protects the cold-trapped stuff in the atmosphere below.)

But this pretty much only applies to solid moon surfaces where exposure times are generally *LONG*. Given plausible ice+ammonia compositions for frosts, and the solar spectrum, somebody with the skills and knowledge can (and I'd assume has) published detectability lifetimes for ammonia containing surface frosts in the outer solar system, but I've never seen numbers.

Something like the plumes is entirely different. I can imagine the stuff spread out along the E-ring to have lost NH3 to photolysis, but I can't imagine that stuff in the Enceladus diffuse plume or the narrower jets to be severely ammonia depleted.

I have total confidence that the Cassini mission will try during primary and extended missions to get the best info they can on plume composition and trace vapor/ice limits.

Posted by: jmknapp Dec 1 2005, 12:23 PM

QUOTE (BruceMoomaw @ Nov 30 2005, 08:33 PM)
That last quote was actually a misinterpretation by Emily Lakdawalla of what Spilker told her, which is that there was some evidence from particle distribution that the E Ring particles were coming off Enceladus as a whole rather than from the vents.  That particular theory very quickly became inoperative; it's the vents, all right.
*


Seems like you went through this before, and it turns out that Emily Lakdawalla's quote is dead-on accurate & there is little room for misinterpretation--just furious backpedaling.

The article also reports:

"A different in-situ instrument, the Cosmic Dust Analyzer (CDA), had measured Saturn's E Ring particles during previous trips through the ring plane. The vaporous atmosphere detected by UVIS and INMS does not match the particulate nature of the E ring, Spilker said." http://www.planetary.org/news/2005/0730_Enceladus_South_Polar_Stripes_Spew.html

It doesn't say what those particles are generally--I suppose ice? So I guess the trick is to detect ice particles coming out of the fountains or else figure out how they could form out of the vapor later?

Posted by: jmknapp Dec 1 2005, 01:08 PM

Closeups of the tiger stripes are intriguing but a little hard to sort out because of the "crater effect" or whatever it's called where features can seem to flip between convex or concave with each blink of the eye. I think the following (approx. south pole marked with a circle) shows raised ridges on each side of the fissure, as if material has spread out from the fissure?



Knowing that the sun is to the left a bit helps to disambiguate things, based on the shadows.

Another stripe detail:


Posted by: dvandorn Dec 1 2005, 01:14 PM

Based on the shadows, the sun is on the *right*... unless you posted the image upside-down to the way you were looking at it when you made the comment.

-the other Doug

Posted by: jmknapp Dec 1 2005, 01:22 PM

QUOTE (dvandorn @ Dec 1 2005, 09:14 AM)
Based on the shadows, the sun is on the *right*... unless you posted the image upside-down to the way you were looking at it when you made the comment.

-the other Doug
*


I don't think so. Here's an expanded view which shows a bit of the terminator in the lower right. A particuarly long shadow stretching to the right is marked by the arrow:



So the sun would be to the left.

Posted by: ugordan Dec 1 2005, 02:04 PM

QUOTE (jmknapp @ Dec 1 2005, 02:22 PM)
So the sun would be to the left.
*

That last image certainly put the things into context, it's really hard to figure out what's high and what's low from small sections of that image you posted. In fact, if the geometry of the image is what I think it is, the sun is precisely on the left hand side, illumination being parallel to the image scanlines.

Posted by: dvandorn Dec 1 2005, 02:18 PM

In the context image, your point is clear... but geez, everything that looked like a hole in the original image turns out to be a bump! That's totally counter-intuitive to someone who's been looking at cratered terrains his whole adult life... blink.gif

-the other Doug

Posted by: jmknapp Dec 1 2005, 02:40 PM

QUOTE (dvandorn @ Dec 1 2005, 10:18 AM)
In the context image, your point is clear... but geez, everything that looked like a hole in the original image turns out to be a bump!  That's totally counter-intuitive to someone who's been looking at cratered terrains his whole adult life...  blink.gif
*


YMMV, but I find it easier to see correctly if rotated like so:



So the sun is coming in from the top in that view.

It's like ice has flowed from the rift and spread outward, complete with striations going out quite a way, reminiscent of the mid-atlantic ridge perhaps?


Posted by: David Dec 1 2005, 02:47 PM

Enceladus is the "shiniest" moon in the solar system, with an albedo of .99. Is it jumping the gun to assume that this is because it is continually being re-frosted with the material from these plumes? If not, are there any differences in reflectivity -- e.g., is the south pole "shinier" than the north pole? Or does the material just float all the way around the moon and coat it pretty much evenly?

Posted by: ugordan Dec 1 2005, 02:55 PM

QUOTE (David @ Dec 1 2005, 03:47 PM)
Enceladus is the "shiniest" moon in the solar system, with an albedo of .99.  Is it jumping the gun to assume that this is because it is continually being re-frosted with the material from these plumes? If not, are there any differences in reflectivity -- e.g., is the south pole "shinier" than the north pole?  Or does the material just float all the way around the moon and coat it pretty much evenly?
*

Those are perfectly logical assumptions. Any snow/ice lying around long enough is bound to become dirty due to constant micrometeoroid bombardment. It has long been realized that the south hemisphere is whiter and younger than the north hemisphere, which does show some signs of dust contamination. There are even recent Cassini global false color mosaics that show the difference in appearance of the southern and northern regions. Apart from being fresh ice, the ice in the tiger stripes is noticeably bluer which says the ice grains are coarser and hence younger because long exposure to cosmic radiation destroys the fine crystalline structure.

At least so I've been told...

Regarding the boulders in the highest resolution image, what are the odds they were expelled during an explosive eruption in the past?

Posted by: Bill Harris Dec 1 2005, 04:09 PM

QUOTE (jmknapp @ Dec 1 2005, 08:40 AM)
It's like ice has flowed from the rift and spread outward, complete with striations going out quite a way, reminiscent of the mid-atlantic ridge perhaps?


*


That is my mental image of what is happening on Enceladus, too. But I'm not sure if there are rifts and speading centers a la a mid-ocean ridge, but rather a bouyant plume of ice/water that domes the surface, causing tension cracks, and the water vaporizes in the low surface pressure at various and variable openings in the fractures to create the plumes. On the few images of these stripes I've looked at I haven't seen much that impresses me as spreading centers, nor have I seen corresponding subduction zones. "YMMV"

This is yet another odd world.

--Bill

Posted by: tasp Dec 1 2005, 05:01 PM

How fast are these plumes turning Enceladus inside out?

How much material falls back on to the surface, and how much is permanently lost to the E ring?

If the Cassini extended mission lasts till the next Saturnian equinox, perhaps photos of Enceladus' shadow on other moons will tell us more too. And Enceladus passing throught the shadows of other moons might help too.

Any chance of a radio occultation of Cassini's transmissions by the plumes?

Posted by: ugordan Dec 2 2005, 08:10 AM

QUOTE (tasp @ Dec 1 2005, 06:01 PM)
Any chance of a radio occultation of Cassini's transmissions by the plumes?
*

And if the occultation can be made simultaneously with the gravity/mass distribution measurements all the better. I'm not sure DSN can do both at the same time, can it?

It depends on the speed the vapor/dust is expelled, but Enceladus has a very low surface gravity as is. Any venting is more likely to resemble cometary jets than Io-like plumes. It's also fairly visible in the CICLOPS color-coded image showing the extent to which the plumes rise. Encleadus gravity is so weak that any material expelled far enough is likely to go into Saturn orbit instead, hence the E-ring.
The fresh south hemisphere implies that at least some stuff falls back down, but the relatively dirtier north hemisphere suggests that the stuff escapes into space rather than ballistically being deposited there also.

Posted by: dvandorn Dec 2 2005, 08:14 AM

The question that comes to my mind is:

If "warm" ice is convecting towards Enceladus' surface and then fountaining out of these south polar vents in significant quantities, could this have been happening for a very large percentage of Enceladus' existence?

Do we have any clue of how much mass is entrained in the E ring? And can we even estimate the rate of mass lost from Enceladus due to this process? Because, for example, even if it's only losing a few tons of material a day, after billions of years, such venting would significantly reduce the mass and size of the body. And what would happen to an icy moon that has lost a significant amount of mass from within -- wouldn't there be signs of global crustal compression?

I guess it depends on what's heating the interior ice and forcing convection of "warm ice" to the surface. Since tidal heating seems unlikely for such a small body, perhaps it's radiogenic? Maybe Enceladus happened to form around a rocky core that, for some as-yet-unguessed reason, had an anomalous amount of radiogenic minerals within it?

If that's the case, then maybe Enceladus started out a lot bigger and has been losing mass -- and size -- for billions of years. Otherwise, you'd have to think that the activity we're seeing now is relatively rare, and we're lucky to be seeing it...

-the other Doug

Posted by: jmknapp Dec 2 2005, 11:18 AM

Interesting comments from Dr. Carolyn Porco:

"We suspect it could be caused by cold vents that lead from somewhere in the subsurface, perhaps as far as 1 kilometer down. Water ice is sublimating (changing directly from a solid to a gas state) and the vapors are coming off and building up to high pressure." http://dsc.discovery.com/news/briefs/20051128/enceladus_spa.html

Since the e-ring has been determined to be particulate, maybe these sublimating water vapor jets have enough pressure to pick up ice particles and send them into space with escape velocity?

Based on a http://cmi.yale.edu/cgi-bin/bh_calc.cgi I get 240 m/sec for escape velocity at the surface of Enceladus.

Also from the article:

"What's puzzling us is how it's getting hot enough," Porco said. "We're still in a quandary over how you'd get this much energy."

Posted by: ugordan Dec 2 2005, 11:50 AM

QUOTE (jmknapp @ Dec 2 2005, 12:18 PM)
Interesting comments from Dr. Carolyn Porco:

"We suspect it could be caused by cold vents that lead from somewhere in the subsurface, perhaps as far as 1 kilometer down. Water ice is sublimating (changing directly from a solid to a gas state) and the vapors are coming off and building up to high pressure." http://dsc.discovery.com/news/briefs/20051128/enceladus_spa.html

So they're more or less abanoning the idea of there actually being liquid water below?

QUOTE (jmknapp @ Dec 2 2005, 12:18 PM)
Based on a http://cmi.yale.edu/cgi-bin/bh_calc.cgi I get 240 m/sec for escape velocity at the surface of Enceladus.

Views of the solar system says 212 m/s, so that's probably about it. It doesn't mean the ice particles need to have this velocity to escape Enceladus, this figure is a theoretical speed needed for an object to reach infinity from the surface. In reality, Saturn's gravitation is bound to take over long before that so the real escape velocity could be substantially lower than that.

Posted by: jmknapp Dec 2 2005, 11:57 AM

QUOTE (ugordan @ Dec 2 2005, 07:50 AM)
So they're more or less abanoning the idea of there actually being liquid water below?
*


The article also states, not quoting Porco directly:

"Another possibility is that Enceladus' energy source is even hotter than suspected and the water ice is actually melted into an underground liquid that is creating hot springs, similar to the geysers found at Yellowstone National Park and elsewhere on Earth."

However Porco seemed to be highlighting the sublimation theory foremost ("We suspect...").

Posted by: ugordan Dec 2 2005, 12:15 PM

Regarding the source of the heat, there is a relatively recent theory that someone mentioned a while ago, probably by someone on this forum, but for the life of me I can't remember who or where it was mentioned.

Basically, it speculates the extra heat on Enceladus comes from a "secondary spin-orbit resonance", and if I understand correctly, comes from Enceladus' orbital eccentricity coupled with its tri-axial shape and its wobbling during each orbit that produces extra heating (hundreds of times more), apart from the usual tidal bulging. It states Enceladus has "just the right shape" for this resonance and that it's probably a relatively short term (geologically speaking) effect.

EDIT: I can understand why Carolyn would prefer the "less" exciting speculation and her being cautious, jumping around saying they've found liquid water on another world without rock-solid proof wouldn't be very wise. It's the same as with the Feb 17 plume images, they didn't scream out loud they may have found plumes, they waited for much more convincing proof. That doesn't mean the evidence so far actually prefers the warm ice theory.

Posted by: BruceMoomaw Dec 2 2005, 12:19 PM

QUOTE (dvandorn @ Dec 2 2005, 08:14 AM)
The question that comes to my mind is:

If that's the case, then maybe Enceladus started out a lot bigger and has been losing mass -- and size -- for billions of years.  Otherwise, you'd have to think that the activity we're seeing now is relatively rare, and we're lucky to be seeing it...

-the other Doug
*


This just might explain why Enceladus has the highest density -- and thus the biggest rocky core relative to its size -- of any of the smaller Saturnian moons.

Posted by: jmknapp Dec 2 2005, 01:21 PM

Regarding Enceladus' density, Wikipedia quotes the current Cassini-derived estimate of 1.61 g/cm^3.

Conversely, in 1994 an article was published in http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1994Icar..109..241D&db_key=AST&data_type=HTML&format= where the abstract states:

"Using the observed shape alone, without any other assumptions other than that the satellite is in hydrostatic equilibrium at its present orbital radius, we place an upper bound on the mean density of 1.12 +/- 0.05 g/cu cm. Thus, the mean density of Enceladus is probably little more than that of water-ice and we conclude that this satellite is markedly deficient in rock."

My how things change.

The abstract continues:

"If the mass of a satellite is unknown, but the satellite is differentiated and has a deep mantle of known composition, then we show that measurement of the shape alone can lead to a determination of the satellite's mass, mean density, and moment of inertia. Application of this method to Enceladus, assuming that the satellite has a deep mantle of water-ice of density 0.93 g/cu cm, gives the result that the mean density of the satellite is 1.00 +/- 0.03 g/cu cm. This result fills the one remaining gap in our knowledge of the structure of the Saturnian satellite system.We now know the mean densities of all the primary Saturnian satellites in the sequence from the coorbital satellites, Janus and Epimetheus, through to the outer satellite Iapetus (the densities of the small, secondary satellites in Trojan-type orbits are still unknown)."

Gotta be careful of the "we now knows" I guess.

Posted by: Bill Harris Dec 2 2005, 02:11 PM

I seem to recall reading that the mass of material in the rings is equivalent to a small satellite, ~200Km in size. Can't re-locate that reference, so I'm not certain.

Attached is an image from the JPL site of one version of the plume mechanism (sorry, didn't bookmark that page, either).

Interesting place...

--Bill

Posted by: volcanopele Dec 2 2005, 06:39 PM

I'll try to answer some of the questions posed in the last day:

ugordan: the escape velocity used by Views of the Solar System is based on the Voyager-era derived mass of Enceladus. As jmknapp pointed out, the closer Cassini flybys have really helped to pin down the mass, which proved to be much higher than expected. Thus the calculated density rose from 1-1.1 g/cc from the Voyager-era mass to 1.61 g/cc using the Cassini-derived mass. So the escape velocity is 240 m/sec. And as BruceMoomaw tried to point out, this indicates that Enceladus has a much larger rock fraction than the other satellites (only Dione at 1.4 g/cc comes close; interesting coincidence that density corresponds to geologic activity, hmm).

And good on ya for remembering the secondary spin-orbit resonance, more on that later... But yes, that is one possibility.

jmknapp: regarding the jetting mechanism, there are a couple of hypotheses that may account for our observations. First, http://photojournal.jpl.nasa.gov/catalog/PIA07725. This is the possibility mentioned by Dr. Porco. This hypothesis has the main advantage of not needing as much heat as the second possibility, so you don't need nearly as much energy. The layer heating the ice from below could be a mix of water and ammonia (and thus heated to 170K or so) and the sublimating ice can still be pure water vapor, as was seen by INMS. The second hypothesis is that http://photojournal.jpl.nasa.gov/catalog/PIA07726. This gas then jets out into space. This possibility requires enough heating to produce pure liquid water, and thus quite a bit more energy would be required. Right now both possibilities, at least as evaluated by late August, are possible, though these new images may require a reassessment. Until we can say one way or the other, we have to keep both possibilities open.

Composition: believe it or not, we ALREADY flew through the plume back in July. So we have INMS measurements of the gases, a UVIS occultation, and CDA data on particle flux and sizes. Summaries of these results, as released thus far, can be found at:

http://photojournal.jpl.nasa.gov/catalog/PIA07723
http://photojournal.jpl.nasa.gov/catalog/PIA03553
http://photojournal.jpl.nasa.gov/catalog/PIA03552
http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20050830_CHARM_Esposito.pdf (transcript of the talk can be found at http://saturn.jpl.nasa.gov/multimedia/products/docs/CHARM_20050830_transcript.doc )

In summary, what everyone had been calling at atmosphere is actually this plume, not a traditional atmosphere so to speak.

tasp: 1) How fast are these plumes turning Enceladus inside out? Depends on how long this has been going on. We know the mass flux (to within an order of magnitude), so I would presume such calculations could be done. at least to find out how much mass Enceladus could have lost.
2) How much material falls back on to the surface, and how much is permanently lost to the E ring? The material likely escapes Enceladus' grasp and goes directly into the E-ring. however, the bright surface of Enceladus, and its fairly uniform grain size distribution except for very young locations, suggests that Enceladus is coated with E-ring particles.
3) If the Cassini extended mission lasts till the next Saturnian equinox, perhaps photos of Enceladus' shadow on other moons will tell us more too. And Enceladus passing throught the shadows of other moons might help too. All you really need is Saturn really. Would be interesting to see what the plumes look like when Enceladus is eclipsed by Saturn.
4) Any chance of a radio occultation of Cassini's transmissions by the plumes? Maybe in the extended mission, though I can think of better measurements to pull off.

Posted by: jmknapp Dec 2 2005, 08:20 PM

QUOTE (edstrick @ Nov 30 2005, 02:24 AM)
Since multiple observations were taken over a moderate range of viewing azimuths, if the plumes were in fact time-invarient, it will be possible to do computed tomographic reconstructions of their 3-D structure.  Not perfect, since it would be "limited angle tomography", but extremely useful.
*


Taking Ed's cue I attempted to do something like this in limited fashion, to try to pin down the location of one of the larger plumes. It's hard to be really accurate, counting pixels and all, but the result of the exercise came out well. If anyone's interested I put the calculations on a page at: http://cassinicam.com/plume

Posted by: David Dec 2 2005, 08:35 PM

I'm wondering if it's appropriate to talk about "plumes"? I may have the terminology not quite right, but in hearing "plumes" I think about geysers and things like the Ionian volcanoes, which erupt intermittently. The impression I'm getting is more of continuous venting of material. Do we have any data to support either possibility -- that the steam is being emitted from Enceladus continuously, or that it gets burped out from time to time? The images of the plumes suggested localization, but I couldn't tell if they implied intermittency.

Posted by: tallbear Dec 3 2005, 02:14 AM

[quote=jmknapp,Nov 29 2005, 07:18 AM]
We're set for a Christmas rerun:



THE DEC 25 OBSERVATION IS NOT GOING TO HAPPEN

Posted by: BruceMoomaw Dec 3 2005, 11:48 AM

One thing to keep in mind is all those dark speckles flocking around the fresher-looking crevasses. They positively jump out at you in many of the better photos, and http://www.aas.org/publications/baas/v37n3/dps2005/450.htm confirms that the team has been thinking the same thing I thought about when I first saw them:

"Among the most mysterious newly-discovered features are small, sub-kilometer-sized dark spots and circular pits that sometimes cluster in a honeycomb like patterns near faults and scarps. Their origin is unknown, but perhaps the pits and dark spots identify sites of explosive venting of subsurface volatiles through fractures or volcanic conduits."

