Enceladus @ AGU Options

[volcanopele]

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.

[belleraphon1]

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

[dvandorn]

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

[volcanopele]

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

[Floyd]

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.

[Doc]

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?

[dvandorn]

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

[Floyd]

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?

[Phil Stooke]

"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

[Juramike]

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?

[Doc]

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.

[Juramike]

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

[Fran Ontanaya]

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?

[Doc]

....as it subducts and crystallizes....

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

[Fran Ontanaya]

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.