There was a news briefing at AGU this afternoon covering the results of the Enceladus flybys this fall:

Saturn's Dynamic Moon Enceladus Shows More Signs of Activity
http://saturn.jpl.nasa.gov/news/press-rele....cfm?newsID=889

The imaging team has released several image products showcasing the results from the encounter:
http://ciclops.org/view_event/98/Enceladus_Shifting_Terrain

I do hope you all enjoy the two large mosaics that accompany this image release:

Tiger Stripes...Magnified!
http://ciclops.org/view/5409/Tiger_StripesMagnified
http://photojournal.jpl.nasa.gov/catalog/PIA11135

A Tectonic Feast
http://ciclops.org/view/5310/A_Tectonic_Feast
http://photojournal.jpl.nasa.gov/catalog/PIA11133

I guess the big results from ISS is the "new" theory that the tiger stripes are analogous to the mid-ocean ridges on earth, spreading centers that act as axes where new terrain is produced.

Absolutely beautiful .... not just the imaging, but the analysis...

"Enceladus has Earth-like spreading of the icy crust, but with an exotic difference -- the spreading is almost all in one direction, like a conveyor belt," said panelist Paul Helfenstein, Cassini imaging associate at Cornell University in Ithaca, N.Y. "Asymmetric spreading like this is unusual on Earth and not well understood."

Craig

If Enceladus' crust is actively spreading, then either 1) the entire moon is expanding, or 2) it has subduction zones. Where would any of y'all think the subduction zones are located?

Also, doesn't subduction and surface spreading absolutely require a soft mantle upon which the crust floats? Where is all the heat coming from to keep Enceladus everything from molten (at the core, and by that I mean liquid water) to very elastic in the mantle?

Suppositions, anyone? (For myself, I wonder if there's any way that Enceladus could have been impacted by an extrasolar AL26 mass sometime within the last several thousand years, which could be powering a short-term period of activity within the moon...)

-the other Doug

The spreading is likely compensated via compression along the margin of the south polar terrain.

dvandorn
Where is all the heat coming from to keep Enceladus everything from molten (at the core, and by that I mean liquid water) to very elastic in the mantle?

There is an article in the current Natue by Roberh H. Tyler that may explain it. The article is mainly about Europa, but describes a tidal force that may also work for Enceladus. I don't have the expertise to evaluate the paper.

Abstract link of Nature article.
Data from recent space missions have added strong support for the idea that there are liquid oceans on several moons of the outer planets, with Jupiter's moon Europa having received the most attention^. But given the extremely cold surface temperatures and meagre radiogenic heat sources of these moons, it is still unclear how these oceans remain liquid. The prevailing conjecture is that these oceans are heated by tidal forces that flex the solid moon (rock plus ice) during its eccentric orbit, and that this heat entering the ocean does not rapidly escape because of the insulating layer of ice over the ocean surface. Here, however, I describe strong tidal dissipation (and heating) in the liquid oceans; I show that a subdominant and previously unconsidered tidal force due to obliquity (axial tilt of the moon with respect to its orbital plane) has the right form and frequency to resonantly excite large-amplitude Rossby waves in these oceans. In the specific case of Europa, the minimum kinetic energy of the flow associated with this resonance (7.3 10¹⁸ J) is two thousand times larger than that of the flow excited by the dominant tidal forces, and dissipation of this energy seems large enough to be a primary ocean heat source.

Yes, that paper was out a while ago and neatly explains a heat source needed to power Enceladus. But coming back to tectonics; the fact that there is no evident subduction on Enceladus reminds me about Antarctica. Surrounded by ridges but no subductive zones, analogous? Volcanopele's suggestion above is plausible but how exactly does compression compensate for spreading?

Exactly, Doc. There are limits to the compressibility of ice. Compression can compensate over a short term -- hundreds of years, maybe. But if this crustal spreading has been going on for millions of years, unless it's dramatically slower than it appears to be, I'd have to think that you'd quickly pass the limit at which compression would compensate for the spreading. You'd either have to be raising enormous ice mountains somewhere, or you'd have to have subduction going on somewhere. Neither of which is apparent in the imaging.

-the other Doug

How about compression with fracturing with ice moving up and down--the overall thickness of the ice sheet increases and eventually piles up at the circum polar range?

"how exactly does compression compensate for spreading?"
"There are limits to the compressibility of ice."

No no no... crustal compression means folding or thrust faulting, not 'compressibility of ice'.

Phil

Tectonic compression,
a blue-white impression

ice crust faultin'
ridges are vaultin'

mountains are thrusting
snowfall is dusting

viscous relaxation,
what a sensation

putting crust down deep
in the ocean to sleep

homeboyz are rootin
the skeeter's been shootin'

Liquid H 2 the O?
or just gas down below?

Please enlighten us Phil. To tell you the truth the whole thing looks muddled up to me. As the team announced, "the spreading is there, but we don't know the geology." But we cannot rule out the material properties factor i.e how an icy crust behaves with tectonics.