Surely these are vents, in which case the plumes are actual expelled liquid material rather than just water vapor boiling off the surface of an area of warmed solid ice. As for their darkness: I understand that there's probably a small amount of methane mixed with the liquid, and thus some of it is mixed in with the part of that expelled liquid that refreezes into ice immediately around the top of the vent -- after which solar and Saturnian radiation could turn the trapped methane into dark carbon compounds (or just plain elemental carbon), as is supposed to be the case with Pluto's dark patches.

Posted by: jmknapp Dec 3 2005, 01:27 PM

[quote=tallbear,Dec 2 2005, 10:14 PM]
[quote=jmknapp,Nov 29 2005, 07:18 AM]
We're set for a Christmas rerun:
THE DEC 25 OBSERVATION IS NOT GOING TO HAPPEN

*

[/quote]

Oh... pity. Are they too busy with Titan approach stuff at that time?

Posted by: The Messenger Dec 3 2005, 03:09 PM

QUOTE (jmknapp @ Dec 2 2005, 06:21 AM)
Regarding Enceladus' density, Wikipedia quotes the current Cassini-derived estimate of 1.61 g/cm^3.

Conversely, in 1994 an article was published in http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1994Icar..109..241D&db_key=AST&data_type=HTML&format= where the abstract states:

"Using the observed shape alone, without any other assumptions other than that the satellite is in hydrostatic equilibrium at its present orbital radius, we place an upper bound on the mean density of 1.12 +/- 0.05 g/cu cm. Thus, the mean density of Enceladus is probably little more than that of water-ice and we conclude that this satellite is markedly deficient in rock."

My how things change.

The abstract continues:

...We now know the mean densities of all the primary Saturnian satellites [/b] in the sequence from the coorbital satellites, Janus and Epimetheus, through to the outer satellite Iapetus (the densities of the small, secondary satellites in Trojan-type orbits are still unknown)."

Gotta be careful of the "we now knows" I guess.
*

I keep arguing with Bruce, with everybody really, that these constant revisions of solar masses and/or gravity anomalies are symptomatic of a weak second-order gravimetric effect that is STILL causing gross underestimates of outer planet and moon masses.

This is why it is so imparative that Cassini makes the gravity runs as scheduled. If the concept is correct, every moon of Saturn will yield unrealistic gravity anomalies upon closest-approach - much like Ganymede, only worse.

Posted by: ugordan Dec 3 2005, 06:33 PM

Since I haven't seen anyone else do it, here's an animated GIF I put together from the 4 images that used the same exposure (which best shows details in the plumes). The images were resized to compensate for the changing distance. The GIF rapidly runs back and forth and vividly shows the parallax effect of Cassini's viewing geometry. I don't believe the changes in the appearance of the plumes are due to their temporal variability, they appear to be constant.

If you concentrate on the Saturnshine lit right side of Enceladus, you get a better feel of the rotating view. Since we're viewing the tiger stripes broadside (so they're nicely sorted out distance-wise), I think it's fairly obvious that we're seeing plumes from all the tiger stripes because the ones farther from the limb would exhibit more parallax than the near ones. It's evident there's a wide range of parallax motion. Most of the plumes seem to be located on the far side of the point around which Cassini's view is rotating, which seems to fit with the fact more tiger stipes actually are on the far side of the limb.


Posted by: Decepticon Dec 3 2005, 06:37 PM

^ Now thats cool!

Nice job. cool.gif

Posted by: David Dec 3 2005, 08:58 PM

QUOTE (ugordan @ Dec 3 2005, 06:33 PM)
I don't believe the changes in the appearance of the plumes are due to their temporal variability, they appear to be constant.

If you concentrate on the Saturnshine lit right side of Enceladus, you get a better feel of the rotating view. Since we're viewing the tiger stripes broadside (so they're nicely sorted out distance-wise), I think it's fairly obvious that we're seeing plumes from all the tiger stripes because the ones farther from the limb would exhibit more parallax than the near ones.
*


Thanks! That is a very helpful summation of the images. But looking at it, I get the impression that the three bursts that I can associate with the tiger stripes are actually double -- which you might expect if the bursts are not coming from the middle of the stripes, but in parallel lines down each side of the stripes. Is this possible, or is it just an illusion?

Posted by: jmknapp Dec 3 2005, 09:26 PM

QUOTE (volcanopele @ Dec 2 2005, 02:39 PM)
I'll try to answer some of the questions posed in the last day:

ugordan: the escape velocity used by Views of the Solar System is based on the Voyager-era derived mass of Enceladus.  As jmknapp pointed out, the closer Cassini flybys have really helped to pin down the mass, which proved to be much higher than expected.  Thus the calculated density rose from 1-1.1 g/cc from the Voyager-era mass to 1.61 g/cc using the Cassini-derived mass.  So the escape velocity is 240 m/sec. 
*


Thanks for addressing that. When ugordan mentioned that the escape velocity might be lower once the escaping object came under the influence of Saturn, I thought it was a good point. But I guess that since Enceladus is in free fall already around Saturn, the latter is out of the picture gravitationally for such calculations & the escape velocity remains at 240 m/sec.

I did calculate what the gravity towards Saturn of a particle on the surface of Enceladus would be, and was surprised to see that Saturn has almost six times as much gravitational pull on said particle as Enceladus itself does.

ugordan: nice animation. That must have taken quite a bit of care to align the images.

Posted by: volcanopele Dec 4 2005, 06:08 PM

QUOTE (tallbear @ Dec 2 2005, 07:14 PM)
THE DEC 25 OBSERVATION IS NOT GOING TO HAPPEN
*

Unless you have heard something I haven't, I wouldn't exactly put the nail on that coffin JUST yet. That may change...

Posted by: mars loon Dec 4 2005, 10:48 PM

QUOTE (ugordan @ Dec 3 2005, 06:33 PM)
Since I haven't seen anyone else do it, here's an animated GIF I put together from the 4 images that used the same exposure (which best shows details in the plumes). The images were resized to compensate for the changing distance. The GIF rapidly runs back and forth and vividly shows the parallax effect of Cassini's viewing geometry. I don't believe the changes in the appearance of the plumes are due to their temporal variability, they appear to be constant.
*

Thats beautiful. thank you biggrin.gif

Posted by: JRehling Dec 5 2005, 02:49 AM

QUOTE (The Messenger @ Dec 3 2005, 07:09 AM)
I keep arguing with Bruce, with everybody really, that these constant revisions of solar masses and/or gravity anomalies are symptomatic of a weak second-order gravimetric effect that is STILL causing gross underestimates of outer planet and moon masses.
*


I'm glad that you specified "with everybody really". Bruce is not a messiah of the physics establishment, just part of the congregation.

What you suggest is a major revision in physics. What is the track record of major revisions in physics proposed on Internet chat rooms? Indulge me a moment to address the medium and not the message: You have a better chance of winning the lottery, being hit by lightning, and bitten by a rattlesnake all at the same time than revising the field of physics rather than being dead wrong.

To address the content in one small way, if the mass of Jupiter were grossly underestimated, then the Galileo probe would have arrived at grossly higher velocities than expected, and it would have undergone grossly higher accelerations and temperatures -- destroying it.

To address the content in a larger way: If you have something that wouldn't be shredded in a moment by a competent physicist, you should submit it to a peer-reviewed publication. Yes, prevailing mindsets can be resistant to new ideas, but if the prevailing physics is dead wrong and yours is dead right, then everyone would be obliged to accept your idea. To the contrary (to address your content in one more way), if the prevailing physics was highly erroneous in these matters, it would not be remotely possible to have a tour like Galileo's and Cassini's keep putting the probe in the desired location on orbit after orbit. If Ganymede's mass were very much different than expected, Galileo would have left the desired path very quickly and by very much. It didn't. So I think you have an idea that only avoids refutation if you keep it vague, nonquantitative, and sheltered from serious critique. And as such, it subtracts from any forums in which you discuss it.

Posted by: tfisher Dec 5 2005, 03:28 AM

Hear, hear, JRehling. Standard physics is well tested far past the point where any anomalies could be identified with Cassini trajectory. Frankly, any speculation about modifications to gravitational theory are pointless unless general relativity is the starting point. Messenger, I'm guessing that you don't have that background? If you do, great, start a thread and throw out some equations. I'm about to finish a Ph.D in mathematics, and I've always wanted a good excuse to learn GR at a deeper level. I'd be happy to follow along your line of thinking. Otherwise, maybe leave the unfounded (and misguided) speculation elsewhere?

Posted by: ugordan Dec 5 2005, 08:21 AM

QUOTE (jmknapp @ Dec 3 2005, 10:26 PM)
Thanks for addressing that. When ugordan mentioned that the escape velocity might be lower once the escaping object came under the influence of Saturn, I thought it was a good point. But I guess that since Enceladus is in free fall already around Saturn, the latter is out of the picture gravitationally for such calculations & the escape velocity remains at 240 m/sec.
*

Don't be so quick to dismiss my reasoning. If your argument held, it would mean Enceladus would not experience and tidal effect due to it being in free fall towards Saturn. That neglects the effect of there being quite a reasonable difference between the pull on the near and the far side of Enceladus. You can't apply an inertial system and expect to have all measured forces accounted for. You said it yourself, Saturn's gravity is much stronger than Enceladus' and only because *locally* the centripetal force of Encleadus' orbit cancels the much stronger Saturnian gravity doesn't mean the effect holds at any other distance. If the particles can get just far enough that their orbital speed (which they basically inherited from Enceladus) is too small or too large (if the particles are directed away from Saturn), they will quickly escape from Enceladus. As I said before, a particle needs not 240 m/s to escape to a height of 500 km (yes, I pulled the figure out of thin air) above the moon and be essentially free. The escape velocity is just a theoretical value, applies only to a one-body system with no other forces in play.

I hope you can now see where I'm getting at. cool.gif

Oh and about the GIF, thanks, but really it wasn't all that difficult to do...

Posted by: ugordan Dec 5 2005, 09:27 AM

I probably misunderstood what The Messenger was trying to say, but I got the impression he wasn't suggesting new physics. I thought he meant that we're hasty to deduce a moon's mass from a single gravitational run or so, assuming the mass distribution is uniform, having no lumps as in Ganymede's case. In this case, if we were to fly above such a lump, we'd assume a greater overall density for the moon. Hence the need for more gravity passes to establish a good understanding on the moon's interior.

But, I might be wrong, Messenger might have indeed proposed all new physics and I got him completely wrong...

Posted by: edstrick Dec 5 2005, 11:02 AM

The Voyager estimates of masses and densities for some of the moons were very marginal. The actual dataset used was the tracking data from 2 flybys, plus the rather noisy Pioneer 11 flyby a year before Voyager 1. (Pioneer was very close to Solar Conjunction and the signals were going through the solar corona.)

Added to the spacecraft tracking (and position location in images) was the historical record of optical observations of the moons. Precision is low, but the long time base gives moons time to show subtle orbital interactions due to resonances. Without checking old xeroxes, I think Dione's mass was rather well determined from it's resonant orbital mechanics effect on Enceladus.

The larger inner moons masses were relatively well determined, the smaller ones were more marginally measured. Cut a moon's diameter in half and it's mass is 1/8'th the original. Iapetus, Hyperion and Phoebe were so far out they interact little and they were all far from Voyager's trajectories.

I don't know how the final pre-Cassini mass estimates compared with analyses from 15-20 years earlier, but in all of those cases, to some extent, the error bars on mass estimates are educated guesses, based on adding random noise to models to judge sensativities for each case, and based on in part very educated guesses on the level of systematic errors in the data.

Cassini's long orbital tour gives more ability to separate weak gravitational effects on the trajectory and better separate different satellites masses, giving on it's own a better set of data for moon mass estimates. And for any satellite, a really close flyby gives the first chance for a really accurate mass, and for all but the outer sats, a chance to crudely estimate the difference between the oblate and tidally elongated shape of the satellite and of it's gravity field. A moon that never differentiated and has "rock" and "ice" uniformly distributed from core to surface will have a gravity field that matches it's shape. A moon that fully melted and has all the "rock" as a well defined core will have it's physical shape more flattened and stretched than the gravity field, as the deeply buried rocky core's gravity will be nearly spherical. Local gravity anomalies, if found, are "icing on the cake", datawise.

But the main point here is that searching for any deep meaning in differences between pre-Cassini moon masses and the new results is an effort in futility.

Posted by: jmknapp Dec 5 2005, 12:09 PM

QUOTE (ugordan @ Dec 5 2005, 04:21 AM)
Don't be so quick to dismiss my reasoning. If your argument held, it would mean Enceladus would not experience and tidal effect due to it being in free fall towards Saturn.


That's why I included the caveat "for such calculations," meaning for escape velocity. Tidal effects are higher order.

QUOTE (ugordan @ Dec 5 2005, 04:21 AM)
If the particles can get just far enough that their orbital speed (which they basically inherited from Enceladus) is too small or too large (if the particles are directed away from Saturn), they will quickly escape from Enceladus.


The orbital speed of Enceladus relative to Saturn is 12,640 m/sec. Seems like 240 m/sec (less than 2% of the total) is a reasonable amount needed to "de-orbit" from Enceladus, so to speak.

Posted by: jmknapp Dec 5 2005, 12:20 PM

QUOTE (edstrick @ Dec 5 2005, 07:02 AM)
But the main point here is that searching for any deep meaning in differences between pre-Cassini moon masses and the new results is an effort in futility.
*


To me the most interesting aspect is that the 1994 peer-reviewed Icarus paper (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1994Icar..109..241D&db_key=AST&data_type=HTML&format=) quoted an *upper bound* of 1.17 g/cm^3 for Enceladus and used sweeping language like "we now know that..." based on presumably scientific methods. So while physics itself may not be changing the quality and reliability of analysis varies quite a bit.

Posted by: ugordan Dec 5 2005, 12:22 PM

QUOTE (jmknapp @ Dec 5 2005, 01:09 PM)
The orbital speed of Enceladus relative to Saturn is 12,640 m/sec. Seems like 240 m/sec (less than 2% of the total) is a reasonable amount needed to "de-orbit" from Enceladus, so to speak.
*

Again, what I'm stating is that it's very likely even a much smaller delta-V is sufficient to escape from Enceladus. I don't know how much smaller, but my gut feeling tells me it out to be a significant reduction. Not and order of magnitude, but 2 or 3 times smaller might seem reasonable.
I might program a simple simulation of particle trajectories of several different speed ranges, but I'm way too lazy to do that at the moment.
Especially if it turns out someone else already did just that...

EDIT: A crude method might be using Orbiter and setting your location on the Enceladus' south pole and applying a short burn upward and seeing where it gets you. Of course, you'd need to change the current best mass estimate in the config file first. I'll play around with it a bit later, I'm currently at work...

Posted by: JRehling Dec 5 2005, 02:19 PM

No one has mentioned this yet: A gas might escape from a body without the plumes themselves exceeding escape velocity. Stand atop the LEM and throw a child's balloon of earth air *downward*, and it will pop, and the gas will eventually all escape, despite the downward initial motion.

H2O should simply diffuse away from the source on Enceladus and escape according to exponential decay. Of course, in this case, there is apparently also considerable upward velocity (or the diffusion would be more wide than high), but don't discount gas diffusion as part of the package.

Posted by: ugordan Dec 5 2005, 02:36 PM

Gaseous diffusion you're talking about has everything to do with the temperature of the gas. Temperature is a statistical figure describing the average kinetic energy of all the molecules. This translates into an average speed for molecules of certain mass. The average velocity can be much less than the moon's escape velocity, but there will always be a "tail" in the high end of the energy distribution where a small number of molecules will have a speed in excess of the moon's escape velocity. These molecules can escape the moon. Thus the exponential decay, the hotter the gas, the faster the decay is because more molecules have speeds above escape velocity. Statistically, there will be molecules escaping even for very low average energies (temperatures), but it will take a longer time.
I wasn't saying the plumes need to be directed upwards for the vapor to escape, it might as well be warm ice sublimating in all directions. The observed plumes, however, seem to be pretty well collimated.
Whether we're talking about water molecules or ice particles, there exists a lower bound on their speed that will allow them to escape into space. In any case, the theoretical 240 m/s escape figure holds for every direction, though I believe the mechanism I was talking about (Saturn helping out) benefits from upwardly directed plumes.

Posted by: tasp Dec 5 2005, 02:49 PM

Would solar UV ionize the gas? Then Saturn's magnetic field would 'sweep' gasses from Enceladus.

Although weak at Saturn's distance from sun, Poynting-Robertson effects (hope I spelled it right) would also be operative on small particles and would accelerate material from Enceladus.

Is the E-ring continuous around Saturn? Or does it 'parse' in the vicinity of Enceladus. (I'm thinking of horse shoe orbits for the E-ring particles)

Posted by: ugordan Dec 5 2005, 02:57 PM

QUOTE (tasp @ Dec 5 2005, 03:49 PM)
Would solar UV ionize the gas?  Then Saturn's magnetic field would 'sweep' gasses from Enceladus.

AFAIK, it certainly would. I think the observation of a massive increase in oxygen atoms prior to Cassini SOI indicates that water is efficiently broken down by UV rays. Still, that mechanism is probably too slow to immediately help with stripping away of the plumes.
QUOTE
Although weak at Saturn's distance from sun, Poynting-Robertson effects (hope I spelled it right) would also be operative on small particles and would accelerate material from Enceladus.

Is that the one with thermal radiation being re-emitted unevenly? I believe that's an even slower effect, especially since water ice particles don't absorb very much light and hence heat up very little.

Posted by: jmknapp Dec 5 2005, 03:53 PM

QUOTE (tasp @ Dec 5 2005, 10:49 AM)
Would solar UV ionize the gas?  Then Saturn's magnetic field would 'sweep' gasses from Enceladus.

Although weak at Saturn's distance from sun, Poynting-Robertson effects (hope I spelled it right) would also be operative on small particles and would accelerate material from Enceladus.

Is the E-ring continuous around Saturn? Or does it 'parse' in the vicinity of Enceladus.  (I'm thinking of horse shoe orbits for the E-ring particles)
*


I believe the thinking is that particles from the solar wind, or Saturn wind, would sweep away gas particles. But seems like macroscopic ice particles would be a bit more of a problem?

Jason recently posted two excellent links to Dr. Esposito's talk back on August 30. Here is one slide:



see: http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20050830_CHARM_Esposito.pdf

The accompanying http://saturn.jpl.nasa.gov/multimedia/products/docs/CHARM_20050830_transcript.doc says:

"I’m moving to my next slide, which is about the first clues that Cassini had of something more exciting happening on Enceladus. On the 17th of February 2005 we had a close fly-by of Enceladus, and Cassini’s magnetometer detected a bending of the magnetic field around Enceladus. Now, this may seem a minor or maybe even a negligible discovery, but the magnetic field, like an electromagnet, or the earth’s magnetic field, is excluded by conductors. It looks as though there was a conducting layer surrounding the moon Enceladus, which the investigators hypothesized may have been a result of ions formed by sunlight falling on the atmosphere of Enceladus, if it had one. So the idea was that there might be an atmosphere around Enceladus and light from the sun would be removing electrons from some of the atoms in the atmosphere, making them conductors, and those conductors would exclude the magnetic field from the region around Enceladus. This was a very exciting indirect indication that Enceladus might have an atmosphere, and the project decided to reinvest some of its resources and to re-plan a later fly-by of Enceladus, which was scheduled for July of this year, to fly very, very close to the moon, to fly within 175 kilometers of the surface of Enceladus."

