Enceladus Plume Search, Nov. 27 Options

[jmknapp]

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

[jmknapp]

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.

[ElkGroveDan]

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

Looks like they need to use the serendipity filter.

[dilo]

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

[jmknapp]

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:

[mgrodzki]

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

[dilo]

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.p...findpost&p=5657

[Decepticon]

Images Up.



http://saturn1.jpl.nasa.gov/multimedia/ima...heQ=0&storedQ=0

[SFJCody]

http://saturn1.jpl.nasa.gov/multimedia/ima...6/N00043446.jpg

One plume per stripe?

[alan]

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

[ugordan]

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? ) 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!



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

[Bjorn Jonsson]

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.

[tedstryk]

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.

[Decepticon]

Is there enough pics for a animation?

[Bjorn Jonsson]

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...

[jmknapp]

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:



link to raw image

[Mariner9]

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.

[Bill Harris]

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

[jmknapp]

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

[ugordan]

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.

[ugordan]

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!

[JRehling]

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?

[jmknapp]

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?

[ugordan]

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.

[tasp]

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!

[Orlin Denkov]

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

[volcanopele]

^^ Fixed

[jmknapp]

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.

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!

[The Messenger]

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.

[tedstryk]

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.

[jmknapp]

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



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.

[tasp]

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

[jmknapp]

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



raw image

[canis_minor]

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.

[Bill Harris]

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

[scalbers]

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/enc..._cyl_www_P4.jpg

[The Messenger]

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



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?

[Bjorn Jonsson]

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/imag...eiImageID=54839

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=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'.

Attached thumbnail(s)

 

[dilo]

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! )...
Anyway, I tried to code with colors the huge dinamic range covered by 3 images taken with increasing exposure time (N00043435/36/37):

Attached thumbnail(s)

 

[volcanopele]

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.

[jmknapp]

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:

[mars loon]

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

[Guest_Sunspot_*]

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

[mars loon]

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

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.

[akuo]

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?

[David]

Looks like they need to use the serendipity filter.

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

Maybe they can name Enceladus' south pole "Serendip"?

[EccentricAnomaly]

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/imag...cfm?imageID=808 for a depiction of the E-ring.

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?

[edstrick]

Regarding the plume images: Feb 20, I posted:
http://www.unmannedspaceflight.com/index.p...topic=691&st=45

"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....?

[ugordan]

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

[Guest_BruceMoomaw_*]

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

[edstrick]

(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.

[Gsnorgathon]

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.

[edstrick]

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.

[ugordan]

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.

[jmknapp]

We're set for a Christmas rerun:



The above is oriented with north pointing straight up.

[ugordan]

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...

[jmknapp]

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:
N00043446.jpg
N00043428.jpg

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.

[The Messenger]

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.

[Omega]

Regarding possible artifact--

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.

[volcanopele]

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.

[jmknapp]

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?

[scalbers]

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?

[Rob Pinnegar]

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?

[Marz]

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?

[JRehling]

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).

[tedstryk]

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.

[jmknapp]

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.

[Guest_BruceMoomaw_*]

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?&...t;P32A-04"

http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P32A-05"

[tedstryk]

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.

[The Messenger]

...
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?&...t;P32A-04"

http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;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.

[edstrick]

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.

[edstrick]

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...)

[Webscientist]

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

[Bill Harris]

> from the Utter Loonacy Department...

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

<VBSEG>

--Bill

[jmknapp]

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.

[ugordan]

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

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

[jmknapp]

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

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):

NAC image of south polar region

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_En...ripes_Spew.html

[Guest_BruceMoomaw_*]

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.

[mars loon]

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

[The Messenger]

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.

[ugordan]

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?

[deglr6328]

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.

[edstrick]

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.

[jmknapp]

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_En...ripes_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?

[jmknapp]

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:

[dvandorn]

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

[jmknapp]

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.

[ugordan]

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.

[dvandorn]

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...

-the other Doug

[jmknapp]

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... 

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?

[David]

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?

[ugordan]

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?

[Bill Harris]

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

[tasp]

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?

[ugordan]

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.

[dvandorn]

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

[jmknapp]

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/20051...eladus_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 web calculator 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."

[ugordan]

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/20051...eladus_spa.html

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

Based on a web calculator 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.

[jmknapp]

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...").

[ugordan]

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.