I think the Ganymede grooved terrain was predicted to be due to tectonic faulting.

The groove pattern is set up as a series of harmonic waves, with smaller subharmonics inside the bigger harmonics. And the whole thing is faulted up as a series of overlapping thrust faults. The harmonic wavelength depending on the temperature gradient of the icy crust and the strain rate.

See: Collins, Head, and Pappalardo Geophys Research Lett. 25 (1998) 233-236. "The role of extensional instability in making Ganymede's grooved terrain: insights from Galileo high-resolution stereo imaging."
Totally available for free here: http://icuc.wheatonma.edu/~gcollins/papers...al_GRL_1998.pdf

An even more in-depth discussion is in: Bland and Showman Icarus 189 (2007) 439-456. The formation of Ganymede's grooved terrain: Numerical modeling of extensional necking instabilities". doi: 10.1016/j.icarus.2007.01.012
(Pay for article: abstract here)


And finally, Bland and Showman applied this to Enceladus:
Bland et al. Icarus 192 (2007) 92-105. "Unstable extension of Enceladus' lithosphere". doi: 10.1016/j.icarus.2007.06.011
Totally available for free here: http://www.lpl.arizona.edu/~showman/public...d-etal-2007.pdf

So I would imagine that the spreading centers could activate the thrust faults and just accordion up the terrain.

-Mike

Q. Does the surface ice become amorphous due to the exposure to solar radiation? If so, wouldn't that help to perpetuate tectonics by releasing heat as it subducts and crystallizes under pressure?

There is no subduction zone as far as the images are concerned (yet).

But even if the 'accordion' terrain doesn't slide below another plate, it must sink in place under its own weight. As new ice is pushed and folded, the lower layer goes down and down and either the liquid mantle melts it or it crystallizes. In Earth it happens when two continental masses too buoyant to subduct collide.

I would expect the ice to melt in the relatively warmer ocean below (if it does subduct anyway). And the wheels of tectonics goes round & round

I'm really not sure but...

At the center of the plate, the ice plate should be the thickest. To maintain isostasy, any amount pushed up would need to be counteracted by material underneath. (Like an iceberg).

The difference (I think) with terrestrial tectonics is that water becomes more dense as it melts. So if you pile up material in the center of an ice plate (by folding or mushing together for example), the thickest part of the base will begin to melt and would diffuse away.

So conceptually, (I'm making this up, I have no literature references) there is a limit how much thicker a floating ice plate could become in relation to the rest of the crust. If it becomes too thick, the base should melt (it would be in a different thermal layer) and diffuse into the ocean. (I suppose the phase change would also suck up energy and cause local cooling as well?)

So I think there is a limit as to how thick a floating ice plate can get. (I think it also means an Olympus Mons-type cryovolcanic construct on floating crust is also not stable, it'll eventually get pulled back down by having the roots melt away)

-Mike

[EDIT: Ha! Doc beat me to it!]

Bingo Mike! I think you can call that subduction on an icy moon. Asymmetrical ridge pushing folded ice along and how do you recycle the piled up ice; let the ocean below melt it(as stated by mike above). This alone can allow stable tectonics to thrive on Enceladus (a good hunch though)

When I finally figured out (with Phil's appreciated help) that Jason was speaking of tectonic compression features, I felt sorta stupid. Well, we all have days like that...

I guess one of the $64,000 questions here is the length of time we think this particular era of crustal spreading has been going on. And to have any real idea of the amount of new crust formed in a given timeframe, we need to know how fast the spreading is taking place. I think we may lack the data to confidently answer either of those questions.

However, you can toss assumptions into the equations, and get ideas of various ranges. That lets you put together a variety of models, which you can then figure out how to test.

If compressional forces are piling up a very thick mass of hard ice in a given region, and yet we don't seem to see constructional forms (i.e., mountain ranges or big, obvious bulges), we've got to conclude that the thickest part of the pile is sinking into an elastic mantle. It may not be subducting as a plate under another plate, but if it's being recycled into the mantle, it's at least a subduction-like process.

I'm not sure that it's actually melting into the mantle. I think we may have a somewhat unique "warm ice" process going on at several places in the mantle, where the material is solid but has elastic properties. I have no idea when or how the phase change from either amorphous or crystalline water ice to this warm ice with elastic properties takes place -- I'm not even sure the physics of that proposed state of ice are all that well thought-out right now. (Obviously, if anyone has any references to the current thinking on the subject, I'd love to see them... )

But I think we need to know more about properties of the Enceladan mantle that underlies such subduction-like processes before we can model what's happening, how long it's been happening, and how fast it's happening. And as much as I'd enjoy getting enough information to answer these questions, I'm not positive that Cassini is capable of gathering enough of the right kind of data to do so.

-the other Doug

Perhaps I should simply study the video more and such, though it might also be nice to see a narrated version that discusses more of the features that match up.