...

"I’m moving to my next slide, which is a demonstration of the effect that was seen by the magnetometer, and what you’re seeing here are these yellow arrows and lines and all that yellow stuff is truly invisible, it’s a representation of the Van Allen belts of Saturn, which are trapped by the magnetic field. As the moon, Enceladus, which you can see on the right, moves around Saturn and moves through that magnetic environment, mostly without any trouble, except if there might be some cloud near Enceladus."

"Here are the artists’ drawings, and you can see on the diagram how the lines of magnetic force have changed their location. The lines of magnetic force are excluded by the neutrals in the cloud, which is shown here near the southern pole of Enceladus; that’s a point we didn’t know yet, so that already in February we had some indications that Enceladus was unusual and the project used some of its capability to readjust the mission to aim for a close fly-by of the moon in the month of July. So, Cassini was redirected to fly within 179 kilometers of Enceladus on the 14th of July 2005."

Posted by: jmknapp Dec 5 2005, 05:21 PM

QUOTE (ugordan @ Dec 5 2005, 08:22 AM)
EDIT: A crude method might be using Orbiter and setting your location on the Enceladus' south pole and applying a short burn upward and seeing where it gets you. Of course, you'd need to change the current best mass estimate in the config file first. I'll play around with it a bit later...
*


I'd be intersted to know how that comes out, but I tried a little simulation myself, modeling the three-body situation of Enceladus, Saturn, and a small particle ejected from the south pole. I used three different initial velocities: 100, 200 and 300 m/sec:





So the escape velocity models out as between 200-300 m/sec as expected.

Posted by: volcanopele Dec 6 2005, 07:24 PM

Press Release: NASA's Cassini Images Reveal Spectacular Evidence of an Active Moon
http://ciclops.org/view.php?id=1714

4-frame movie showing plumes:
http://ciclops.org/view.php?id=1702

Posted by: jmknapp Dec 7 2005, 11:02 AM

Exploring this ice sublimation theory some more...

If ice is subliming in sufficient quantities to create powerful jets sending water vapor and/or particles into space at hundreds of meters per second, it seem that the pressure at the vents must be very high, well over atmospheric pressure on Earth?

Also, the temperature of the jets must be over 273K, no?

That translates to a lot of heat, which would tend to melt the sides of the tiger stripes where the vents issue.

The Cassini team has released a color image of the area:



Dr. Esposito of the Cassini team referred to the bluish tinge of the tiger stripes in the above image:

QUOTE
The parallel stripes were unexpected from earlier images and show two things; they show that there’s been a recent, we don’t know exactly how recent, many millions and even billions of years, but a recent event which has covered up the southern region of Enceladus and that has stretched the surface so that it’s broken to form these fissures, which we see as tiger stripes.  The color of those features is a bluish hue, and that bluish hue says that the ice that’s in the fissures, in those tiger stripes, that the ice there is relatively young and may be fresh, at least geologically fresh.  So with this information, there is geologic evidence of recent resurfacing and splitting of the icy surface of Enceladus.


Could the hot plumes of water vapor be melting the tiger stripes and entraining atomized water droplets in the stream? Moreover, the plumes would quickly cool and wouldn't that tend to make the water vapor and/or droplets condense as ice or even snow?

Maybe the plumes act as enormous snow machines:



Ski Enceladus! biggrin.gif

Posted by: edstrick Dec 7 2005, 12:01 PM

"...the 1994 peer-reviewed Icarus paper ..."

The analysis predicting the moon's mass based on it's tidally-deformed shape was probably correct (Peter Thomas is a damn competent researcher), but based on a false assumption. We don't know what that assumption is yet.

The amount of tidal deformation varies with the distribution of mass in the moon. The amount is different if the "rock" in the moon is uniformly mixed with the ices, compared with all the rock forming a core and all the ices forming a mantle. That sort of variation shows up in the difference between their two estimated densities: 1.12 vs 1.00. Clearly, that's not enough to explain the discrepancy.

Another possibility is that Enceladus is not hydrostatically relaxed. There may be an ongoing convection pattern, maybe rising at the south pole and descending at the north (probably more complicated, but..), resulting in dynamically supported topography. The Tharsis bulge on Mars may be supported by a still active, though very slow and sluggish, monster convective plume in Mars' mantle.

What *is* important is that unless there was a booboo in the calculations (can't rule that out), the discrepancy is TELLING us something. The measured shape of the gravity field, compared with the density and more accurate global shape measurements will probably provide enough additional information to blow some candidate explanations out of the (ammonia)water, and make one a "prefered solution".

Posted by: ugordan Dec 7 2005, 12:19 PM

QUOTE (jmknapp @ Dec 5 2005, 06:21 PM)
I'd be intersted to know how that comes out, but I tried a little simulation myself, modeling the three-body situation of Enceladus, Saturn, and a small particle ejected from the south pole. I used three different initial velocities: 100, 200 and 300 m/sec:

So the escape velocity models out as between 200-300 m/sec as expected.
*

I played with Orbiter a bit and it turns out you were right. Ejecting from the pole and perpendicular to the orbital plane really doesn't change distance from Saturn considerably so Saturnian gravity more or less remains cancelled out. The situation becomes interesting when you're ejecting the "particle" inwards towards Saturn or outwards. In the case of 100 m/s I quickly winded up back on the surface, but in the case of 200 m/s there was a period of normal deceleration due to Enceladus' gravity and then, some hundreds of km above the surface, the speed started to pick up again and it was clear I wasn't going back down.

In short, I probably underestimated the strength of Enceladus' gravity, though it's obvious things get interesting well below nominal escape velocity if the emission direction is "right". The plumes are however neatly emitted from the pole so this effect really doesn't come into play.

Posted by: jmknapp Dec 7 2005, 02:57 PM

QUOTE (ugordan @ Dec 7 2005, 08:19 AM)
The situation becomes interesting when you're ejecting the "particle" inwards towards Saturn or outwards. In the case of 100 m/s I quickly winded up back on the surface, but in the case of 200 m/s there was a period of normal deceleration due to Enceladus' gravity and then, some hundreds of km above the surface, the speed started to pick up again and it was clear I wasn't going back down.
*


Interesting... just checked & I get the same result. Away from Saturn the escape velocity comes out as about 190 m/s and towards Saturn about 205 m/s.

A space.com article yesterday, http://space.com/scienceastronomy/ap_051206_saturn_moon.html, has an interesting bit:

QUOTE
It's unclear what causes the geologic activity, but scientists think it's due to internal heating caused by radioactivity or tides.


But Enceladus is tidally locked to Saturn & so would not experience tides? Or maybe it might... there's a little wobble longitudinally, about 1 degree in extent every Enceladus day, based on current SPICE models. Would that little wobble be enough to generate heating?

The space.com article referred to a talk yesterday given at the American Geophysical Union conference in San Francisco. A list of the http://www.agu.org/meetings/fm05/fm05-sessions/fm05_P31D.html includes the following:

QUOTE
Calculation of Loss Rate from Saturn's E Ring

* Leisner, J S (jleisner@ess.ucla.edu) , IGPP/UCLA, Box 951567, Los Angeles, CA 90095
Russell, C T (ctrussell@igpp.ucla.edu) , IGPP/UCLA, Box 951567, Los Angeles, CA 90095
Dougherty, M K (m.dougherty@imperial.ac.uk) , Imperial College, London, Blackett Lab Space & Atmos Physics, London, SW7 2AZ United Kingdom
Blanco-Cano, X (xbc@geofisica.unam.mx) , UNAM, Ciudad Universitaria Instituto Geofisica, Coyoacan, DF 4510 Mexico
Strangeway, R J (strange@igpp.ucla.edu) , IGPP/UCLA, Box 951567, Los Angeles, CA 90095
Strangeway, R J (strange@igpp.ucla.edu) , Imperial College, London, Blackett Lab Space & Atmos Physics, London, SW7 2AZ United Kingdom
Bertucci, C (c.bertucci@imperial.ac.uk) , Imperial College, London, Blackett Lab Space & Atmos Physics, London, SW7 2AZ United Kingdom
Bertucci, C (c.bertucci@imperial.ac.uk) , UNAM, Ciudad Universitaria Instituto Geofisica, Coyoacan, DF 4510 Mexico


Ionization--whether through solar photons, impacting particles, or charge exchange--is a significant loss process in the neutral E ring. When these particles are ionized, they are energized by the electric field associated with the corotating magnetized plasma. If the pick-Up energy is great enough, the particles generate ion cyclotron waves with a magnetic field amplitude that is determined by the energy of the pick-Up ions. These waves, with frequencies near the local water-group gyrofrequencies, were first seen in the E ring by the Pioneer 11 and Voyager 1 magnetometers, but it is Cassini's large coverage of radial distance, local time, and latitude within the E ring that allows us to use this spacecraft's magnetometer to conduct a comprehensive study of these waves. Using a simple plasma profile and modeling the E ring as a mass source in the equatorial plane, we use the ion cyclotron waves to calculate the ionization rate through the ring. We find that between 3.5 and 7 Rs, the E ring loses a total of 6e^26 neutrals, or 18 kilograms of H2O, per second. This total is about one fourth of the amount expected from previous models and the radial erosion rate agrees qualitatively with E ring density profiles, except at 4.6 Rs. Using the ion cyclotron waves, our calculations show a discontinuity in the erosion rate at that equatorial distance and all latitudes. This discontinuity can not be explained with the assumed smoothly varying plasma profiles or production rates.


Assuming that the E ring is in steady state, it must be picking up at least 18 kg/sec from the vents?

Also:

QUOTE
Saturn's E ring as seen by Cassini

* Kempf, S (sascha.Kempf@mpi-hd.mpg.de) , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Beckmann, U , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Burton, M , Jet Propulsion Laboratory, Oak Grove Drive, Pasadena, CA United States
Helfert, S , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Helfert, S , Helfert Informatik, Bits Road, Mannheim, 60000 Germany
Srama, R , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Moragas-Klostermeyer, G , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Roy, M , Jet Propulsion Laboratory, Oak Grove Drive, Pasadena, CA United States
Gruen, E , Max Planck Institute Nuclear Physics, Saupfercheckweg 1, Heidelberg, 69117 Germany
Gruen, E , HIGP Univ. Honolulu, Honolulu, Honolulu, Hawaii United States


The Cosmic Dust Analyzer (CDA) onboard the Cassini mission measures the properties of micron sized dust particles in the environment of Saturn. Since its arrival at Saturn in July 2004, the CDA detector recorded many thousand dust impacts within the E ring of Saturn. Data analysis revealed enhanced dust densities until a radial distance of at least 16 Saturn radii. The dust densities and the mass distribution of the ring particles is investigated. Furthermore, compositional measurements indicate a clear dominance of water ice particles in the size range between 0.5 and 2 micro meter. Dust charge measurements are in agreement with Cassini plasma measurements. The discovery of the extended E ring changes the former understanding and modeling based on remote sensing observations. The dust rate measurements during the Enceladus flyby on July 14, 2005, support a strong active dust source on the southern hemisphere of Enceladus.

Posted by: ugordan Dec 7 2005, 03:20 PM

QUOTE (jmknapp @ Dec 7 2005, 03:57 PM)
But Enceladus is tidally locked to Saturn & so would not experience tides? Or maybe it might... there's a little wobble longitudinally, about 1 degree in extent every Enceladus day, based on current SPICE models. Would that little wobble be enough to generate heating?
*

That wobble might explain the unusually high amount of internal heat production, as part of one theory which I mentioned before - the secondary spin-orbit resonance. The wobble itself *might* be due to libration as with our own moon where the rotational period stays constant, but the orbital speed changes (the orbit is slightly eccentric!) so Enceladus does not always show exactly the same face to Saturn.
There's however the "usual" tidal mechanism also - Enceladus' orbit is, as I said, eccentric so at certain points its closer to Saturn than at others. This introduces a change in the height of the tidal bulge and the oscillations produce continuos "flexing" that also tends to heat Enceladus up, though much less than needed amount to explain the observed activity.

Posted by: dvandorn Dec 7 2005, 09:41 PM

I assume the effects of the other moons have been taken into account when it comes to tidal heating of Enceladus? After all, Io isn't heated by its tidal interaction with Jupiter -- its internal heat is pumped by tides from the other three Galilean moons.

Also, if Enceladus has been regenerating the E ring for aeons, its mass has decreased significantly. Wouldn't this loss of mass tend to decrease tidal heating?

My bet is on radiogenic heating. It'll be interesting to see just what kind of model would make sense in terms of the type of composition you'd need for such heating.

Perhaps the Saturn system is so volatile (in more ways than one) because it somehow became anomalously Al-26 enriched *after* Saturn had gotten fairly far along in its accretion phase...?

-the other Doug

Posted by: ugordan Dec 7 2005, 10:05 PM

QUOTE (dvandorn @ Dec 7 2005, 10:41 PM)
I assume the effects of the other moons have been taken into account when it comes to tidal heating of Enceladus?  After all, Io isn't heated by its tidal interaction with Jupiter -- its internal heat is pumped by tides from the other three Galilean moons.
*

Are you sure Io isn't heated by Jupiter alone? I figured the other moons served only to maintain the eccentricity of its orbit, due to their resonances stabilizing Io's orbit as is?
If what you're saying is true, why isn't Europa being heated to the same level as Io?
As for Enceladus, AFAIK Dione serves to maintain the eccentricity of Enceladus orbit as the two are in a 2:1 resonance.

Posted by: dvandorn Dec 7 2005, 11:02 PM

Well, yes -- it's all relative. The other Galileans pull Io out of an orbit that would otherwise get circularized, and thus makes it travel up and down through potential energy levels of Jupiter's gravitational field. And Io heats more than the other moons because the gravity gradients in Jupiter's field are greater closer in, and Io orbits closer to Jupiter.

So, yes, the primary source of the energy is Io's interaction with Jupiter's gravitational field. But the "pumping" that keeps Io moving up and down through Jupiter's gravity gradient is provided by the other three Galilean moons. It's the five-body tug-of-war that keeps Io hot... but yes, indeed, the energy is mostly drawn out of Jupiter's gravitation.

And Europa *is* tidally heated. Just not nearly as much as Io. All of the Galileans gain at least some internal heat from their complex interactions with the other three Galileans and Jupiter. But the effect grows less and less intense as you move away from Jupiter, and the gravity gradients between the extremes of the moons' orbits becomes less (thereby allowing for the drawing of much less energy from movement up and down the "well").

My point in re Enceladus is that there *must* be some kind of similar tug-of-war happening to produce the amount of tidal heating needed to melt interior ice. So, I was basically asking just how out-of-circular Enceladus' orbit might be, how much of that can be attributed to the influences of the other Saturnian moons, and what amount of tidal heating might come from that?

From some not-very-specific comments I've seen, it seems that the orbit is too circular, Enceladus is too small, and the distance from Saturn is too great for orbital eccentricities to account for what seem to be the observed heating effects. My sense is also that the other potential non-radiogenic sources of heating aren't enough to make the kind of heat we seem to be seeing.

So, the logic points me towards radiogenic heating. Now, what do we need to do to get supporting data that will help us actually limit this dataset? Is Cassini capable of discovering the why's, or do we need to design another probe to get to the bottom of it...?

-the other Doug

Posted by: silylene Dec 8 2005, 02:54 AM

QUOTE (dvandorn @ Dec 7 2005, 11:02 PM)
From some not-very-specific comments I've seen, it seems that the orbit is too circular, Enceladus is too small, and the distance from Saturn is too great for orbital eccentricities to account for what seem to be the observed heating effects.  My sense is also that the other potential non-radiogenic sources of heating aren't enough to make the kind of heat we seem to be seeing.

So, the logic points me towards radiogenic heating. 


The 'loaded die' hypothesis
I wish to propose that if the interior of Enceladus is non-symmetrically inhomogeneous, that this in itself should be cause significant tidal heating. For example, if there was a lump of a high density material within the moon, but off-center, this should cause a very high amount of tidal heating, located above the dense lump.

My hypothesis does not require that the lump itself offer radioactive heating, nor does it require that the lump carry remnant heat from a long-ago meteorite impact.

Simply, if a dense large lump is present off-center in the moon, the lump will interact gravitationally with Dione, Saturn and other moons to cause the ices and rock above and around it to heat significantly due to unequal gravitational flexing as the moon attempts to wobble on its axis.

Perhaps careful monitoring of the trajectory as Cassini passes close to Enceladus will detect whether the mass within this moon is nonsymmetrically distributed.

Has anyone attempted to model the effect of gravitational heatnig on a moon with a nonsymmetric mass distribution?

Posted by: jmknapp Dec 8 2005, 05:44 PM

QUOTE (dvandorn @ Dec 7 2005, 07:02 PM)
My point in re Enceladus is that there *must* be some kind of similar tug-of-war happening to produce the amount of tidal heating needed to melt interior ice.  So, I was basically asking just how out-of-circular Enceladus' orbit might be, how much of that can be attributed to the influences of the other Saturnian moons, and what amount of tidal heating might come from that?
*


Enceladus actually has a bit more eccentricity than Io (0.0045 vs. 0.0041), is closer to the central planet (238,000 km vs. 422,000 km) and gets tidally squeezed at a higher rate (revolution period 1.37 day vs. 1.77 day).

On the other hand, Saturn is over three times less massive than Jupiter (5.7e26 kg vs. 1.9e27), Enceladus is only one seventh the diameter of Io (505km vs. 3643 km) and is 800x less massive (1.08e20 kg vs. 8.93e22 kg).

Some back-of-the-envelope calculations (caveat emptor):

According to http://en.wikipedia.org/wiki/Tidal_force, the tidal force is given by:

F = 2GMmr/R^3

where:

G = 6.6742e-11 m^3/sec^2/kg
M = mass of central body
m = mass of orbiting body
r = radius of orbiting body
R = radius of orbit

Periapsis-Apoapsis:

Io: 419,971-423,429 km
Enceladus: 236,877-239,019 km

Plugging in the numbers:

Io: F = 1.11e21 N periapsis, 1.09e21 N apoapsis
Enceladus: F = 1.54e17 N periapsis, 1.50e16 N apoapsis

So the absolute magnitude of the tidal force is almost 10,000x greater on Io, assuming no mistake.

Posted by: mike Dec 8 2005, 06:01 PM

Given that Europa is less massive than Io, it seems to me that it would take less force to exert the same amount of overall temperature differential.

Posted by: nprev Dec 8 2005, 06:41 PM

Is the average density of Enceladus significantly greater than, say, Dione or Tethys? If so, Enceladus today may be a remnant of its former self with a relatively large rocky core, making a combination of tidal & radiogenic heating more likely.

