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Enceladus Plume Search, Nov. 27
David
post Dec 1 2005, 02:47 PM
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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?
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ugordan
post Dec 1 2005, 02:55 PM
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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?


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Bill Harris
post Dec 1 2005, 04:09 PM
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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


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tasp
post Dec 1 2005, 05:01 PM
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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?
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ugordan
post Dec 2 2005, 08:10 AM
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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.


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dvandorn
post Dec 2 2005, 08:14 AM
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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


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jmknapp
post Dec 2 2005, 11:18 AM
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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."


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ugordan
post Dec 2 2005, 11:50 AM
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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/20051...eladus_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 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.


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jmknapp
post Dec 2 2005, 11:57 AM
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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...").


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ugordan
post Dec 2 2005, 12:15 PM
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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.


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Guest_BruceMoomaw_*
post Dec 2 2005, 12:19 PM
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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.
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jmknapp
post Dec 2 2005, 01:21 PM
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Regarding Enceladus' density, Wikipedia quotes the current Cassini-derived estimate of 1.61 g/cm^3.

Conversely, in 1994 an article was published in Icarus 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.


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Bill Harris
post Dec 2 2005, 02:11 PM
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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


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volcanopele
post Dec 2 2005, 06:39 PM
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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, the water ice could be heated from below enough to allow for enhanced sublimation, which would carry along with it entrained particles. 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 there is a pressurized chamber of liquid water which is connected to the surface via fractures, to the tiger stripes. 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/prod...RM_Esposito.pdf (transcript of the talk can be found at http://saturn.jpl.nasa.gov/multimedia/prod..._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.


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jmknapp
post Dec 2 2005, 08:20 PM
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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: Enceladus plume location


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