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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."
AlexBlackwell
The artilces are now downloadable from the Science website.
AlexBlackwell
QUOTE (AlexBlackwell @ Dec 14 2006, 09:00 AM) *
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:

QUOTE
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 biggrin.gif
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.
AlexBlackwell
QUOTE (Littlebit @ Dec 15 2006, 04:41 AM) *
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?
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
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?
AlexBlackwell
QUOTE (remcook @ Dec 19 2006, 12:30 AM) *
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
AlexBlackwell
QUOTE (volcanopele @ Dec 18 2006, 07:12 PM) *
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 cool.gif
AlexBlackwell
laugh.gif 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...
AlexBlackwell
QUOTE (nprev @ Dec 21 2006, 08:43 AM) *
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."
volcanopele
The current thinking is that Saturn's moons formed relatively quickly, allowing short-lived radionuclides like Al-26 and Fe-60 to produce greater short term heating when a moon like Enceladus than it normally would have. For Enceladus, with a greater percentage of rock than any of the other moons except Titan, this would have allowed for the melting of the interior. While the resonance with Dione would not have generated enough heat to heat up the interior by itself, it would be enough to prevent Enceladus from freezing.
AlexBlackwell
QUOTE (volcanopele @ Dec 21 2006, 09:33 AM) *
The current thinking is that Saturn's moons formed relatively quickly, allowing short-lived radionuclides like Al-26 and Fe-60 to produce greater short term heating when a moon like Enceladus than it normally would have. For Enceladus, with a greater percentage of rock than any of the other moons except Titan, this would have allowed for the melting of the interior. While the resonance with Dione would not have generated enough heat to heat up the interior by itself, it would be enough to prevent Enceladus from freezing.

And, I believe, added constituents such NH3, CH3OH, H2SO4, etc. could help depress melting points.
nprev
QUOTE (AlexBlackwell @ Dec 21 2006, 11:44 AM) *
And, I believe, added constituents such NH3, CH3OH, H2SO4, etc. could help depress melting points.

Particularly the NH3, I'd imagine.

What other supporting evidence could there be for rapid moon formation, and why would the Saturn system have formed more rapidly than those of the other gas giants? Offhand, I'd say that the small size of most of Saturn's moons is a good argument for rapid accretion, but then again you could also argue that Titan swept up most of the available material & starved its siblings.

In any case, localized radioisotope enrichment just seems like a bit of a stretch to me (no offense to anyone intended), and it's odd that Enceladus should apparently be the sole recipient of this bounty. Surely there must be a simpler possible mechanism.

What about collision scenarios? Is it possible that Enceladus is the ice-shrouded core of a larger body that was disrupted (with attendant residual heat), part of which went on to form the ring system?
AlexBlackwell
QUOTE (AlexBlackwell @ Dec 21 2006, 08:29 AM) *
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 cool.gif

I should clarify my somewhat awkard wording. I find the Kieffer et al. model attractive and elegantly simple, not simplistic. However, the point that Matson et al. make about the production of C2H2, C3H8, etc. is cogent.
AlexBlackwell
QUOTE (AlexBlackwell @ Dec 21 2006, 09:32 AM) *
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."

As Spencer et al. note in an LPSC abstract earlier this year, one of the drawbacks to a tidal heating cause is that nearby Mimas, which has a greater orbital eccentricity, does not display any apparent internal activity. However, I found an interesting paper/preprint on the web by Varga, (1.7 Mb MS Word document), which notes at the end:

QUOTE
The tidal heating can warm up and melt the inner part of the Enceladus at the depth interval 15-160 km. This way, the fountain-like plumes detected by the Cassini mission can be explained. In case of reduced of reduced inhomogenety (expressed in lower mean density values), the tide generated heat moves to the deeper parts of the moon. Possibly that is the reason why in case of other moons of Saturn there is no similar volcanic activity observed.

Tidal friction due to irregular axial rotation along eccentric orbit can be an another source of tidal heating, which needs further investigations.
AlexBlackwell
For the sake of completeness, I thought I would post a link to another, older Enceladus-related thread, which mentions another important paper published earlier this year in Nature.
AlexBlackwell
For those with access to Icarus, here's a newly posted paper in press:

Enceladus: Present Internal Structure and Differentiation by Early and Long Term Radiogenic Heating
Icarus, In Press, Accepted Manuscript, Available online 8 January 2007
Gerald Schubert, John D. Anderson, Bryan J. Travis and Jennifer Palguta
PDF
AlexBlackwell
In the January 25, 2007, issue of Nature, our own John Spencer and co-author David Grinspoon have a News and Views piece:

Planetary science: Inside Enceladus
John Spencer and David Grinspoon
Nature 445, 376-377 (2007)
Full text