Since the moon formed closer to Saturn than the other major icy satellites, its final composition may have been comparatively metal-rich due increased levels of residual formation heating from Saturn (using an analogy to Mercury here...)

Just a thought. I just found this site; what a terrific forum!!!

Posted by: JRehling Dec 8 2005, 08:21 PM

QUOTE (dvandorn @ Dec 7 2005, 01:41 PM)
My bet is on radiogenic heating.  It'll be interesting to see just what kind of model would make sense in terms of the type of composition you'd need for such heating.

Perhaps the Saturn system is so volatile (in more ways than one) because it somehow became anomalously Al-26 enriched *after* Saturn had gotten fairly far along in its accretion phase...?

-the other Doug
*


Here's a thought. Suppose Enceladus isn't enriched in absolute abundance of Al-26, compared to, say, Rhea -- assume they have the same abundance. A priori, you would expect much greater heating to have taken place in Rhea. And it looks like that didn't happen.

Consider Callisto -- a still bigger icy moon, which we know not to be well differentiated. It's non-icy materials, and its Al-26, is scattered throughout its mass. It seems to be about as dead/cold/no-past-heating as Rhea.

Suppose that the difference is that Enceladus was once heated, leaving all of its Al-26 to drop into a central core, where the temperature would then be much higher than anywhere in Rhea (or Callisto). That initial heating could have been tidal. So while Enceladus got a furnace and a chimney, the larger icy satellites had their scattered Al-26 just drive a low simmer that never really did anything.

Posted by: deglr6328 Dec 8 2005, 11:48 PM

What about K-40 abundances? All the Al-26 should have decayed long long ago. 1/2 life is only ~.7 Myr.

Posted by: nprev Dec 9 2005, 12:24 AM

Just out of curiosity, was there any anomalous magnetometer data during the last close flyby that did <b>not</b> seem to be related to passage through the polar plumes? If Enceladus possesses enough radioactives to sustain volcanic activity, then it should have a similar abundance of good old Fe-56 in its core, some of which might be molten...

Posted by: jmknapp Dec 9 2005, 11:32 AM

QUOTE (nprev @ Dec 8 2005, 08:24 PM)
Just out of curiosity, was there any anomalous magnetometer data during the last close flyby that did <b>not</b> seem to be related to passage through the polar plumes?
*


Here's what they saw in February:



So the disturbance was beyond passing through the plume per se, but I think still attributed to the south polar atmosphere.

Dr. Nick Achilleos said of this diagram:

QUOTE
Slide number 26 finally shows us some magnetometer data that was taken during a flyby of the satellite Enceladus by Cassini earlier this year.  Enceladus orbits Saturn at a distance of about four Saturn radii.  So it’s much closer in than Titan.

The panel on the left here shows you the magnetic field vectors, which were measured by the magnetometer during the first flyby in February this year.  You can see that at closest approach to Enceladus there was this strong distortion in the field. Again, you see a pattern that looks as though the field is draping itself around Enceladus. 

What’s interesting about this is that the field draping indicates there’s an obstacle, which is much larger than Enceladus itself.  On the basis of this measurement we did, the project allowed a much closer flyby of Enceladus the 14th of July this year.  So that very close flyby, which was about 750 kilometers above the surface of Enceladus, all the instruments on Cassini saw some kind of interaction due to the presence of Enceladus in Saturn’s magnetosphere. 

On the right, there is a schematic diagram of what’s happening with the field.  As hot plasma in Saturn’s magnetosphere flows past Enceladus, because of this frozen-in condition that we’ve been talking about, the field distorts itself and drapes itself around the Enceladus obstacle.  The obstacle shown in this picture here is this haze of gas, which is essentially steam, water surrounding Enceladus, but observations taken since then indicate that the source of the out-gassing on Enceladus is asymmetric.  There seems to be a lot more steam coming from the southern hemisphere of Enceladus.  So we’re refining this picture all the time.


http://saturn.jpl.nasa.gov/multimedia/products/docs/CHARM_20050927_transcript.doc
http://saturn.jpl.nasa.gov/multimedia/products/pdfs/20050927_CHARM_Achilleos.pdf

Posted by: dvandorn Dec 9 2005, 04:10 PM

So, they're saying that the deflection of Saturn's magnetic field lines around Enceladus is due to the cloud of water ice particles jetting out of the south polar region? And not due to anything intrinsic about Enceladus' magnetic properties?

I wonder -- they talk about it being caused by "a cloud of steam" or water vapor. And yet, isn't the cloud actually ice particles and *not* vapor? Would ice particles have the same effect on magnetic fields as vapor?

And since when does water ice or water vapor have that kind of effect on magnetic fields? I thought water, and ice, were rather neutral when it came to magnetism. They're not metals, after all.

This picture hasn't come together yet...

-the other Doug

Posted by: jmknapp Dec 9 2005, 05:01 PM

QUOTE (dvandorn @ Dec 9 2005, 12:10 PM)
So, they're saying that the deflection of Saturn's magnetic field lines around Enceladus is due to the cloud of water ice particles jetting out of the south polar region?  And not due to anything intrinsic about Enceladus' magnetic properties?

I wonder -- they talk about it being caused by "a cloud of steam" or water vapor.  And yet, isn't the cloud actually ice particles and *not* vapor?  Would ice particles have the same effect on magnetic fields as vapor?

And since when does water ice or water vapor have that kind of effect on magnetic fields?  I thought water, and ice, were rather neutral when it came to magnetism.  They're not metals, after all.

This picture hasn't come together yet...
*


You're right about that, but as for the cloud of gas/particles, this is from the http://saturn.jpl.nasa.gov/multimedia/products/docs/CHARM_20050830_transcript.doc:

QUOTE
So the cosmic dust measurements, the direct measurements of dust near Enceladus also show some evidence for eruption, and the idea would be, like on a comet, as the gases are shot out from the comet vent or a jet on the comet, they also carry small grains of ice, and as the jet moves away from the comet, the gas and the ice separate a little bit.  And it’s seen here that the maximum of dust is some 25 seconds later than the maximum of water that was seen by the neutral mass spectrometer both on the Cassini spacecraft as it flew by Enceladus, so near closest approach, but not exactly at closest approach.  The amount of water vapor there at the spacecraft and the amount of dust peaked at the different points, so this is saying to us that we have a very complicated and unsymmetric source of water and dust near the south pole of Enceladus, most likely associated with geologic activity, where water vapor and associated molecules of nitrogen and grains of ice are being shot out of a fissure, what the camera team called the “tiger stripes.”


And also:

QUOTE
...the magnetic field, like an electromagnet, or the earth’s magnetic field, is excluded by conductors.  It looks as though there was a conducting layer surrounding the moon Enceladus, which the investigators hypothesized may have been a result of ions formed by sunlight falling on the atmosphere of Enceladus, if it had one.  So the idea was that there might be an atmosphere around Enceladus and light from the sun would be removing electrons from some of the atoms in the atmosphere, making them conductors, and those conductors would exclude the magnetic field from the region around Enceladus.

Posted by: dvandorn Dec 9 2005, 05:11 PM

So.... are they excluding the possibility that the magnetic effects are being caused by an Enceladan intrinsic magnetic field (caused by a hot metal core)? And if they are, is it because the magnetometer readings don't indicate an intrinsic field, or because they don't *expect* Enceladus to have a magnetic field, so they're just not thinking that we could be seeing one?

A lot of our perceptions are bounded by our expectations, after all.

-the other Doug

Posted by: jmknapp Dec 9 2005, 07:29 PM

QUOTE (dvandorn @ Dec 9 2005, 01:11 PM)
So.... are they excluding the possibility that the magnetic effects are being caused by an Enceladan intrinsic magnetic field (caused by a hot metal core)?  And if they are, is it because the magnetometer readings don't indicate an intrinsic field, or because they don't *expect* Enceladus to have a magnetic field, so they're just not thinking that we could be seeing one?
*


I don't know if they've addressed that publically anyway. Maybe Enceladus is way too small (diameter about the distance between Los Angeles and San Jose) to have such a core?

On another matter regarding the plumes: may we assume that if the plumes are sometimes sending ice particles into space with escape velocity, the plumes also sometimes emit particles with less than escape velocity that will impact somewhere downrange on the moon?

If so, then is there then effectively a rain of ice particles impacting the surface, resulting in accumulation and even erosion in the preferred impact spots? After all, the particles would impact with about the same velocity they left the plume with, up to 240 m/sec. Truly a "hard rain" falling.

Posted by: The Messenger Dec 9 2005, 07:31 PM

QUOTE (dvandorn @ Dec 9 2005, 10:11 AM)
So.... are they excluding the possibility that the magnetic effects are being caused by an Enceladan intrinsic magnetic field (caused by a hot metal core)?  And if they are, is it because the magnetometer readings don't indicate an intrinsic field, or because they don't *expect* Enceladus to have a magnetic field, so they're just not thinking that we could be seeing one?

A lot of our perceptions are bounded by our expectations, after all.

-the other Doug
*

This is not the first time a magnetic field has shown up where it was least expected: Ganymede was a big eye opener. The difference I can see, is as you have elluded: There is a possible pretender to a magnetic core in the water/ice/vapour that is escaping. This is just a guess, and not a well rooted one.

Posted by: BruceMoomaw Dec 10 2005, 06:36 AM

QUOTE (jmknapp @ Dec 9 2005, 07:29 PM)
On another matter regarding the plumes: may we assume that if the plumes are sometimes sending ice particles into space with escape velocity, the plumes also sometimes emit particles with less than escape velocity that will impact somewhere downrange on the moon?

If so, then is there then effectively a rain of ice particles impacting the surface, resulting in accumulation and even erosion in the preferred impact spots? After all, the particles would impact with about the same velocity they left the plume with, up to 240 m/sec. Truly a "hard rain" falling.
*


It's been assumed since Voyager that this is the likely explanation for Enceladus' extremely high albedo -- it's refrosting itself, all over its surface. (Presumably, when such impacts vaporize some of the ice in the particles, it quickly refreezes again a short distance away.)

Posted by: jmknapp Dec 10 2005, 07:29 PM

QUOTE (BruceMoomaw @ Dec 10 2005, 02:36 AM)
It's been assumed since Voyager that this is the likely explanation for Enceladus' extremely high albedo -- it's refrosting itself, all over its surface.  (Presumably, when such impacts vaporize some of the ice in the particles, it quickly refreezes again a short distance away.)
*


Seems like the process is becoming a lot more constrained, knowing that the ice jets are coming from (approximately) the south pole.

A while back ugordan and I were discussing the escape velocity & I came up with a little simulation predicting what would happen to a particle emitted from the south pole. I refined the program some more recently, and now it allows prediction of the path of a particle emitted from any point on the surface, and in any direction (azimuth and elevation from the "vent" location). Some interesting asymmetries are evident resulting from the moon's rotation and the three-body situation involving Saturn's strong gravity.

Take the case of a vent located at 180W, 80S (10 degrees from the south pole) and with a plume direction of 70 degrees elevation. Rotating that plume 360 degrees in 5-degree steps (think of it kind of as a rotating sprinkler) results in a surprising asymmetrical pattern. This map shows the predicted impact points, varying the plume velocity from 0-220 m/sec (approx. escape velocity) in 1 m/sec steps (click for larger version):

http://cassinicam.com/plumesim/p180.jpg

The impact points are preferentially towards the center, which is the anti-Saturn (180-degree) meridian, offset to the west a little due to the moon's rotation.

That's just the result with the plume at one location, but interestingly a simlar pattern persists even if the plume location is moved (click on any for a larger version):

0W 80S:
http://cassinicam.com/plumesim/p0.jpg

90E 80S
http://cassinicam.com/plumesim/p90.jpg

270E 80S:
http://cassinicam.com/plumesim/p270.jpg

Not sure if it's my imagination, but it seems like the patterns line up with some of the features on Enceladus. Interesting anyway.

Posted by: ugordan Dec 10 2005, 08:08 PM

QUOTE (jmknapp @ Dec 10 2005, 08:29 PM)
Not sure if it's my imagination, but it seems like the patterns line up with some of the features on Enceladus. Interesting anyway.
*

Outstanding work, Joe!
I'm curious: are those Matlab simulations or something you programmed for yourself?

One thing that could be done to further investigate the patterns would be to integrate across all tiger stripes and all angles/speeds and see if the "fallout" map would preferentially exclude some areas of the moon and then compare with the albedo and crater saturations of those regions.
That would probably be grounds for a scientific paper, though smile.gif

What you did already suggests there indeed are some areas in the north that should be very depleted of snowfall.
Once again, great work!

Posted by: jmknapp Dec 11 2005, 01:41 AM

QUOTE (ugordan @ Dec 10 2005, 04:08 PM)
I'm curious: are those Matlab simulations or something you programmed for yourself?

One thing that could be done to further investigate the patterns would be to integrate across all tiger stripes and all angles/speeds and see if the "fallout" map would preferentially exclude some areas of the moon and then compare with the albedo and crater saturations of those regions.
*


I programmed it in C, using the CSPICE library to determine the positions of Enceladus and Saturn over time, and a Runge-Kutta integration to determine the particle trajectory. One thing about doing a lot of integrations though across the tiger stripes or whatever: each map takes a few hours to generate on my machine! So I'm wondering how exactly to plan it out that would give relevant results and not waste a week of number-crunching.

Posted by: nprev Dec 11 2005, 10:27 AM

Thanks for the magnetometer vector slide, Jim; most interesting!

One thing that I find particularly striking is that the field vectors--rough as the data may be for this comment-- do not seem to be "dragging" due to Enceladus' orbital motion, as you might presumably expect if the field source was in fact the eruption cloud. In fact, such an effect might also be expected from transfer and diffusion of eruption material (and charge) throughout the E-ring; instead, the field X-Y plane seems orthogonal to Saturn and therefore centered on the mass of the moon itself. Might that imply an endogenic, internally generated field for Enceladus as a reasonable interpretation of this dataset instead, or am I overestimating the effects of orbital motion and cloud behavior?

Posted by: ugordan Dec 11 2005, 12:27 PM

QUOTE (jmknapp @ Dec 11 2005, 02:41 AM)
I programmed it in C, using the CSPICE library to determine the positions of Enceladus and Saturn over time, and a Runge-Kutta integration to determine the particle trajectory. One thing about doing a lot of integrations though across the tiger stripes or whatever: each map takes a few hours to generate on my machine! So I'm wondering how exactly to plan it out that would give relevant results and not waste a week of number-crunching.
*

It's been a while since I studied different integration methods, but isn't there a time step that's adjustable for different levels of precision? Or are the stability constraints too tight? I'm probably talking jibberish here... rolleyes.gif
Out of curiosity, what machine are you running the sims on?

Posted by: jmknapp Dec 11 2005, 03:17 PM

QUOTE (ugordan @ Dec 11 2005, 08:27 AM)
It's been a while since I studied different integration methods, but isn't there a time step that's adjustable for different levels of precision? Or are the stability constraints too tight? I'm probably talking jibberish here...  rolleyes.gif
Out of curiosity, what machine are you running the sims on?
*


Yes, I used a step of 3 seconds, which is maybe a little overkill but not sure how far I can push it. I'll look into that. The main time hog is calling the SPICE library every step to find the positions of Saturn & Enceladus--very accurate but definitely overkill. Maybe the whole thing could be sped up quite a bit.

It's running currently on a Linux system.

Posted by: ugordan Dec 11 2005, 03:27 PM

QUOTE (jmknapp @ Dec 11 2005, 04:17 PM)
Yes, I used a step of 3 seconds, which is maybe a little overkill but not sure how far I can push it. I'll look into that. The main time hog is calling the SPICE library every step to find the positions of Saturn & Enceladus--very accurate but definitely overkill. Maybe the whole thing could be sped up quite a bit.
*

I don't think you would lose much in terms of accuracy if you simply assumed Enceladus was in a circular orbit with a given orbital period. Given the angular momentum, it would be a no-brainer to calculate Enceladus' position at any given time.
That way there'd be no need for extensive calls to the SPICE library at all. All those function call overheads would simply go away...

Posted by: Rob Pinnegar Dec 11 2005, 04:13 PM

Just as a sidenote to the main thread: There's been some discussion here regarding a small iron core for Enceladus. Assuming for the moment that the observed magnetic anomaly is internally generated, it seems to me that a small _solid_ iron core, below the Curie temperature, is a better contender for the source of the magnetic field than a liquid core.

I just have trouble believing that a dynamo could get going in such a small body (and that the core could be molten). Of course I haven't been through the physics so this is just conjecture, but it seemed worth mentioning.

So the question then becomes, why don't we see similar fields at Tethys, Dione or Rhea? This argues _against_ internal generation of the field.

Posted by: nprev Dec 11 2005, 06:01 PM

QUOTE (Rob Pinnegar @ Dec 11 2005, 09:13 AM)
So the question then becomes, why don't we see similar fields at Tethys, Dione or Rhea? This argues _against_ internal generation of the field.
*



There are at least three major differences between Enceladus and the other icy satellites relevant to this discussion:

1. Enceladus is on the receiving end of orbital resonance 'squeezing' from Saturn and Dione, and it doesn't have much overall mass to dissipate the resultant heat.

2. Enceladus formed closer to Saturn than the other icy moons cited, and therefore may be enriched in metals (including radionuclides) by comparison, and again has limited mass for use as a heat-sink.

3. Cassini proved that Enceladus alone among the icy moons has a present internal heat source of sufficient magnitude to drive surface vulcanism.

Systemically, these factors (if valid; the actual magnitude of #1 and any other tidal interactions seem poorly defined right now, and only #3 is a confirmed fact) may yield the right combination to produce at least a semi-molten iron core.

Posted by: Jeff7 Dec 11 2005, 06:32 PM

Another passing thought/wild theory:
What if Iapetus' dark side was caused by something similar to Enceladus' plumes - a massive "tiger stripe" on the one side, which is now the ridge? It could have spewed out material long ago, coating one side of the moon with dark material, and at the same time, formed a volcanic ridge, similar to the mid-Atlantic ridge on Earth.

Posted by: hendric Dec 12 2005, 05:37 AM

QUOTE (jmknapp @ Dec 11 2005, 09:17 AM)
Yes, I used a step of 3 seconds, which is maybe a little overkill but not sure how far I can push it. I'll look into that. The main time hog is calling the SPICE library every step to find the positions of Saturn & Enceladus--very accurate but definitely overkill. Maybe the whole thing could be sped up quite a bit.

It's running currently on a Linux system.
*


Well, SOP for optimizing a slow calulation like that is to simply do it once, create a lookup table, and reference that every time. If each run starts at the same time, they can use the same Saturn/Enceladus location information. In fact, you could even create the lookup table off-line and load it at the start of each execution.

Posted by: nprev Dec 12 2005, 07:17 AM

I just had a thought: What if there is a "flux tube" between the Saturnian magnetosphere and Enceladus, as there is between Jupiter & Io? If so, perhaps the south pole of Enceladus is the impact point, which might add another increment of local heating and explain why vulcanism is apparently confined to that location.

