Enceladus Plume Search, Nov. 27 Options

[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.