Enceladus Flyby Options

[tedstryk]

This might be a better example than just pointing out the poles....a side by side comparison of the terrain around each polar region.

[scalbers]

Greetings,

Thought I'd mention some speculations about a couple of items in this topic. I wonder if it would be that surprising to be able to obtain enough tidal heat to warm Enceladus? Note its orbital eccentricity of .004 and the significant tidal distortion of its equatorial radius in the X and Y directions (5-10km?). A rough guestimate of the tidal flexing within each orbit might then be 10-20m? Would this be of the same order of magnitude as the tidal flexing on Europa? On the other hand, this line of reasoning would suggest much more tidal heating of Mimas, having a more elliptical shape and greater orbital eccentricity.

Will be interesting to hear more of VP's investigation of the glare/artifact that looks like a plume. My speculation is that it would not be caused by the sun if it is well correlated by Enceladus' position. Also, given that it appears to emanate from the S polar region and not other portions of the crescent, it's difficult for me to imagine a type of glare pattern that would do this, since the crescent should have relatively uniform illumination over its area.

[volcanopele]

Will be interesting to hear more of VP's investigation of the glare/artifact that looks like a plume. My speculation is that it would not be caused by the sun if it is well correlated by Enceladus' position. Also, given that it appears to emanate from the S polar region and not other portions of the crescent, it's difficult for me to imagine a type of glare pattern that would do this, since the crescent should have relatively uniform illumination over its area.

It is not my investigation at all. But from my understanding this caused by a camera artifact, essentially a scattered light issue. This problem is very apparent on images of targets that are: overexposed and high phase. One possible example with Saturn is here:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=46019

In terms of heating, don't forget the evidence that Enceladus' orbit has migrated back and forth, interestingly across the Dione 2:1 resonance.

Artifact seen in:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=29948 [Enceladus, same position on crescent, not on surface, weaker since not overexposed)

[Gsnorgathon]

That's an interesting tidbit about Enceladus wandering back and forth through the Dione 2:1 resonance. I hadn't heard that. Do you have a link handy?

[volcanopele]

That's an interesting tidbit about Enceladus wandering back and forth through the Dione 2:1 resonance. I hadn't heard that. Do you have a link handy?

Enceladus is currently in a 2:1 resonance with Dione and the crinkled terrain along the edges of south polar terrain suggest that Enceladus' shape has changed over time, from despinning and upspinning. Such shape and spin rate changes are most likely the result of Enceladus' orbit changing, likely hovering aroung the 2:1 resonance.

[john_s]

I don't know if the remaining Enceladus flyby of the regular mission will inspect this or not -- given the geometry, I give it an odds-on "I doubt it".

Next we have to see if the north pole area also has a warm spot of any kind.

Actually, we are very lucky with the remaining Enceladus flyby on Rev. 61- we fly in over the north pole and out over the south pole, so we'll get a great look at the south pole. What's especially handy for those of us on the CIRS teams is that Enceladus enters Saturn eclipse right around closest approach, so the ISS VIMS and UVIS teams won't be able to do very much and we'll get most of the observing time. We'll certainly be revising our observing plans based on the new discoveries.

[Guest_BruceMoomaw_*]

Well, that means YOU'RE very lucky, but the same can't be said for scientific observations of Enceladus as a whole during the next flyby. Once again, several more flybys are obviously needed during the extended mission.

[john_s]

Well, that means YOU'RE very lucky, but the same can't be said for scientific observations of Enceladus as a whole during the next flyby.  Once again, several more flybys are obviously needed during the extended mission.

True enough- we are the beneficiaries of what's an unfortunate circumstance for the overly flyby science. The fact that Enceladus was going to be entering eclipse near closest approach was only discovered after this flyby had already been scheduled. The other remote sensing instruments will get great data on the north pole on approach, but of course that's not where the action is. We all hope there will be additional Enceladus flybys in the extended mission, and these new discoveries will make that a high priority.

[scalbers]

I shamelessly stole Steve Albers' cylindrical projection of Enceladus (thanks!) and made this polar projection of the southern hemisphere (just using the polar coordinates function in Photoshop):

[attachment=930:attachment]

Incidentally, the saturnshine images from the last imaging sequence on this orbit show new territory west of Voyager 2 coverage which could be added in to the northern hemisphere of the map.
Phil

Thought I'd mention that I've updated my Enceladus map again today, now having several more of the detailed south polar images.

[Gsnorgathon]

Don't be so shy, Steve! Put in the link!

[Guest_BruceMoomaw_*]

I talked this afternoon with Dr. Linda Spilker -- and it turns out that the puzzle
of that quote from her in the Planetary Society article (
http://www.planetary.org/news/2005/encelad...ctive_0730.html )...

"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."

...is explained by the simple fact that Emily Lacdawalla misquoted her, and
in fact apparently made the last sentence up out of thin air. (Upon being
informed of what she was being quoted as saying, Spilker promised to give
Emily a Talking To.) So here's the real story:

What she actually told Lackdawalla is that the E Ring particles, as analyzed by Cassini's Cosmic Dust Analyzer, are basically just water ice, while the stuff that Cassini detected at Enceladus' south pole is water VAPOR. Period. End of story. No meaningful compositional difference to indicate that the E Ring particles are not coming originally from Enceladus' southern vents -- in fact, she agreed that it's very hard to explain the E Ring particles as having been knocked off Enceladus by meteoroid impacts, for the simple reason that in that case we'd see comparable rings centered around the orbits of Saturn's other icy moons.