Presumably this would still require Enceladus to have an endogenic magnetic field, though. huh.gif

Posted by: BruceMoomaw Dec 12 2005, 08:21 AM

QUOTE (Jeff7 @ Dec 11 2005, 06:32 PM)
Another passing thought/wild theory:
What if Iapetus' dark side was caused by something similar to Enceladus' plumes - a massive "tiger stripe" on the one side, which is now the ridge? It could have spewed out material long ago, coating one side of the moon with dark material, and at the same time, formed a volcanic ridge, similar to the mid-Atlantic ridge on Earth.
*


The trouble with this is that the dark material is so perfectly aligned with Iapetus' forward-facing side -- which is stretching coincidence unless the dark material instead spiralled in from an orbit further beyond Saturn and impacted Iapetus' forward face. My betting is still that the dark material is exogenic -- especially since a good thermal explanation has now been devised for why it extends farther in low-latitude regions but doesn't extend all the way to the poles:

http://www.aas.org/publications/baas/v37n3/dps2005/745.htm
http://www.aas.org/publications/baas/v37n3/dps2005/491.htm

http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05%2Ffm05&maxhits=200&="P11B-0107"

Posted by: BruceMoomaw Dec 12 2005, 08:24 AM

QUOTE (nprev @ Dec 12 2005, 07:17 AM)
I just had a thought: What if there is a "flux tube" between the Saturnian magnetosphere and Enceladus, as there is between Jupiter & Io? If so, perhaps the south pole of Enceladus is the impact point, which might add another increment of local heating and explain why vulcanism is apparently confined to that location.

Presumably this would still require Enceladus to have an endogenic magnetic field, though. huh.gif
*


First, it doesn't explain the total lack of any sign of such activity at Enceladus' north pole. Second, Saturn's magnetic field is vastly less intense than Jupiter's -- and its charged-particle magnetosphere is much less intense than THAT, thanks to the fact that the rings are conveniently placed to soak up most of the planet's high-energy trapped radiation. Third, the heat source for Enceladus is coming from inside out -- not outside in (in which case it would be spread evenly across the surface).

Posted by: edstrick Dec 12 2005, 09:28 AM

A significant current along a magnetic flux-tube would show up strongly in magnetometer data, I believe.

Something INSIDE Enceladus is profoundly assymetrical, more or less in a north-south direction. Targeting close flyby's for gravity tracking at both high latitudes and different longitudes at low latitudes will give more or less the only chance for Cassini to search for mass distribution anomalies that might be involved.

"Curiouser and Curiouser", said Alice.

Posted by: jmknapp Dec 12 2005, 12:50 PM

QUOTE (hendric @ Dec 12 2005, 01:37 AM)
Well, SOP for optimizing a slow calulation like that is to simply do it once, create a lookup table, and reference that every time.  If each run starts at the same time, they can use the same Saturn/Enceladus location information.  In fact, you could even create the lookup table off-line and load it at the start of each execution.
*


I could try that, although it would be a bit more involved as a number of rotations matrices, etc. would also have to be precomputed. In any case, I ran gprof on the program & it turns out that the major time hogs are s_cmp() and s_copy() which are in the Fortran-to-C library. The SPICE library was originally developed in FORTRAN and then ported over to C.

Anyway, I optimized the program a little bit as-is and got it to run a bit faster. I ran a simulation where a particle is shot out of one of the three major tiger stripes (coded red, green and blue below) at a random point along the stripe, at a random azimuth (0-360 deg) and random elevation (75-90 deg) and random speed (0-230 m/sec). Repeat 100,000 times and the following pattern of impact points results:

http://cassinicam.com/pint.JPG

Here's the base map:

http://cassinicam.com/enc2000grid.jpg

Not sure if that's consistent with observations of how the ice particles may be distributed, but that's how it comes out. One thing I wonder about is the particles striking towards the north are generally those with greater speeds, up to 230 m/sec or so. So maybe when they impact they could spread secondary particles pretty far afield.

Back in March, Cassini imaged the area shown in the black rectangle above:


http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=34956

Kinda looks like snow covered craters, no?

EDIT: There was a nasty little bug in my original program that was throwing the results off a bit. I've updated the graphic to reflect the fix & added color coding for the individual stripes. --jmk

Posted by: nprev Dec 12 2005, 09:00 PM

QUOTE (edstrick @ Dec 12 2005, 02:28 AM)
A significant current along a magnetic flux-tube would show up strongly in magnetometer data, I believe.

Something INSIDE Enceladus is profoundly assymetrical, more or less in a north-south direction.  Targeting close flyby's for gravity tracking at both high latitudes and different longitudes at low latitudes will give more or less the only chance for Cassini to search for mass distribution anomalies that might be involved.

"Curiouser and Curiouser", said Alice.
*


Would a "weak" (in comparison to Io's) flux tube in fact have been detectable by the Cassini magnetometer remotely? Obviously, if one exists then the spacecraft hasn't flown directly through it yet; just trying to set some constraints on the idea, here. Also, have there been any attempted observations of emissions from likely ionic species that might consititute the plasma of such a tube at the appropriate wavelengths?

The whole reason I'm pushing the flux tube idea and other external systemic effects as putative (and probably synergistic) causes for the observed geological activity is that I find it difficult to accept a significant degree of internally asymmetric mass distribution for Enceladus. One would think that this would produce many pronounced effects on the moon's gross orbital dynamics such as an axial tilt or even a chaotic rotation period like Hyperion's. If I'm not mistaken, Enceladus' inclination with respect to Saturn is zero, which argues for very symmetrical interior mass distribution.

Posted by: volcanopele Dec 12 2005, 09:06 PM

jmknapp, keep in mind that it isn't just plume material falling back that would coat Enceladus, but also E-ring material that would reimpact with Enceladus.

Posted by: nprev Dec 12 2005, 09:08 PM

QUOTE (volcanopele @ Dec 12 2005, 02:06 PM)
jmknapp, keep in mind that it isn't just plume material falling back that would coat Enceladus, but also E-ring material that would reimpact with Enceladus.
*


Aren't E-ring material & plume material synonymous, or has this not been definitively proven as yet?

Posted by: jmknapp Dec 13 2005, 06:44 PM

QUOTE (nprev @ Dec 12 2005, 05:08 PM)
Aren't E-ring material & plume material synonymous, or has this not been definitively proven as yet?
*


I believe he's pointing out that the distribution of ice particles on the surface will be due not only to the direct fallout from the plumes, but also an indirect flux of particles swept up from the E-ring, which might have been ejected with esape velocity long before.

Posted by: jmknapp Dec 14 2005, 12:30 PM

Interesting correlation here... an Enceladus albedo map was published in Icarus in 1994:


http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45NK0NP-3S&_coverDate=07%2F31%2F1994&_alid=345933012&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=fd103faa244518de95759f29cede97b2

Comparing the distribution of particle impact locations that I got previously:



There are some inconsistencies, but very interesting that the predicted main spike northward of particles corresponds very well to the area of high albedo at right.

Posted by: ugordan Dec 14 2005, 03:02 PM

QUOTE (jmknapp @ Dec 14 2005, 01:30 PM)
There are some inconsistencies, but very interesting that the predicted main spike northward of particles corresponds very well to the area of high albedo at right.
*

Very, very interesting. Those spikes at 0 and 180 degrees longitude correspond pretty well to your prediction, especially the 180 deg one. Your sim shows a fair amount of fallout will impact there and IMHO it's no coincidence the moon is brighter at that longitude than at any other region. The 0 deg impacts are sporadical and more scattered implying they're perturbed more easily by other, higher order effects so the impact points don't fit nearly as well.

Posted by: tty Dec 14 2005, 06:12 PM

Note the darker "cores" to the high albedo spikes at about longitude 60 and 210. They suggest that either there relatively fewer particles with low velocities or that the low-velocity particles have a different composition.

tty

Posted by: dilo Dec 14 2005, 09:37 PM

QUOTE (tty @ Dec 14 2005, 06:12 PM)
Note the darker "cores" to the high albedo spikes at about longitude 60 and 210. They suggest that either there relatively fewer particles with low velocities or that the low-velocity particles have a different composition.

tty
*

I have impression that albedo is rather related to particles origin: the central/polar tiger stripe (blue color code) generate darker material compared to the other two... ones ohmy.gif
Anyway, amazing work, jmknapp!

Posted by: hendric Dec 14 2005, 10:27 PM

Ugordan,
You should write up a paper and submit it to arxiv.org! wheel.gif

Posted by: jmknapp Dec 14 2005, 11:35 PM

QUOTE (dilo @ Dec 14 2005, 05:37 PM)
I have impression that albedo is rather related to particles origin: the central/polar tiger stripe (blue color code) generate darker material compared to the other two... ones
*


Another possibility: maybe the central stripe isn't as active, or not active along its entire length? The sim assumed that each stripe was active across its length. Nice to see Voyager data come in so handy!

Hendrik:
BTW, what is arxiv? Does one need academic/institutional affiliation to submit a paper?

Posted by: jamescanvin Dec 15 2005, 12:02 AM

QUOTE (jmknapp @ Dec 15 2005, 10:35 AM)
BTW, what is arxiv? Does one need academic/institutional affiliation to submit a paper?
*


It's a pre-print server - and no I don't think you need to be affiliated with anywhere to submit to it. If you do write this stuff up (and I think you should) you should think about submitting it to a proper peer reviewed journal as well.

James

Edit: Actually maybe you do need affiliation, I'm not so sure after a quick look at some of the submission pages...

Posted by: edstrick Dec 15 2005, 10:24 AM

nprev: "The whole reason I'm pushing the flux tube idea and other external systemic effects as putative (and probably synergistic) causes for the observed geological activity is that I find it difficult to accept a significant degree of internally asymmetric mass distribution for Enceladus. ...."

A weak flux tube with low current will also provide weak heating of Enceladus. There's also questions of how current would penetrate couples through the surface to the presumably conductive interior. I recall discussions of that in reference to Io, and I think I recall discussions of searching for nighttime glows on Io where flux tube current passes through conductive spots triggering possible auroral or arcing emissions. What they found or concluded I don't remember.

How much or little flux tube is possible comes out of quantitative comparisons of models of atmosphere and plume neutral and ionized gas interactions with Saturn's magnetic field, and the observations along flyby trajectories by Cassini. I'm not about to guess, but at a purely arm-waving-arguement level, the interaction seems to point to the plume and it's volume in space dominating the interaction.

Right now, the expectations are that Enceladus is pretty fully differentiated and any "rock"... more likely rock-mud core (at least at it's surface interface with the mantle) should be pretty fully relaxed and pretty level with the local gravitional potential. But the question remains why is the activity most at the south pole, and generally least at the north pole. Very very odd. My best weak guess is a single-cell convection pattern in the mantle ice, resulting in sinking under a stable crust in the north and upwelling in the south. But that might make features more like the coronae on Miranda. Our biggest hope for understanding this if we don't detect a flux tube (and that should equally heat the north and south in the simplest models), is gravity measurements on the internal mass distribution to make tidal heating models less-unconstrained.

Posted by: BruceMoomaw Dec 15 2005, 11:24 AM

One prediction which is becoming fairly consistent in planetary geology is that small worlds are likely to develop single-cell mantle convection cycles, which could also explain the asymmetry between the Moon's and Mars' two hemispheres (plus the Tharsis Bulge). In the case of Enceladus, one thing that intrigues me is the "patchiness" of its resurfacing -- which seems to me to imply that Enceladus has undergone a limited number of heating episodes during the Solar System's history, with its spin axis wandering between such episodes. This, of course, is what you might expect from a history of complex tidal interactions between Enceladus and the other moons.

Posted by: edstrick Dec 15 2005, 11:34 AM

One thing that seems clear is that during "heatups", some areas get totally resurfaced, while other areas have very soft mobile ice only a few kilometers under a really cold brittle crust that relaxes and develop those spectacular crazing and cracking patterns beautifully shown in the pic posted a page or two back. The control of the cracking by the topography to my mind suggests that convex surfaces like crater rims were colder and stayed cold enough to be stiff to greater depth than concave surfaces on crater floors etc. This should be very amenable to quantative modeling to produce the same geometry fracture pattern.

What's particularly significant, though, is that areas like these had soft ice under the brittle crust that was *NOT* convecting or flowing so as to horizontally deform the brittle crust with tectonic processes. It's a pretty pure relaxation-only process. So some soft ice was mobile, probably convection, some was not.

Go Figure. Sustained geological analysis will help unravel the relative sequence of events, first on local, then on global scales. Absolute times and durations we won't know, or only guess at.

Posted by: jmknapp Dec 15 2005, 02:15 PM

Here's an animation of the Christmas flyby:

http://cboh-t.cboh.org/~jmk/enceladus25dec_ob.avi

According to pointing info in the SPICE kernels, the only ORS observations are slated to occur briefly during the inbound, low-phase leg. But Jason has indicated that this is perhaps under review. I don't know what the tradeoffs are, but it seems a shame to pass up the opportunity to do more observations of the plumes, maybe over a greater range of time as the moon rotates outbound.

Posted by: ugordan Dec 15 2005, 02:49 PM

jmknapp: Seriously, you should think about writing a (short) paper on your sim. I don't know if others did similar simulations, but the results are intriguing. Perhaps with a little bit of playing around with distributions of venting locations you could make an even better fit to the albedo map. The paper doesn't have to be a hundred pages long, a few pages might do, describing your reasoning, steps and discussing your tentative results a bit.

Someone's bound to do this anyway and you might feel bad when they take the credit for it. unsure.gif

Posted by: elakdawalla Dec 15 2005, 04:40 PM

QUOTE (ugordan @ Dec 15 2005, 06:49 AM)
jmknapp: Seriously, you should think about writing a (short) paper on your sim. I don't know if others did similar simulations, but the results are intriguing. Perhaps with a little bit of playing around with distributions of venting locations you could make an even better fit to the albedo map. The paper doesn't have to be a hundred pages long, a few pages might do, describing your reasoning, steps and discussing your tentative results a bit.

Someone's bound to do this anyway and you might feel bad when they take the credit for it.  unsure.gif
*

If Joe is interested in that, the Lunar and Planetary Science Conference would be a good way to go. They publish two-page "abstracts," and their deadline is January 3. --However, if he does want to publish anything, of course, that is really "doing science" which means he would have to strictly avoid even giving the impression that he has looked at any Cassini Enceladus data that has not been delivered to the PDS. That would be a no-no at this point! I think the early Enceladus data should be part of the next PDS release, right? And I hear there is a publication coming up, perhaps in Science, perhaps in January, which should free other scientists' tounges a bit...

--Emily

Posted by: jmknapp Dec 15 2005, 05:34 PM

QUOTE (elakdawalla @ Dec 15 2005, 12:40 PM)
....would have to strictly avoid even giving the impression that he has looked at any Cassini Enceladus data that has not been delivered to the PDS.  That would be a no-no at this point!  I think the early Enceladus data should be part of the next PDS release, right?  And I hear there is a publication coming up, perhaps in Science, perhaps in January, which should free other scientists' tounges a bit...
*


Yeah, I seem to recall something about that in the MER documents, something like any early released information is not to be published for a set time? Of course, "published" may be a nebulous concept, what with web-publishing and all. I suppose it means not in peer-reviewed venues? Or perhaps, strictly, anywhere?

Posted by: elakdawalla Dec 15 2005, 05:43 PM

QUOTE (jmknapp @ Dec 15 2005, 09:34 AM)
Yeah, I seem to recall something about that in the MER documents, something like any early released information is not to be published for a set time? Of course, "published" may be a nebulous concept, what with web-publishing and all. I suppose it means not in peer-reviewed venues? Or perhaps, strictly, anywhere?
*

Yeah, now that preliminary versions of "data" are available on the Web for the rovers and Cassini it's much harder for people on science teams to police access to data. Obviously scientists who are not on the imaging teams for either mission look at the pictures and take note of things that they eventually want to publish about. But they absolutely cannot publish scientific analysis of anything that is out there -- raw images, press released images, whatever -- until the data is formally released to the community via the PDS. In the past, when people have published about press released images, they have been "drummed out of the business" (in the words of one scientist I talked with recently). Once it's in the PDS though, it's fair game. This is to give the scientists who've worked to bring the instruments to space a first chance to publish about the data, and also to calibrate the data properly based on their unique knowledge of the instruments before it's made available to scientists who don't have that unique knowledge and experience.

--Emily

Posted by: AlexBlackwell Dec 15 2005, 07:25 PM

QUOTE (elakdawalla @ Dec 15 2005, 05:43 PM)
Yeah, now that preliminary versions of "data" are available on the Web for the rovers and Cassini it's much harder for people on science teams to police access to data.  Obviously scientists who are not on the imaging teams for either mission look at the pictures and take note of things that they eventually want to publish about.  But they absolutely cannot publish scientific analysis of anything that is out there -- raw images, press released images, whatever -- until the data is formally released to the community via the PDS..

Before anyone who is not familiar with the practices gets scared, I think a bit of clarification is in order. Since there are no formal rules in place preventing anyone from publishing results based on publicly-released data, then stating that someone "absolutely cannot publish scientific analysis of anything that is out there...until the data is formally released to the community via the PDS" is, forgive me, a bit of hyperbole.

Of course, as you noted in the portion of your post that I elided, scientists risk condemnation from colleagues (viz., "drummed out of the business") for "claim jumping" or even outright rejection by a publisher. However, no one can be prevented from submitting something for publication based on data, in whatever form and however unreliable, that is in the public domain, regardless of whether it has been formally validated and placed on one of the PDS Nodes. Period. I may be mistaken but my understanding of NASA's current (and recent) policy regarding data is that all data are in the public domain, and while the data acquirers have a brief period of time for validation, usually six months, they have absolutely no proprietary rights.

Now Cassini may have been grandfathered in under the "old" system (e.g., Voyager-era where some teams did have proprietary rights) but I'd have to check the specific policy. However, I'm pretty sure MER operates under the current rules that others missions (e.g., Discovery, MGS, Mars Odyssey, etc.) fall under.

Having said all that, I would never trust a paper based on data that have not been validated. And I have a hard time believing that any respectable peer-reviewed journal would even consider, let alone accept, any such submission. Nevertheless, there is absolutely nothing to prevent someone from trying.

Posted by: Phil Stooke Dec 15 2005, 09:28 PM

In the specific case that was being discussed here, the mathematical modelling is perfectly legitimate for publication, assuming it was done correctly. And it would be quite reasonable to add to it a note about making a formal comparison with Cassini images when they become available, "but a preliminary examination of unvalidated data suggests...".

Phil

Posted by: jmknapp Dec 15 2005, 09:36 PM

QUOTE (Phil Stooke @ Dec 15 2005, 05:28 PM)
In the specific case that was being discussed here, the mathematical modelling is perfectly legitimate for publication, assuming it was done correctly.
*


How about this though--it's based on the locations of the tiger stripes, the existence and nature of which was not known previously--the locations determined from un-PDS'ed "raw" images from the ISS team. So would that then make any results tainted?

As for correctness--you are right--a little more verification is called for.

Posted by: The Messenger Dec 15 2005, 10:39 PM

QUOTE (elakdawalla @ Dec 15 2005, 10:43 AM)
...This is to give the scientists who've worked to bring the instruments to space a first chance to publish about the data, and also to calibrate the data properly based on their unique knowledge of the instruments before it's made available to scientists who don't have that unique knowledge and experience.

