[X] My Assistant

[Guest_AlexBlackwell_*]

From the December 15, 2006, issue of Science:

A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume
Susan W. Kieffer, Xinli Lu, Craig M. Bethke, John R. Spencer, Stephen Marshak, and Alexandra Navrotsky
Science 314, 1764-1766 (2006)
Abstract
Supporting Online Material

See also the accompanying News of the Week article "A Dry View of Enceladus Puts a Damper on Chances for Life There" by Richard Kerr.

EDIT: See also the related Space.com story.

2nd EDIT: See "Scientists propose alternate model for plume on Enceladus."

This post has been edited by AlexBlackwell: Dec 14 2006, 07:19 PM

[Guest_AlexBlackwell_*]

The artilces are now downloadable from the Science website.

[Guest_AlexBlackwell_*]

From the December 15, 2006, issue of Science:

A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume
Susan W. Kieffer, Xinli Lu, Craig M. Bethke, John R. Spencer, Stephen Marshak, and Alexandra Navrotsky
Science 314, 1764-1766 (2006)
Abstract
Supporting Online Material

I like the concluding sentences from the paper:

As an alternative to the shallow boiling water 'Cold Faithful' model [Porco et al., 2006], we propose that the south pole of Enceladus is a colder world with a 'Frigid Faithful' plume emanating from degassing clathrates. This model accounts in a simple and unified way for the gas composition of the plume and the variability of fluxes over space and time. It provides a plausible advective heat transfer process as heat absorbed as latent heat of decomposition of clathrate is redeposited near the surface as latent heat of condensation of ice.

"Frigid Faithful"? I like it

[Littlebit]

I am a little confused by the motivation here: They say it is premature to send a mission to Enceladus searching for life, but I don't see that they have offered any proof that the Porco model is wrong. Any mission launched with the intent of resolving the difference should likely be equipped with some way to look for life.

In any case, the issue may be resolved by Cassini, which will be passing within 30km of Enceladus: If the temperature gradient at the vents is approaches 0C, we should be able to detect it.

[Guest_AlexBlackwell_*]

I am a little confused by the motivation here: They say it is premature to send a mission to Enceladus searching for life, but I don't see that they have offered any proof that the Porco model is wrong.

That's simple: Kieffer et al. do not claim to have disproved Porco et al. What the former offer in their paper is an alternative model (part of the multiple working hypotheses concept at the heart of the scientific method), which fits with the observables. And as long as there is a plausible model still standing that does not have as a sidelight the possibility of life on Enceladus, then yes, it is indeed premature to spend the billions of dollars (or euros) to send a life-detection mission there.

Does anyone happen to remember the obscure Viking biology experiments?

This post has been edited by AlexBlackwell: Dec 21 2006, 06:34 PM

[qraal]

Hi All

Sorry for a highly prejudicial view, but I think looking for life in Enceladus is dumb. Or rather it's ridiculously unlikely - the energy sources are too weak to sustain a biosphere. At best a bit of prebiotic chemistry, but Titan has scads of that so it's a better target by far. The clathrate hypothesis means the hype is utterly unjustified.

Adam

[Guest_AlexBlackwell_*]

Matson et al. have an interesting paper in press with Icarus:

Enceladus' plume: Compositional evidence for a hot interior
Dennis L. Matson, Julie C. Castillo, Jonathan Lunine and Torrence V. Johnson
Icarus, In Press, Corrected Proof, Available online 18 December 2006
Abstract

[volcanopele]

Thanks for posting the link, Alex. I wanted to wait to comment on the Kieffer et al. paper until after that article was posted on the web. The Matson et al. article brings up I think the chief arguments for a liquid water origin of the Enceladus plumes as opposed to a clathrate origin. The key to the plume's origin is in the minor components: ammonia, propane, and acetylene. These minor components strongly suggest that the major components (water, CO2, methane, and nitrogen) have been thermally altered before they were ejected into space. Matson et al. suggest that the nitrogen in the plume originated as ammonia. At some point in Enceladus' history, the suspected water/ammonia mixture of the lower layers of the ice mantle percolated through cracks in Enceladus' rocky core, and the ammonia broke down into nitrogen. The water/ammonia mixture also would have contained CO and CO2, which when combined with water and the molecular hydrogen released from the ammonia, would have created methane. The presence of higher order hydrocarbons strongly suggest catalytic reactions, again supporting interaction between a liquid water lower mantle and hot, rocky core.

All this work again suggests that the origin of the material in Enceladus' is in a liquid lower mantle, not from clathrates in the upper mantle, as suggested by Kieffer et al.

[Gsnorgathon]

Thanks for the explanation, Jason. Those of us up here in the peanut gallery appreciate it.

I have to admit, phrases like "hot, rocky core" in conjunction with Enceladus still set up a bit cognitive dissonance, geysers notwithstanding.

[remcook]

One question I have about the Matson et al. paper: what happened to the CO? Does it not come out together with the water, or is all the CO within the production region converted to CH4?

[Guest_AlexBlackwell_*]

One question I have about the Matson et al. paper: what happened to the CO? Does it not come out together with the water, or is all the CO within the production region converted to CH4?

I finally had a chance to read the relatively short paper (5 pages).

Matson et al. reiterate the INMS data reporting "4 ±1% N2 or CO" as well as noting that "[o]bservations from UVIS, VIMS place significant limits on any CO gas in the plumes." And the authors also note that "[t]he conditions required to form molecular nitrogen are also favorable for the Fischer–Tropsch synthesis of methane from carbon monoxide." They give a well-known reaction: CO + 3H2 -> H2O + CH4

[Guest_AlexBlackwell_*]

All this work again suggests that the origin of the material in Enceladus' is in a liquid lower mantle, not from clathrates in the upper mantle, as suggested by Kieffer et al.

Though I not an expert in this area, I have to admit the Kieffer et al. model is attractive, perhaps because it is simplistic. Having said that, maybe their model is too simplistic. As you noted, Matson et al. raise some good points, and I could well imagine that in the absence of the INMS, CAPS, and UVIS data on the plumes, one could be tempted to grab hold of the clathrate model. Many an elegant model has been slayed by those pesky little observables

[Guest_AlexBlackwell_*]

No sooner do I revive this thread with the two items above, then I notice Emily's latest blog entry. Now that's timing!

[nprev]

Any new thoughts in the planetary science community regarding just how Enceladus' core could get (and stay) hot?

Last I heard, the moon's tidal 'squeezing' from Dione didn't seem to provide enough energy, which is presumably why alternate plume generation mechanisms like the clathrate model that require less energy input are under active consideration...

[Guest_AlexBlackwell_*]

Any new thoughts in the planetary science community regarding just how Enceladus' core could get (and stay) hot?

Last I heard, the moon's tidal 'squeezing' from Dione didn't seem to provide enough energy, which is presumably why alternate plume generation mechanisms like the clathrate model that require less energy input are under active consideration...

I wouldn't say that tidal heating of Enceladus to account for the observed high temperatures at its south pole has been discounted. In fact, even Spencer et al. [2006] state: "It is also possible that Enceladus is in an oscillatory state, as has been proposed for Io and Europa [reference omitted]. In that case, its eccentricity and tidal heating rate may have recently been much higher, and perhaps the moon is still cooling down from that period."