When I suggested the following model of what is actually going on, she also
agreed with it: Water/ammonia mixture, with a little methane mixed into it (as also detected by Cassini), is spurting from vents near Enceladus' south pole -- indeed, the vaporizing methane is probably largely responsible for driving the venting. (The vents are probably those little dark speckles, near the cracks, that look like Enceladan blackheads -- at any rate, http://www.aas.org/publications/baas/v37n3/dps2005/450.htm says so.
Presumably their blackness results from the fact that a small amount of the
methane has been locally radiolyzed into dark carbon compounds, just as on
Pluto.)

The NH3 vapor is almost immediately broken down by solar UV into nitrogen
and hydrogen, thus explaining both the lack of NH3 in Cassini's gas analyses
and the surprisingly large amount of molecular hydrogen in it. This also
means that most of the 28 AMU gas detected by Cassini is indeed nitrogen
rather than carbon monoxide. (It turns out that Cassini's UVIS can measure both
CO and atomic N --http://www.aas.org/publications/baas/v37n3/dps2005/452.htm -- but, like a
fool, I forgot to ask Spilker about its findings. However, we already know
that most of the atomic N in Saturn's magnetosphere is concentrated around the E
Ring, with very little coming from Titan's atmosphere.)

The H2O and CH4 are broken down by solar UV a lot more slowly, although
Spilker said that Cassini did indeed detect a large amount of OH in the
cloud from water breakdown. (The carbon released from the decomposing methane -- or, alternatively, carbon monixide also released from Enceladus' subsurface -- may react with the atomic oxygen to produce the carbon dioxide gas detected by Cassini, which is rather hard to explain otherwise given the low temperatures.) However, a lot of the water vapor spreads all the way around Enceladus -- and, in fact, out into the E Ring -- without breaking down, and then refreezes as those tiny particles of ice in the Ring, which are relatively even in size (unlike the power-law distribution of various-sized fragments of material in all the other rings, including the F and G Rings). Spilker actually dropped another tidbit backing this up: the CDA team has seen what may be an asymmetry in the distribution of water-ice particles around Enceladus, but are still trying to decide whether it's statistically significant.

A lot of that refreezing water vapor also forms the pure-water, fine-particle "snow" that seems to coat all of Enceladus evenly, including the parts that haven't ever had surface geological activity. (And is it possible that those big "boulders" that so surprised everyone in the very high-resolution photo of the active region are actually "snowballs", made of water-ice particles that are clumping together in the immediate vicinity of the vents -- maybe because of influence by the local high concentration of ammonia vapor?)

A neat picture -- although, of course, it does nothing to answer the billion-dollar questions: what's driving the heating that causes the vents, and just how much liquid water/ammonia mixture is there in the subsurface underneath them? There is, by the way, one hell of a lot of other interesting new stuff from Cassini/Huygens in the new DPS abstracts, brief as they are. More on those other items later.

[Guest_Richard Trigaux_*]

(The vents are probably those little dark speckles, near the cracks, that look like Enceladan blackheads -- at any rate, http://www.aas.org/publications/baas/v37n3/dps2005/450.htm says so.

Possible evidence for past cryovolcanism includes kilometer-scale ridges and linear arrays of rounded domes that appear to have extruded through preexisting surface fractures. Some wrinkled, flow-like features with lobate margins are found near the ridge and dome features, but it is unclear if they are volcanic flows or tectonically folded grooved-terrain. ...  Among the most mysterious newly-discovered features are small, sub-kilometer-sized dark spots and circular pits that sometimes cluster in a honeycomb like patterns near faults and scarps. Their origin is unknown, but perhaps the pits and dark spots identify sites of explosive venting of subsurface volatiles through fractures or volcanic conduits.

Has someone pictures of these features? These pictures must exist somewhere, otherwise there would be no comments about them. Thanks.

[Guest_BruceMoomaw_*]

It occurs to me that what I said about Emily Lackdawalla was very tactlessly phrased -- but, unfortunately, it remains true. Linda Spilker DID say that Ms. Lackdawalla completely misinterpreted what Spilker had told her, and she did say that she hadn't said anything remotely suggesting that the E Ring particles definitely did not come from the vents. She did say that the science teams themselves are still not entirely certain on that point -- but she added (as other members of this group have also said) that in that case it's very hard to see why Saturn's other moons aren't also producing comparable numbers of E Ring particles.

As for the Enceladan "blackheads", they turn up on a very large number of photos in all three close flybys -- they jumped out at viewers immediately even during the first, "untargeted" close flyby in February. Here's a few examples:

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=45675

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=45662

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34984

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=34976

[David]

It occurs to me that what I said about Emily Lackdawalla was very tactlessly phrased

Emily Lakdawalla (not Lackdawalla or Lacdawalla) recently joined this list. It would be a shame if she were to decide to leave it due to a failure of tact. While I appreciate the clarification of Spilker's comments, I think that criticisms or disputes between named individuals, unrelated to the facts themselves, might better be handled privately.

[Guest_BruceMoomaw_*]

Ms. Lakdawalla has indeed -- understandably -- hit the ceiling about my accusation. Another conversation I've just had with Dr. Spilker confirms that the key erroneous statement "So the vents are not the source of the E Ring" was indeed made to Ms. Lakdawalla, in just those words, by Dr. Spilker herself, because it was the tentative very early conclusion of the Cassini science team themselves (which they have now completely changed their minds about, on what would seem to be the obvious grounds that Saturn's other moons aren't throwing off E Rings of their own).

So Ms. Lakdawalla concocted exactly nothing; the error was entirely one by her source. My profound apologies to her. (Also, my embarrassed apologies for managing to misspell her name twice in a row. You'd think that someone whose name is misspelled as often as mine is would be a bit more sensitive to that point as well.)