--Emily
*

In this case, I think jmknapp is stepping around their toes, but not on them. His approach is original and well conceived. Give the devil his due.

THE SYSTEM makes the assumption that the PI's are in the best position to validate and evaluate their own data. Most of the time this should work well, but on several of the current missions I have serious doubts.

For example, the WMAP team has been camped on the 2d & 3rd year data for 2-3 years. Literally hundreds of papers have been written that are based on first-year results, and some of these present serious challenges to the initial WMAP team findings.

So why the silence? Serious challenges have been raised, and I am of the opinion the WMAP team members are sitting on data they are either unwilling or unable to defend. As long as this stalemate continues, how can the science progress?

Posted by: elakdawalla Dec 15 2005, 10:55 PM

QUOTE (jmknapp @ Dec 15 2005, 01:36 PM)
How about this though--it's based on the locations of the tiger stripes, the existence and nature of which was not known previously--the locations determined from un-PDS'ed "raw" images from the ISS team. So would that then make any results tainted?
*

All I'm really saying is that I'd rather be on a ship to another planet at the moment that a certain team leader discovers any of the raw Cassini data being published in any form of scientific analysis by anybody not on the team.

That being said, http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm05&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm05%2Ffm05&maxhits=200&="P22A-06" by a scientist who spoke generally about the geophysical effects of a modeled plume on "mini-moons" including Miranda, Enceladus, Mimas, and Proteus. It's a nice example of not talking about the gorilla in the room.

--Emily

Posted by: ynyralmaen Dec 15 2005, 11:32 PM

QUOTE (jmknapp @ Dec 15 2005, 11:36 PM)
How about this though--it's based on the locations of the tiger stripes, the existence and nature of which was not known previously--the locations determined from un-PDS'ed "raw" images from the ISS team. So would that then make any results tainted?

*


I reckon that's still not playing by the rules.

I'd suggest that the best way to approach this would be to wait for the first publication(s) on the tiger stripes to appear. Once a paper has been published that maps the relevant features' positions and discusses their nature, then, as long as you properly cite that paper as the basis for your model, I would think that it would be OK.

Of course, you still wouldn't be able to include in the paper actual Cassini data until they're on the PDS, but for the purposes of your simulation, the images themselves are probably not necessary, as long as the sources' likely positions have already been provided elsewhere. I guess the comparison albedo map could be based on Voyager data or any relevant publications that contain such a Voyager data-based map.

One thing that's almost as bad as publishing work based on data that's still proprietary (at least when it's not yours! smile.gif ) is to knowingly publish work based on that of others while not citing the original authors' paper. The instrument science teams quite rightly expect and deserve credit for these initial discoveries, as without their instruments, the data wouldn't be available in the first place, and overall (I know there are exceptions), I think that this way of doing things is pretty fair to all.

Finally, the other thing you could do is to contact the relevant science team suggesting a collaboration, offering coauthorship to them. You may be pleasantly surprised.

Posted by: AlexBlackwell Dec 16 2005, 12:14 AM

QUOTE (ynyralmaen @ Dec 15 2005, 11:32 PM)
I reckon that's still not playing by the rules.

What "rules"? And how are these rules to be enforced? And by who?

QUOTE (ynyralmaen @ Dec 15 2005, 11:32 PM)
One thing that's almost as bad as publishing work based on data that's still proprietary...

Please show me a reference that the data in this case (especially raw, unvalidated data publicly released on the Internet) are proprietary.

Posted by: TheChemist Dec 16 2005, 12:33 AM

QUOTE (elakdawalla @ Dec 16 2005, 12:55 AM)
All I'm really saying is that I'd rather be on a ship to another planet at the moment that a certain team leader discovers any of the raw Cassini data being published in any form of scientific analysis by anybody not on the team.
*

Although that would be a huge boost for manned spaceflight, I don't see it happening. This lovely chap (or a cooperator or team member) would probably get to review the article smile.gif

I agree with Alex that the rules are not there, but in this case the peer-review proccess or the Editor would act as the publishing "police". Having said that, one can simply stick to the simulations, as Phil suggested, there is nothing wrong about publishing that part, I would guess.

Posted by: AlexBlackwell Dec 16 2005, 02:01 AM

QUOTE (TheChemist @ Dec 16 2005, 12:33 AM)
I agree with Alex that the rules are not there, but in this case the peer-review proccess or the Editor would act as the publishing "police". Having said that, one can simply stick to the simulations, as Phil suggested, there is nothing wrong about publishing that part, I would guess.

I think the upshot of this is that if one wishes to, say, become Carolyn Porco's research assistant, or not worry about an ISS team member being one of the anonymous peer reviewers (a very real possibility!), or not fear being poisoned during the http://www.lpi.usra.edu/meetings/lpsc2005/menu.html, etc., then Emily's point in this instance about not publishing before the ISS team is well taken tongue.gif

If, on the other hand, one doesn't care about that sort of thing, then there is absolutely nothing to prevent one from attempting to publish, especially if the supporting data (unvalidated though it may be) has been publicly released.

Posted by: Bill Harris Dec 16 2005, 02:16 AM

The world of Publish or Perish is a jungle and caution is advised. However, Joe has made a brilliant observation based on general data and he should get the deserved recognition for it.

--Bill

Posted by: Phil Stooke Dec 16 2005, 03:54 AM

I can't help thinking that the Cassini folks really need to clear this up. Frankly, we're in rather poorly understood ethical ground here, because we are living through a transition in the way data are distributed.

The old idea was that NASA data were proprietory for a while to give the team who did the hard work to get the images the first chance to use them. Then it would be publicly available. That in itself was different from the more distant past, and other areas of science, where data might be proprietary for ever, basically.

But now the MER, Cassini etc. projects are releasing raw images almost as soon as they get them. The Mars Odyssey Themis team just started doing the same. Apparently MRO will as well.

(aside: NEAR said they would, but chickened out a few days before going into orbit... Calvin Hamilton and I were playing with the daily releases of images just like we are today with Cassini and MER, until it was shut off)

The catch is, the "raw data" are not really raw. Images are contrast stretched and heavily jpegged. Now, I may be wrong, but I have believed all along that when they say 'should not be used for scientific analysis until a validated copy is deposited in PDS' or words to that effect, what they mean is - it's full of jpeg artifacts and has no photometric validity, so you can't trust the pixel values. For instance, you can't calculate albedo or optical depth, and small details may be artifacts. I really don't think anybody intends that the images are totally out of bounds. In fact I think that would be unenforceable as well as just plain foolish.

But maybe we need clarification here. If the Cassini imaging team really don't want people doing anything scientific with the daily releases, they need to be specific about it on their website. If it's only the caution about pixel values they are intending, they could be more specific about that. But frankly, we live in an era where any one of us could discover a new satellite or whatever in these images, and there's nothing to stop that happening once the images are on the web. If the team really doesn't want that, they shouldn't put it out there!

Phil

Posted by: The Messenger Dec 16 2005, 04:14 AM

Two more saliant points:

1) Historically, Cassini-Huygens is a mission that the probes themselves, and not whoever discovers whatever in the data who will get the primary credit.

2) It should not take nearly as long today for mission principles to reduce the primary data - yes, there is a lot more data, and perhaps smaller teams working on it, but the number crunching rarely takes weeks upon weeks. The Deep Impact team is just hand wringing when they say it will take forever to crunch the data. They didn't get the resolution they needed to draw the conclusions they are trying to reach.

3) The moons of Saturn are turning out to be an enormous puzzle - these pages are helping the principles, they are reading, and they have thousands of eyes and hours they would not have otherwise, scrutinizing the images. In an absolute paradox to point 2), there is too much processed data for the hired eyes to pick up everything. This will be especially truo of the MRO and Messenger, and if the PI's do not realize the internet is their friend, they could sit on precious data for decades and never know it.

Posted by: tallbear Dec 16 2005, 05:00 AM

The discussion going on here is interesting... but to attempt publications could change
a policy of posting these 'raw' JPEG'ed images for the public in near real time....

Of course we all get a kick out of the science extracted from JPEGs ... finding ray guns
and such on mars based on image compression artifacts...

I doubt that any reputable peer reviewed journal would touch an article using images
that do not have a clear 'OK' from the ISS Team Leader.

The modeling discussed might seem to lie in a grey area but it is based on ISS data
( and also CIRS data ) so I doubt that it is publishable until the images go into the
PDS in about one year or until articles by various Teams are officially published.

It took a lot of work and politicking to get the Cassini Images put out into public hands
in near real time, it would take one instance of abuse to basically have that stream of
images cut off to the public and to only be able to see images that go thru the official
PIO image press release process.

Enjoy the images... please don't push buttons that could shut off that stream of images
for all to enjoy.... a lot of people worked hard to get that public image URL.

Posted by: elakdawalla Dec 16 2005, 05:46 AM

QUOTE (Phil Stooke @ Dec 15 2005, 07:54 PM)
But maybe we need clarification here.  If the Cassini imaging team really don't want people doing anything scientific with the daily releases, they need to be specific about it on their website.  If it's only the caution about pixel values they are intending, they could be more specific about that.  But frankly, we live in an era where any one of us could discover a new satellite or whatever in these images, and there's nothing to stop that happening once the images are on the web.  If the team really doesn't want that, they shouldn't put it out there!
*

I can promise you that the Cassini imaging team absolutely doesn't want people doing anything scientific with the raw releases (or the press released images for that matter). In fact the team doesn't want any of you to be doing what you are doing with the raw releases. (I've been yelled at a few times, which is why I'm so sensitive about this, and I'm letting you know that that's among the potential consequences of crossing the team.) Unlike with MER the raw image releases are happening over the objections of the team, and the team believes that no responsible scientist is even looking at the raw images (which I find hard to believe--I couldn't resist if I were them). But JPL is going ahead and releasing them, and they are wishing us armchair image processors godpseed; both the public information people and the project science people for Cassini are in support of public access to the raw images. I think it's interesting that JPL didn't manage to do the same with Deep Impact, even though that was also a JPL mission. They have that horribly clunky viewer at the "View Near Real Time Images..." link on their http://deepimpact.jpl.nasa.gov/gallery/images.html page but that's it. Hopefully as time goes on, the Mars data model will become the common one.

EDIT: and let me thank everyone at JPL who has anything to do with the raw image releases. They are allowing us all to follow the mission in a way never achievable before. We are all "doing science" just looking at and thinking about these pictures and discussing them amongst ourselves.

--Emily

Posted by: tallbear Dec 16 2005, 05:50 AM

QUOTE (jmknapp @ Dec 15 2005, 07:15 AM)
Here's an animation of the Christmas flyby:

According to pointing info in the SPICE kernels, the only ORS observations are slated to occur briefly during the inbound, low-phase leg. But Jason has indicated that this is perhaps under review. I don't know what the tradeoffs are, but it seems a shame to pass up the opportunity to do more observations of the plumes, maybe over a greater range of time as the moon rotates outbound.
*



There will be no hi phase imaging for the Dec 25 opportunity. It was not possible
to alter existing sequences on such a short time scale.

The next hi-res hi-phase opportunities occur in 2007 ( DOY 273 and DOY 321 )
and the time for one of those opportunities is 'owned' by ISS at present.

However, the Cassin Tour will be altered in early 2006 and these opportunities may
go away while others may appear. There are a number of other hi-phase opportunities
but they are all at considerably lower resolution inthe Tour as it stands today.

Posted by: lyford Dec 16 2005, 06:44 AM

QUOTE (The Messenger @ Dec 15 2005, 08:14 PM)
..3) The moons of Saturn are turning out to be an enormous puzzle - these pages are helping the principles, they are reading, and they have thousands of eyes and hours they would not have otherwise, scrutinizing the images. In an absolute paradox to point 2), there is too much processed data for the hired eyes to pick up everything. This will be especially truo of the MRO and Messenger, and if the PI's do not realize the internet is their friend, they could sit on precious data for decades and never know it.
*

This reminds me of Steve Squyres statement about how hard it has been to get papers finished when they are still spending time still driving and collecting data. It seems as sensor density increases, there will be bandwidth bottlenecks not only transmitting, but also analyzing, the tremendous amounts of information the probes can now deliver.

Data mining other people's projects to preemptively publish smacks of playing unfair, unless you are data rescuing of a sort by going through NASA vaults as some of our august members apparently have. It seems entirely appropriate for the PI and the team who designed the experiments to have first dibs by withholding the calibrated keystone.

I agree though that the "fair use" statement should be made more explicit if the team has any fears of being upstaged.

(And just think in the future if some AI from a competing university starts doing analysis and announce results before the original team - that would be awkward!)

Posted by: Rob Pinnegar Dec 16 2005, 01:16 PM

QUOTE (elakdawalla @ Dec 15 2005, 11:46 PM)
In fact the team doesn't want any of you to be doing what you are doing with the raw releases.

Hmmm. This is news to me. It would have been helpful to have known it before now. (Hope I didn't step on anyone's toes in the D Ring thread, for example.)

Posted by: Phil Stooke Dec 16 2005, 02:55 PM

Well, this has been really useful. I think we have the clarification we needed. It would have been a lot better if this had been spelled out on the website, though, to avoid potential misunderstandings.

Phil

Posted by: pat Dec 16 2005, 03:37 PM

just to add some clarifications to the publication discussion

You can't attempt to publish anything in a peer reviewed journal when you cannot say what images you are using. The raw images on the JPL site specifically DO NOT indicate what the actual image number of the image is. Without that you cannot identify the image. e.g. the unique identifier for an image looks something like, N1404789881, where N means the Narrow Angle Camera and the number corresponds to the seconds portion of the spacecraft clock at shutter close. Its only when the PDS releases the archive volumes 9-12 months after the image is taken that you can actually correctly identify the image. The identifier on the JPL raw image site, e.g. N00046830.jpg, isn't related to the actual image number in any way and will not be accepted as an identifier by any reputable journal

Until the PDS release of images you also don't have any of the necessary anciliary data for an image e.g. exposure duration, gain state etc. (also until you know the exact time of the image you don't know where the camera was actually pointed)

Until you know basic things like exposure time, the correct image number etc either the editor or reviewers should automatically reject any submitted paper which uses images or unpublished data from images

Also Emily's comment about the ISS Team comming down like the wrath of God on anyone doing anything with the raw images. I'd suggest that its the ISS Team Leader who is following this policy, I'd further suggest that the ISS Team as a whole might very well have been in favour of the rapid release of the raw images. The participation of at least two members of the ISS Team in this forum strongly hints at this. I'd also imagine that many of the ISS Team members are quite enjoying the products being posted online by non ISS Team Members

Posted by: The Messenger Dec 16 2005, 03:53 PM

QUOTE (Rob Pinnegar @ Dec 16 2005, 06:16 AM)
Hmmm. This is news to me. It would have been helpful to have known it before now. (Hope I didn't step on anyone's toes in the D Ring thread, for example.)
*


Even if they are suspicious of the eyeballs, they must be aware of the heightened public interest these images and data are bringing to the program...and as a plus, we are getting educated as to what may or may not be appropriate. Jason was discovered on the internet, and he is a valuable asset on the Cyclops team.

Let me be the Devils advocate: Suppose the images were not being made public, and the Cyclops team, (which is under UNFAIR pressure to find things first) backed off and missed the sputtering on Enceladus. Less priority might have been placed on the follow-up, and the unambiguous evidence of Enceladus' plumes lost forever.

On the other hand, when a certain scientist tried to lie, cheat and steal a Keplarian planet discovery, the internet had his fingers all over it. We need to find these clowns and drum them out, not wait for embarrassing scandals like the Korean government is facing over cloning secrets.

Finally, I think revised science is necessary to understand the phenomenon being unleased by this mission, and I think this can demonstrated with Cassini and Huygens observations. If it does take revised theory to explain what we are observing, mission scientists need these new tools in their bag to make sense of the data.

Posted by: jmknapp Dec 16 2005, 03:59 PM

I should note that I determined the positions of the tiger stripes (latitude and longitude along their lengths) based on a cylindrical projection from http://laps.fsl.noaa.gov/albers/sos/sos.html, who in turn based his map on a previous Voyager-based images from http://www.ssc.uwo.ca/geography/spacemap/enindex2.htm and http://home.arcor.de/jimpage/saturn.html.

It's generally hard to extract latitude and longitude information from the raw images, & Steve apparently did that by eyeball in making his mosaic. If the descriptions of the raw images included the time of observation, it would be possible to find the (tentative) geographic coordinates in an image by using the SPICE kernel data about where the camera was pointed when, etc., but the time of observation is only given to the day on the Cassini website. But I suppose to provide more would be leaking out more "science."

Don't mean to step on anybody's toes--I actually looked a while ago for some kind of usage restriction statement but wasn't able to find the same. http://www.jpl.nasa.gov/images/policy/ says:

QUOTE
Unless otherwise noted, images and video on JPL public web sites (public sites ending with a jpl.nasa.gov address) may be used for any purpose without prior permission, subject to the special cases noted below.


If one searches the PDS Imaging Node for "Enceladus" a link is provided to the http://photojournal.jpl.nasa.gov/target/Enceladus released to the Planetary Photo Journal.

I got an inkling that UnmannedSpaceflight.com might raise a few hackles on the Cassini team based on a http://science.slashdot.org/article.pl?sid=05/11/28/2320211&tid=160&tid=14 on the Enceladus plumes that referenced this thread. A poster in that thread complained:

QUOTE
Since Cassini is so slow in releasing results to the general public, you may be interested in this discussion (including some neat image processing) by amateur astronomers: http://www.unmannedspaceflight.com/index.php?showt [unmannedspaceflight.com] This site usually get a jump on the official Cassini channels of about a week.


Rather inflammatory yes, but it got this response from someone evidently on the Cassini team:

QUOTE
Pardon me, but Cassin is NOT slow to release its results. Some of these images came down in the past two *days*. And I'd like to note that they got posted to the JPL website almost instantly. That's actually rather unfair to us, since there's usually a one *year* propriatary period where the data are the kept by the people who put the work into designing, building, and operating the instrument. Thanks to JPL, anybody off the street can get up at 3 AM to grab the images of the website before we've woken up that morning, let alone gotten our coffees in.

Of course, amateurs are not bound be either rules for peer-review to get published or by NASA's process for press-releases, so their results will often appear on the web sooner than the offical findings. But they should also be treated with a certain measure of skepticism. Also, remember that the images that JPL posts aren't scientific quality.


After getting some feedback about taxpayers who are paying the bills, etc., came this response from the same person:

QUOTE
Yeah, why should the people who didn't devote years of their lives and continue to devote 60-80 hours a week running the instrument be at a disadvantage? Reminds of that story about the chicken who wants to bake bread and no one will help her, but everyone wants to eat it afterward.

The taxpayers have every right to the data. The question is, should they get it at the same time as the people who have spent years making sure that the data arrive at all? By comparison, are you going to insist that the data collected in labs (under government funding) be open to the public as the scientists take it?

If you want to head down this road, you're not going to get data at all. Scienists take a severe hit to their careers to PI an instrument like this. If they didn't get something back, like a period where they could have first pass at making discoveries in the data, you'd be hard-pressed to get anyone to build the things.


And then:

QUOTE
"What would that disadvantage be? Missing the opportunity to be the first to write about something? Do you really believe that you are entitled to that?"

Careers are made or broken based on getting things published first. From a PR standpoint, the amateur community only ruins press-releases. Which is a drag, but not seriously problematic. (Although that can be a damper on a career, too. Getting into the mainstream media for a new result helps when hunting for jobs.) But other scientists are also out there and when they see these things, they jump on them and race to beat us to publication. That can easily wreck a person's entire career.

Why do you think that the general public is entitled to beat the scientists to the results? Are you also prepared to argue that you're entitled to a ride on Air Force One or to look at the CIA's current data?


Seems like there's a bit of negative feelings about amateur analysis of the raw data?

Posted by: ugordan Dec 16 2005, 04:22 PM

QUOTE (jmknapp @ Dec 16 2005, 04:59 PM)
Seems like there's a bit of negative feelings about amateur analysis of the raw data?
*

I would say it more looks like negative attitude towards statements like "Cassini team is slow at releasing data". I can't say I blame them, I also perfectly understand why the people who designed the damn thing should be able to get credit for discoveries first.
I don't believe they're against raw data processing folks like us, it's more against someone claiming afterwards they discovered something before the PI did...
Frankly, I don't see what the purpose of the raw images would be if not to allow ordinary guys like us to play around with data.
Another thing, though - what if the discovery process went the other way: someone on a forum like this actually discovers something that escaped the official team, someone on the inside team finds this out and publishes the finding as it was their own, without due(?) credit to the "ordinary" guy. That wouldn't be very nice, either...

Posted by: volcanopele Dec 16 2005, 06:04 PM

Okay, as one of the ISS people here on this forum, I will try to clarify our position. First, as Emily pointed out, we had the raw images page on the JPL site thrust upon us, there was definitely some strong criticism of that decision by a number of people on the team. The reasons vary, but primarily it took away the ability of the team to announce major discoveries, to an extent. For example, everyone here knew about the spokes (and the rumors abounded within the space science press) for a week before we actually announced it. This jives with what John said on Slashdot, it puts us at a disadvantage when we are scooped by someone who woke up earlier than us. Now, in many ways, I both agree and disagree. I have no problems with some of the stitching work done here. Due to the JPEG artifacts and stretching, I don't feel I have anything to worry about, and it provides good press for Cassini. However, I do agree that any talk of publication based on the images on the raw images page or from press release images is down right unethical and because of the way the data has been manhandled before you see them (except my mosaics, they are treated with TLC or someone gets a caneing), that no peer-review publication would even touch it. But, that may not stop someone from working on a paper then submitting it once the data officially hits the PDS. And in that way, we are put at a disadvantage.

Now, what does this mean for people like jmknapp. Well, his work isn't as constricted. His work was a simulation of particle fall out from a plume near the south pole of Enceladus. Now, you would get into some problems with the fact that the seed locations were along tiger stripes that have not been described in a peer-review publication. So, I would dissuade you from going forward until that publication is out. But still, it may be best to make the simulation more generic, maybe simulate fallout patterns from random locations in the region, and see where that gets you.

Posted by: volcanopele Dec 16 2005, 06:04 PM

I'm moving all the Cassini Titan tour tweaks discussion to another thread.

Posted by: AlexBlackwell Dec 16 2005, 06:09 PM

QUOTE (ugordan @ Dec 16 2005, 04:22 PM)
Frankly, I don't see what the purpose of the raw images would be if not to allow ordinary guys like us to play around with data.

Perhaps that's one ancillary (though perhaps unintended) benefit, but I always thought the primary purpose of releasing raw, unvalidated imagery on the Internet was for public relations (viz., "You [the public] see the images before we do! This shows how cooperative we are and it also shows we're not airbrushing out the Little Green Men.").

At any rate, I somewhat agree with Phil Stooke regarding this issue. If, for example, NASA/JPL/ISS doesn't want the public "analyzing" the raw, unvalidated imagery (whatever that means), then they should explicitly say so. Frankly, though, I don't think that will happen since, as a practical matter, any such an approach would quite likely blow up in their collective faces. While there may be some latent hostility and grumbling among the team members, the gatekeeping process of peer-review that I alluded to (though "pat" described more clearly) serves as the ultimate check.

That said, I don't think this is much of a problem. Except for the kooks, who are easily dismissed, or honest amateurs with a real interest in the process, there aren't many people rushing in to "analyze" the raw, unvalidated data. For instance, MER released the raw, unvalidated Pancam, Navcam, and MI imagery from the outcrop at Eagle Crater and conducted detailed press briefings months before the first papers were published in Science. In fact, the story at Eagle Crater unfolded slowly over several weeks and, despite the great amount of speculation on the Internet about what it all meant, I didn't see anyone rushing to publish ahead of Squyres et al. Indeed, their recent, detailed publication in EPSL, over a year after the initial discovery, still outstrips all of the discussion I've seen.

I attribute this to a variety of reasons, but chief among them is that the vast majority of the public doesn't have the faintest idea of (or interest in) how to do science in the first place, so they don't even try, lest they embarass themselves in the attempt. And the small segment that does know how to do science "plays by the (unwritten) rules" of etiquette.

Posted by: Sunspot Dec 16 2005, 06:19 PM

Wasn't the orginal intention to have one picture from Cassini released per day, Monday - Friday? ohmy.gif

Posted by: AlexBlackwell Dec 16 2005, 06:26 PM

QUOTE (volcanopele @ Dec 16 2005, 06:04 PM)
Okay, as one of the ISS people here on this forum, I will try to clarify our position.  First, as Emily pointed out, we had the raw images page on the JPL site thrust upon us, there was definitely some strong criticism of that decision by a number of people on the team.

I agree that the ISS team members get screwed with this approach, while other teams get to hold on to their data ("I demand access to the T8 MAG data now!"), and this would engender some hostility as a result. And you're right, any scientist with ethics wouldn't try to scoop the team in the first place by publishing in peer-reviewed journals.

However, journalists, bloggers, and other armchair amateurs are different. For example, Emily's lament that she had been "yelled at a few times...[for] crossing the ISS team" shows how ridiculous this has become. It's as if the developers of bird seed at an animal food company are complaining to the birds for eating seeds placed in the bird feeder because the sales/marketing department released it against their objections.

Posted by: jmknapp Dec 16 2005, 07:00 PM

BTW, here's an animation of the incoming Enceladus flyby, Dec. 24:

http://cboh-t.cboh.org/~jmk/enceladus24dec_ib.avi

The yellow dot shows where the ORS platform is pointed, based on the SPICE CK kernel. Not sure what the scanning is all about.

Posted by: odave Dec 16 2005, 07:03 PM

Please help me wrap my software engineer brain around the concepts in this discussion.

QUOTE (volcanopele @ Dec 16 2005, 01:04 PM)
Now, you would get into some problems with the fact that the seed locations were along tiger stripes that have not been described in a peer-review publication. 
*


Does this mean that the tiger stripes do not "officially" exist, even though images of them have been released to the public by NASA, until a peer reviewed article has been published? And Joe would have problems because his simulation involves these features that do not "officially" exist?

Not trying to be mean or sarcastic - I just want to understand the process...

Posted by: Bill Harris Dec 16 2005, 07:39 PM

QUOTE
Now, you would get into some problems with the fact that the seed locations were along tiger stripes that have not been described in a peer-review publication.


I don't see why this needs to be officially described. Any sentient being with tentacles and antennae can see plume, fractures and subdued high-albedo areas and put Bwrdd and Iaith together to get Gymraeg. Joe- and a lot of others, I'm sure- are assuming that the plumes are here, so lets assume this is happening, crunch numbers and get _this_.

My 10^6 Zlot, FWIW.

--Bill

Posted by: Rob Pinnegar Dec 16 2005, 08:23 PM

To put another two cents in: There really shouldn't be all _that_ much of a problem regarding major announcements getting scooped, just as long as our noodling around with the raw images doesn't work its way into the mainstream press. There are what, a few hundred people here? That's really not much when compared to the millions, or even tens of millions, who are going to see the official press releases on Google News.

Also, I agree with the folks who have pointed out that most of the amateur analyses being done here would never get through peer review anyways. For example, I've played around with some images for the fun of it, and even written a few Matlab scripts to "analyze" raw images. But the techniques I used consisted of ad hoc, strap-it-on-and-go reasoning, of the sort that I wouldn't send out for peer review if my life depended on it. That can hardly compete with a conference abstract.

There is of course another, less easy to implement, way to alleviate the problem, and that would be to speed up the peer review process. (If you really want to hear an angry, bitter diatribe, try asking me about the paper that I've had in review at "a journal" for over two years now. It has just been scooped by someone else, who independently came up with the exact same idea, submitted it to "a different journal" over a year after mine went in, and then got published first because they got through review in six months. Experiences like this will give you some appreciation for the frustration felt by the imaging team.)

Posted by: volcanopele Dec 16 2005, 08:32 PM

QUOTE (odave @ Dec 16 2005, 12:03 PM)
Please help me wrap my software engineer brain around the concepts in this discussion.
Does this mean that the tiger stripes do not "officially" exist, even though images of them have been released to the public by NASA, until a peer reviewed article has been published?  And Joe would have problems because his simulation involves these features that do not "officially" exist?

Not trying to be mean or sarcastic - I just want to understand the process...
*

They have only been discussed in press releases and at DPS/AGU. So, yes, they "officially" exist. And the plumes have been shown in a press release, so they "officially" exist. Using the exact location of the tiger stripes is a problem because officially, that info isn't available, and you are using press release images to derive those locations. Now, if Joe wanted to do a simulation with random seed locations throughout the south polar region, that would be fine. It may even be more useful... wink.gif

Posted by: odave Dec 16 2005, 08:51 PM

Gotcha - thanks!

Posted by: volcanopele Dec 16 2005, 10:40 PM

Bringing us back to the plumes, the VIMS team has released some specta of the plume/jets taken during the Nov. 27 non-targeted encounter:



http://photojournal.jpl.nasa.gov/catalog/PIA06443

During a non-targeted flyby by the Cassini spacecraft of Saturn's moon Enceladus on Nov. 26, 2005, Cassini's visual and infrared mapping spectrometer measured the spectrum of the plumes originating from the south pole of the icy moon.
The instrument captured a very clear signature of small ice particles in the plume data, at the 2.9 micron wavelength. This image of Enceladus, taken with the visual and infrared mapping spectrometer, shows not only the plume over the south pole, but also the dark side of the moon, silhouetted against a foggy background of light from the E Ring.
The bottom graph shows the measurements of the spectrum, of this background light. It shows a very similar signature of small ice particles to that in the plumes, confirming earlier expectations that Enceladus is indeed the source of the E ring.
Preliminary analyses suggest that the average size of the particles in the plume is about 10 microns (or 1/100,000 of a meter). The particles in the E ring are about three times smaller. The sunlit surface of Enceladus itself, visible as a thin crescent at the bottom of the image, is also composed of water ice, but with a much larger grain size than the plume.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The visual and infrared mapping spectrometer team is based at the University of Arizona.
For more information about the Cassini-Huygens mission http://saturn.jpl.nasa.gov. The visual and infrared mapping spectrometer team homepage is at http://wwwvims.lpl.arizona.edu.

Posted by: Rob Pinnegar Dec 16 2005, 11:48 PM

QUOTE (volcanopele @ Dec 16 2005, 04:40 PM)
Bringing us back to the plumes...

On that note:

Think it'd be a good idea to take all the posts in this thread that relate to publication, and put them in their own thread? This has been one of the better discussions I've seen in this forum, but it _is_ a bit off topic for the Enceladus Plume Search (although of course it didn't start out that way).

Posted by: David Dec 17 2005, 01:06 AM

QUOTE (volcanopele @ Dec 16 2005, 08:32 PM)
Now, if Joe wanted to do a simulation with random seed locations throughout the south polar region, that would be fine.  It may even be more useful... wink.gif
*


Hm. It may be misguided of me to put my two cents in, but here goes anyway: I can see two possible outcomes. Either you get the same patterns from ejections from random places between the pole and South Latitude X degrees. Then fine, it really doesn't matter where they come from. Or they come out differently, and then -- if there's the time to run enough simulations -- you could constrain the locations off the simulation itself, saying "in order to get a pattern mapping to such-and-such an area [assuming that that area officially exists!] the ejection has to take place from an area within this part of the southern polar region.

Now that would probably be pretty arduous, but if jmknapp could *predict* the existence of the tiger stripes based on that, it would be a pretty remarkable result. smile.gif

Posted by: jmknapp Dec 17 2005, 02:13 AM

Ouch--a quick heads up: I'm not exactly sure yet, but I think the results of the sim are suspect.

I've been testing the basic code the last couple days and figured a good test of the integration would be to see if it could simply predict the orbit of Enceladus, using the SPICE-reported positions as a gold standard. If the simulated Enceladus starts out at the same position as the real (SPICE) one, the positions diverge over time such that after 6 hours of sim time, the positions are 200 km different. This error was the about the same regardless of the integration step size, meaning the integration code itself is probably not at fault. What I think is happening is that the orbit of the real Enceladus is precessing which is not modeled by a simple point-mass integration that my sim is doing.

So in the sim the SPICE library calls fix the position of Saturn and Enceladus accurately, but the test particles lack the precession component. With particle times of flight upwards of 10 hours, the error is significant. Anyway, regardless of the cause, the predictor seems to be faulty.

I modified the code to test this further. Instead of using the SPICE library, I determined the Enceladus position also with the integrator. So both Enceladus and the test particles would follow idealized, point-mass trajectories. That sim is still running (takes a long time especially with the added calculations) but I can see by the results so far that the pattern seems to go away. Will post tomorrow...

Posted by: dvandorn Dec 17 2005, 02:35 AM

As far as Joe's work is concerned, let's look at the sources he's used:

1) Albedo maps of Enceladus based on officially released PDS Voyager data (created by Steve Albers). No problems there, though, of course, Albers would have to be cited for his contribution.

2) The existence of ice-particle plumes. Details of the plumes' origins and trajectories are not "officially" available, but their existence is official. So, distribution of plumes observed emanating from the south polar region will have to wait for official PDS release.

3) The existence and location of the tiger stripes. These features were hinted at in Voyager imaging, but are not well enough defined in the Voyager data to locate them well. Correlations between plume sources and tiger stripes will have to wait for the PDS release.

4) The simulator used to predict the trajectories of plume particles. Any peer review will require enough detail on the constraints and limitations of the simulation techniques to provide any kind of confidence in the validity of Joe's findings. This might require more information about the simulation programming than Joe has proprietary rights to provide.

So, it would seem like Joe could write up his paper, leaving blanks for the identifiers he will get from the PDS release, but assuming that the PDS data will not vary significantly from the data he already has at his disposal. As soon as the data is released to the PDS, he can verify it as it relates to his paper, insert the appropriate identifications in the placeholders he's left, and submit it.

However, he's going to have to deal with 4) above, regardless. If the calculations performed by the simulation software are proprietary and he can't demonstrate their validity (and state error margins), then no matter how valid his conclusions are, it'll be hard to get them published...

-the other Doug

Posted by: jmknapp Dec 17 2005, 03:06 PM

QUOTE (dvandorn @ Dec 16 2005, 10:35 PM)
As far as Joe's work is concerned, let's look at the sources he's used:

1)  Albedo maps of Enceladus based on officially released PDS Voyager data (created by Steve Albers).  No problems there, though, of course, Albers would have to be cited for his contribution.
*


The albedo map is by Anne Verbiscer, photometrically calibrated using Voyager and HST data. I used the Albers map to get the stripe locations. The Verbiscer map was published in 1994. More recently (Jan. 2005) she has published another Icarus paper http://2F2005&_alid=346222757&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000039638&_version=1&_urlVersion=0&_userid=709071&md5=1042648117fb661b6eb775b5be870815 which concludes that the albedos need to be increased about 20% from previous values.

QUOTE (dvandorn @ Dec 16 2005, 10:35 PM)
4)  The simulator used to predict the trajectories of plume particles.  Any peer review will require enough detail on the constraints and limitations of the simulation techniques to provide any kind of confidence in the validity of Joe's findings.  This might require more information about the simulation programming than Joe has proprietary rights to provide.
*


I guess we were posting at the same time & you didn't see my post about the prediction program being suspect, but you hit the nail on the head! I reran the sim overnight with the latest "fix" and while the results are quite a bit different, maybe it's not a total washout.

Here's the new distribution map I get:



So there's still a pattern, just not as distinct. I suppose that this result is suspect too, as there may be other problems with the sim. It would be nice if someone could independently try to replicate this simulation, to compare results.

I'm not sure what you're getting at about proprietary rights to the software though. I just programmed the thing from scratch in C--using the SPICE library which is also open source. I could post the source files if anyone's interested.

As for correlation with the Verbiscer albedo map, with the new distribution it's not nearly so clear. Amazing how a program bug matched so well by coincidence! However there may still be a correlation. Here's the result:




It's maybe hard to see the colored dots above, so here's the map with all dots bright yellow:



Anyway, that's where it stands. Again, I'd love to see someone else try this approach & see how it matches up.

Posted by: ugordan Dec 17 2005, 03:30 PM

QUOTE (jmknapp @ Dec 17 2005, 04:06 PM)
I'm not sure what you're getting at about proprietary rights to the software though. I just programmed the thing from scratch in C--using the SPICE library which is also open source. I could post the source files if anyone's interested.
*

I for one would be interested in the source, if you're really willing to provide it openly. Is it a very complex piece of work or is it "manageable"? You could send it to me via e-mail if it'd be more convenient that way.
And no, I wouldn't rip you off by changing the code a bit and saying it was all my work. Just in case you're wondering wink.gif

Posted by: dvandorn Dec 17 2005, 03:44 PM

Okay, Joe -- I wasn't sure whether you were using your own trajectory simulator or whether you were using some third-party software. If it's your own code, you're fine to publish -- but you will, indeed, need to provide enough detail about the code to generate confidence in its trajectory predictions.

I guess I was becoming somewhat confused between your trajectory sims and those run by some others using the freeware Orbiter program. Sorry 'bout that.

-the other Doug

Posted by: Bjorn Jonsson Dec 17 2005, 09:33 PM

This is extremely interesting and I for one would be interested in seeing the source, especially since I have also been using the SPICE library.

BTW wouldn't it be fairly easy to use this program with some modifications to simulate the appearance (or at least the density distribution) of the plumes as seen from Cassini when it imaged the plumes ? I once wrote a program to simulate the appearance of Io's plumes with somewhat interesting results - it was far more simple than this one (I didn't need to take Jupiter's gravity into account for instance) but visually the output was interesting (parts of the attached image are 'overexposed'):

 

Posted by: pat Dec 19 2005, 01:12 AM

QUOTE (jmknapp @ Dec 17 2005, 03:13 AM)
.....What I think is happening is that the orbit of the real Enceladus is precessing which is not modeled by a simple point-mass integration that my sim is doing...

*


Joe, its just a guess but it looks like you might not be allowing for the oblateness of Saturn. You have to modify the Newtonian equation of gravity using (at least) the first two zonal gravitational harmonics, J2 and J4. I know people who also use J6 and C22 (at least I think its C22, it could be one of the other C's).

Essentially the effect of a planet,s oblateness (non sphericity) is to increase the mean motion of orbiting objects (compared to a point mass model) and adds precession to elliptical orbits e.g. at the F ring the pericentre precesses by ~2.7 deg day.

Posted by: pat Dec 19 2005, 01:15 AM

sorry that should be "first two even zonal gravitational harmonics, J2 and J4."

Posted by: jmknapp Dec 19 2005, 12:43 PM

QUOTE (pat @ Dec 18 2005, 09:12 PM)
Joe, its just a guess but it looks like you might not be allowing for the oblateness of Saturn. You have to modify the Newtonian equation of gravity using (at least) the first two zonal gravitational harmonics, J2 and J4. I know people who also use J6 and C22 (at least I think its C22, it could be one of the other C's).
*


Thanks--I'll look into doing that as with a super accurate simulation maybe even the development of the E-ring could be modeled. If I had a supercomputer cluster that is!

As it is though I modified the existing code to integrate both the particle set and Enceladus using the same (point mass) method so that each would be treated the same way & cleaned up the code since several people here have expressed interest in seeing it. And guess what--the original result stands & is perhaps even strengthened! It was my test program, casting doubt on the results, that was in error. The code now passes any test I have thought of and clearly shows the correlation between the predicted particle distribution and the Verbiscer albedo map.

I guess with all the attention here and mini-controversy about using Cassini data, I was a bit too hasty in reviewing the code. In any case, the bottom line is that I stand by the original result, and offer the code here for anyone to review:

http://cassinicam.com/plumesw/psim.zip

Here's the correlation with the Verbiscer map, plotting the impact locations of 200,000 simulated particles coming from the three tiger stripes (coded red, green, blue):

http://cassinicam.com/plumesw/vstripes.jpg

http://cassinicam.com/plumesw/verbiscer.jpg

The match is quite remarkable in some areas.

Here's the south polar view on the Albers map:



I'm currently trying a sim such as Jason suggested, using random plume locations in the south polar region rather than the specific locations of the tiger stripes. I guess that would be of general interest, and not rely on Cassini-derived data so much. So far it looks like the general pattern is more or less the same. If so, a remarkable coincidence that the tiger stripes themselves are lined up the the two northward streams of particles.

Posted by: Bill Harris Dec 19 2005, 01:42 PM

Wonderful! Peer review at this level. It looks as though your observations and the model are holding up well.

One idea: Using the albedo features and the muting of the landscape I wonder if you could model a reverse-trajectory from the supposed impact points back to their source and "predict" the linear plume sources (tiger stripes)? There are a lot of variables and it might not work.

--Bill

Posted by: babakm Dec 19 2005, 02:03 PM

QUOTE (Bill Harris @ Dec 19 2005, 01:42 PM)
One idea: Using the albedo features and the muting of the landscape I wonder if you could model a reverse-trajectory from the supposed impact points back to their source and "predict" the linear plume sources (tiger stripes)?  There are a lot of variables and it might not work.
*


The "backtesting" tack may also be useful in finding the sources for the Albedo features in the Northern hemisphere.

Posted by: ugordan Dec 19 2005, 02:14 PM

Frankly, I don't see how you could backtrace the impacts to their origin. There's no information "preserved" when the particle impacts: you'd need azimuth and elevation angles and particle speed for starters.
Otherwise you have no idea whether the particle came along a short trajectory from the immediate neighborhood or was kicked up from the opposite side of the moon.

Posted by: Bill Harris Dec 19 2005, 03:45 PM

You're right, without knowing anything about the particle trajectory there is no way to trace it back.

--Bill

Posted by: jmknapp Dec 19 2005, 05:06 PM

QUOTE (Bill Harris @ Dec 19 2005, 11:45 AM)
You're right, without knowing anything about the particle trajectory there is no way to trace it back. 
*


I suppose a conclusion might be reached if the overall pattern changed based on whether the plumes were coming from defined stripes vs. from generally all over the south pole region. However, I ran the sim that Jason suggested--with particles coming from the south pole generally--and there is not much difference to the impact map. Here's the result of 200,000 particles from the area between 75S and 90S latitude:



For comparison here is the map based on the tiger stripes, this time with all points colored red:



Not much difference there.

Posted by: Bill Harris Dec 19 2005, 05:48 PM

That is a good datapoint. It seems that the Size and shape of the impact zone is more a function of the "gravitational harmonics" and is not strongly influenced by the number and characteristics of the plumes. This is good because it means that the plumes can change over time without changing the impact area. It is better that it works with generic plumes in a general area without referring to specific fracture systems.

You have done some impressive work on this.

--Bill

Posted by: volcanopele Dec 19 2005, 06:38 PM

Very nice. Still, interesting that it predicts particle fallout along 30 and 210.

Posted by: David Dec 19 2005, 07:01 PM

I notice that on the Verbiscer map there are two albedo "spikes" (areas of higher albedo extending from the Enceladian antarctic northwards), at about 90° W and 140° W, which the simulation doesn't appear to account for -- at least, not as aggressively as it does some of the other "spikes". There's another high-albedo region on the other hemisphere, about 280°-340°, which is poorly filled in by these trajectories.

I wonder if these gaps could be accounted for by varying the initial velocity of the particles, or by assuming additional particle sources?

Posted by: JRehling Dec 19 2005, 07:44 PM

To be part of the spoilsport / peer review on another end: How meaningful is that albedo map? It looks like some dark streaks are relief features -- if ALL of the dark features are albedo features, even ones that were not well resolved, then resurfacing is not a good account of the correlation.

Icy particle snow-out should only correlate with the albedo features if the background albedo of Enceladus is a bit darker due to compositional differences. (Or, less plausibly, due to relief differences, with the bright areas being smothered in a relief-hiding blanket of snow.)

My second note of spoilsportdom would be the general observation that many worlds with tidal heating show patterns with 180-degree periodicity of one sort or another: Europa and Miranda are examples. The correspondence shown in your map would be less amazing if we start with the notion that what's happening on Enceladus would be southern-centered activity of 180-degree periodicity. Namely, it would not be that unlikely that a set of rift complexes would alter the albedo at 30 & 210, and that a snow-out pattern would happen by coincidence to overlie it. It's not LIKELY that it would happen, but the probability of the null hypothesis might not be down to the standard of publication.

Overall, these criticisms add up to the possibility that the albedo of Enceladus (like Dione) is brighter where relief features exist, and that this pattern happens to spatially correlate acausally with your work. That mere possibility is not something that would make your work unworthy of publication -- it's just a possible explanation.

Spoilsporting over... good work!

Posted by: Bjorn Jonsson Dec 19 2005, 11:07 PM

It might be interesting to check what happens if the plume sources are moved towards the equator, especially if you want to 'pretend' you don't know where the plume sources are loacted (since the images showing the plumes will not appear on the PDS until late next year). Heck, I might even try this myself now that I have the source code if I can find any time for this.

Interestingly, some of the brightness variations in the albedo map seem to be simply due to a leading/trailing hemisphere asymmetry. However, I wonder if I'm overlooking something elementary when looking at the albedo map because its left edge is much darker than its right edge and the edges are at 180° longitude ??

Posted by: tallbear Dec 20 2005, 07:34 AM

QUOTE (jmknapp @ Dec 16 2005, 12:00 PM)
BTW, here's an animation of the incoming Enceladus flyby, Dec. 24:

http://cboh-t.cboh.org/~jmk/enceladus24dec_ib.avi

The yellow dot shows where the ORS platform is pointed, based on the SPICE CK kernel. Not sure what the scanning is all about.
*



That observation is a CIRS Thermal mapping of Enceladus using FP3... The masimum phase angled is aobut 105 deg and the sub s/c lat/lon is basically 0/115 during the observation.... while ISS probably has some rider frames that's not the best geometry for viewing small forward scattering plume particles.

Posted by: jmknapp Dec 20 2005, 12:17 PM

QUOTE (David @ Dec 19 2005, 03:01 PM)
I wonder if these gaps could be accounted for by varying the initial velocity of the particles, or by assuming additional particle sources?
*


For direct fallout in the north polar area I think it would have to be the latter (additional particle sources) or lower particle elevations (the sim used 75-90 degrees). The sim already uses particle speeds in excess of the escape velocity. Jason did mention that particles from the E-ring are re-impacting Enceladus, so "escaped" I guess is a relative term. Maybe such secondary fallout accounts for the areas you mentioned? I saw an analysis of ejecta from saturnian satellites (caused by meteor impacts) where 80% of the ejecta eventually re-impacted the moon in a long-term sim.

QUOTE (Bjorn Jonsson @ Dec 19 2005, 07:07 PM)
However, I wonder if I'm overlooking something elementary when looking at the albedo map because its left edge is much darker than its right edge and the edges are at 180° longitude ??
*


I wondered about the same thing. I don't have an explanation as to why the 180 degree meridians on the left and right of the albedo map don't match up.

QUOTE (JRehling @ Dec 19 2005, 03:44 PM)
Icy particle snow-out should only correlate with the albedo features if the background albedo of Enceladus is a bit darker due to compositional differences.
*


Yeah, "dark" and Enceladus don't realy seem to go together, do they? Here's an interesting quote from the 1994 Verbiscer/Veverka paper (emphasis added):

QUOTE
The extremely high albedo of this object precludes the existence of impurities in the layer into which light penetrates (Veverka and Thomas 1986, Buratti 1988). The experimental work of Clark (1981) showed that a fractional coverage of http://en.wikipedia.org/wiki/Montmorillonite grains of frost of only 0.2% resulted in a decrease from 0.98 to 0.90 in the reflection spectrum at 0.65 um. Verbiscer et al (1990) used areal and intimate mixtures to model the surface of icy outer solar system satellites and concluded that the frost itself must be strongly backscattering, unlike any possible terrestrial analogs (Verbiscer and Veverka 1990). The low-temperature, low-gravity conditions on the surface of Enceladus may preserve intricate internal textures of frost grains which produce the remarkable backscattering behavior.

Enceladus and Mimas both have brighter trailing hemispheres, whereas the rest of the major saturnian satellites have brighter leading hemispheres. This effect may be due to the intercations between Mimas and Enceladus and Saturn's diffuse E-ring (Buratti 1988). In addition the southern regions on both satellites are slightly brighter than those at high northern lattitudes.

...

More observations of Enceladus are needed in order to characterize the properties of the surface of this icy satellite. The upcoming Cassini mission to the saturnian system will provide complete spatial and spectral coverage and resolutions will far exceed those of Voyager.


QUOTE (JRehling @ Dec 19 2005, 03:44 PM)
My second note of spoilsportdom would be the general observation that many worlds with tidal heating show patterns with 180-degree periodicity of one sort or another: Europa and Miranda are examples. The correspondence shown in your map would be less amazing if we start with the notion that what's happening on Enceladus would be southern-centered activity of 180-degree periodicity. Namely, it would not be that unlikely that a set of rift complexes would alter the albedo at 30 & 210, and that a snow-out pattern would happen by coincidence to overlie it. It's not LIKELY that it would happen, but the probability of the null hypothesis might not be down to the standard of publication.
*


Could be... would love to see a Cassini-derived albedo map to verify. In lieu of that, maybe I could map the distribution of particles differently (say, with an "isobar" type map) that would show the correspondence better. Seems like the fit in the 210W area is quite good.

Posted by: jmknapp Dec 21 2005, 12:10 PM

A couple of questions about the albedo map were raised:

QUOTE (Bjorn Jonsson @ Dec 19 2005, 07:07 PM)
Interestingly, some of the brightness variations in the albedo map seem to be simply due to a leading/trailing hemisphere asymmetry. However, I wonder if I'm overlooking something elementary when looking at the albedo map because its left edge is much darker than its right edge and the edges are at 180° longitude ??
*


and...

QUOTE (JRehling @ Dec 19 2005, 03:44 PM)
To be part of the spoilsport / peer review on another end: How meaningful is that albedo map? It looks like some dark streaks are relief features -- if ALL of the dark features are albedo features, even ones that were not well resolved, then resurfacing is not a good account of the correlation.
*


Dr. Verbiscer responds in an email, where she refers to a pseudocolor version of the map given in the http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45NK0NP-3S&_coverDate=07%2F31%2F1994&_alid=348320947&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=6821&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=65127993ca8232ccf621fb19d5b870f3:



QUOTE (Anne Verbiscer @ Dec. 20 2005, 11:14 AM)
If you look at the pseudo-color
map (fig 7) in the 1994 paper, the lower left corner is completely black.
There are no Voyager data which cover this region of Enceladus, so it was
left black.  (The caption probably should have included some indication that
this region has no data.)  If you look just north of there, the green region
matches pretty well with the green region on the right, but further north,
it is quite true that the blue/green on the left does not match the yellow
on the right.

My explanation for those mismatches comes from how the map was produced
from 3 Voyager 1 images and 6 Voyager 2 images.  Voyager 1 obtained only
low resolution views of the leading hemisphere and southern latitudes.
Voyager 2 provided the higher resolution images from the northern
latitudes of the trailing hemisphere.  If I remember correctly, the
leading hemisphere portion of the map came predominantly from a single
Voyager 1 image, centered at longitude 98 deg.  Since the entire leading
hemisphere was visible in this image, it was fairly low resolution and
more importantly, one "edge" was along longitude 180 deg.


She goes on to point out that the photometric model breaks down a bit near the limbs of images.

She continues:

QUOTE (Anne Verbiscer @ Dec. 20 2005, 11:14 AM)
The root of the problem here is the photometric model.  It was done
with virtually no near-opposition (low phase angle) data and no high
phase angle data (nothing higher than 43.5 deg).  Obviously, we have
higher phase angle data now from Cassini, but they are in the PDS
(or still the ISS team hands) awaiting the same modeling smile.gif.  [...]
Cassini will provide the high phase angle observations;
all of the low phase angle observations come from HST and last
January's ground-based observation campaign (when *true* opposition
was reached for the saturnian satellites.... if you were sitting on
on of them looking back at the Earth, you would have seen Earth transit
the Sun.)

One last comment about the albedo map... yes, some of
the dark regions do correspond to topography, as shadows get reproduced
as dark regions, not as they would appear at opposition (zero deg.
phase angle).  Cassini should help out with this problem.

Posted by: jmknapp Dec 29 2005, 02:56 AM

Possible plume sighting coming up next month--favorable geometry, except that distance is greater than the Nov. 27th observation. Here is the predicted view:



The solar phase angle will be around 156 degrees. Here's the science plan entry:

BEGIN_TIME: 190898765.184441 (2006 JAN 18 23:25:00 UTC)

END_TIME: 190900565.184442 (2006 JAN 18 23:55:00 UTC)

POINTING_AGREEMENT: S_N_ER_5

PRIMARY_POINTING: UVIS_FUV to Enceladus

REQUEST_ID: ISS_020EN_166W154PH001_PRIME

REQUEST_TITLE: Enceladus spectrophotometry/ phase coverage

REQ_DESCRIPTION: Turn -Y to Enceladus; Do 1x1 mosaic 3 clear + 4 NAC color + 9 NAC polarizer + 4 WAC polarizer filters; turn to WP

Posted by: alan Jan 20 2006, 01:05 AM

Check this out.
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS17/N00048536.jpg
Is that a curved plume or an internal reflection?

Posted by: jmknapp Jan 20 2006, 01:33 AM

QUOTE (alan @ Jan 19 2006, 09:05 PM)
Check this out.
http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS17/N00048536.jpg
Is that a curved plume or an internal reflection?
*


Wow... right out of the south pole. If real, the curvature might be used to derive the speed?




There's a wispy curved trail off to the right maybe too. The curvature looks about the same as the disk, which is possibly suspect.

Posted by: Ames Jan 20 2006, 12:59 PM



Not sure, but could be an arcing Jet. But as we have seen before, image artifacts are difficult to prove/refute unless it we have another image from a different angle/range.

Also a nice diffuse background glow is visable


Nick

Posted by: jmknapp Jan 20 2006, 01:31 PM

QUOTE (Ames @ Jan 20 2006, 08:59 AM)
Not sure, but could be an arcing Jet. But as we have seen before, image artifacts are difficult to prove/refute unless it we have another image from a different angle/range.

Also a nice diffuse background glow is visable
*


Here's an overlay of the image and a simulated Enceladus disk:




Maybe the arc is a ghost image of the crescent.

As for other images, here's one that was less overexposed, along with a histogram-stretched version:




Looks like there might be something in the general direction, although more straight in this image.

Posted by: Myran Jan 20 2006, 10:40 PM

I think they nailed it, its right at the south pole where it should be. Agreeing with jmknapp about the arc it might be one artifact, but I tend to think that the diffuse part are real outgassing.

Posted by: jmknapp Feb 3 2006, 01:08 PM

Interesting abstract for the Lunar & Planetary conference:

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1766.pdf

The abstract describes results from the Visual and infrared Mapping Spectrometer which allowed determination of the sizes of the icy particles covering Enceladus, and the distribution of same. The authors produced a graphic showing the distribution, which matches pretty well (not perfectly) with what I got for the predicted ballistic emplacement of particles from the tiger stripes. Here's their graphic along with my predicted distribution, oriented the same way:



The abstract states:

QUOTE
Our measurements show that the particle size of water ice increases toward younger regions with the largest ones in “fresh” surface material. The smallest particles were generally found in old more or less densely cratered plains and the larger ones in younger tectonically resurfaced areas (e.g. the sulci = ridged and grooved bands). The largest particles (>0.02mm) are concentrated in the so called “tiger stripes” of the south polar area. ... Our findings support the results of [1,7] with amorphous water ice being concentrated in older terrain due to the long-term exposure to incoming radiation, and crystalline water ice in the vicinity of the younger resurfaced regions, esp. the South Pole. Amorphization usually goes along with the destruction of water ice particles, resulting in the decrease of mean particle size.


But given a possible correlation between larger particle sizes and ballistic emplacement from the tiger stripes, might an alternate explanation be that the particles ejected from the tiger stripes that end up ballistically emplaced are larger, and the small particles are a more uniform dust coming from, say the e-ring? Perhaps the largest particles don't have the velocity of the smaller ones, and end up close to their source.

Posted by: volcanopele Feb 3 2006, 07:42 PM

QUOTE (jmknapp @ Feb 3 2006, 06:08 AM)
But given a possible correlation between larger particle sizes and ballistic emplacement from the tiger stripes, might an alternate explanation be that the particles ejected from the tiger stripes that end up ballistically emplaced are larger, and the small particles are a more uniform dust coming from, say the e-ring? Perhaps the largest particles don't have the velocity of the smaller ones, and end up close to their source.
*

hmm, I don't see THAT good of a correlation. The correlation with surface age appears to me to be quite robust, particularly in the other figure from that abstract, which clearly shows larger grains in Diyar Planitia and fracture belts in the anti-Saturnian hemisphere and smaller grains in the cratered regions of that hemisphere, indicating a correlation between grain size and surface age.

Can you remind us again what the colors represent in your graphic?

Posted by: jmknapp Feb 3 2006, 08:51 PM

QUOTE (volcanopele @ Feb 3 2006, 03:42 PM)
Can you remind us again what the colors represent in your graphic?
*


It codes the distribution from the leftmost three tiger stripes. For example, here's the fallout pattern from just the leftmost one:



Or all three as one color:



I suppose it would still be correlated with the younger surfaces, as the particle fallout would be a resurfacing.

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