For those without access, Spencer and Grinspoon discuss the Matson et al. paper in press with Icarus, which was mentioned earlier in this thread.
AlexBlackwell
There is a new, interesting paper in press with Icarus:

Enceladus's South Polar Sea
Icarus, In Press, Accepted Manuscript, Available online 22 February 2007
Geoffrey C. Collins and Jason C. Goodman
PDF (976 K)

For those without access to Icarus, I believe you can find an early version preprint (at least the manuscript that was submitted) here.
martin peters
I would like to discuss an idea about how the tiger stripes of Enceladus are heated. The process is independent of the interior, so the interior need not be warm/hot and it need not contain clathrates. Because this process takes place entirely in the plume/atmosphere and on the surface (no deeper than 100 m below the surface, let’s say), it is really not at all related to clathrates or volcanoes and its relationship to geysers is tenuous at best. It is an example of what we might call ‘cold-interior’ models for Enceladus. I would appreciate someone letting me know if this topic would be of interest to this group, and, if so, whether it would be appropriate to continue here, on another thread, or on a new thread.

Thank you,
martin
ngunn
Welcome martin. I'd like to hear it.
martin peters
The process is a cycle in which 1) a gaseous mixture captures solar energy in photochemical reactions in the illuminated plume and atmosphere, 2) some or all of these reaction products condense on the surface as liquids, 3) liquid materials, perhaps carrying particulate matter along with them, flow down to (or percolate toward) the tiger-stripes region, 4) these materials, as they reach the topographic lows of the tiger stripes, react and/or decompose, releasing their stored energy and producing gaseous and particulate materials, and 5) the chemical reaction/decomposition process is energetic enough to propel materials upward into the observed plume (replenishing those species depleted in the first step).

Although this is a solar hypothesis, an essential element is that there is a mechanism that collects solar energy that is incident on a region of Enceladus many times bigger than that of the tiger stripes. Clearly, the local sunlight falling on a surface unit in the tiger-stripes region can under no circumstances raise the temperature anywhere near as high as observed. Energy is concentrated because an energy-carrying liquid flows toward the central topographic low of the tiger stripes.

One of the main concerns is whether there is enough sunlight for this process to work. For the purpose of estimating, I have chosen to consider only the energy at wavelengths shorter than about 650 nm, because I doubt that photons of longer wavelength could drive the photochemical reactions needed for this hypothesis (I might be wrong about that). Total energy in sunlight incident on Enceladus is about 1500 GW at wavelengths below 650 nm, so it would be necessary for 0.4% of that total to be captured and transferred to the tiger stripes. Much more needs to be said about this, but in this initial description of the model, let me just say that after reading a photometric analysis of Hubble Space Telescope observations of Enceladus, I feel fairly confident we will not be able to rule out the hypothesis with the argument ‘the total energy captured by Enceladus in the spectral range that drives one or more relevant photochemical reactions is less than the 6 GW of thermal energy observed to be radiating from the tiger stripes.'

A second concern of this hypothesis, in my opinion the most likely thing to cause its downfall, is about the existence of substances on Enceladus that have all the needed properties. For the overland travel portion of the cycle, there is need for either a pure substance or a mixture to be low-freezing and high boiling. In the atmospheric phase, some mixture of species must capture and store solar energy. The flowing liquid must also be persuaded by the conditions it encounters at the tiger stripes to release the energy. This is a fairly stringent set of requirements, and it rules out many suites of chemicals right away. I have looked at one chemical system that I believe might come close to making this model work. Some of the atoms in this system are not at the present time identified on Enceladus, and so this system serves only as an illustration. However, it is my view that there are such a very large number of chemical compounds with such a great diversity of properties that it seems hardly the right approach to just assume that none of them could have the right properties and also exist on Enceladus.

I will pause here.
martin
ngunn
OK I'm having trouble with anything condensing out and then remaining liquid for that long on the surface of Enceladus. Also you would still need a separate explanation for the topographic low near the south pole. However it's always worth playing with different perspectives on things. Maybe, even if not on Enceladus, there are places where such strange things happen . . .
dvandorn
In addition, if liquid was flowing on Enceladus in any significant amount, I'd have to think we'd see fluid flow features on the surface. I grant you, we don't have extremely high definition views of the entire moon, but the flow of enough mass to fill those tiger stripes ought to leave large scoured scabland-like terrain behind, and we really don't see that.

In fact, the areas near the tiger stripes that we *do* have high-res images of doesn't look like it's ever seen fluid flow. Extensive cracking of an ice crust, perhaps, but no fluid flow.

Great conceptual idea, Martin, but I don't think it fits all of the observed facts... sad.gif

-the other Doug
martin peters
Thanks to the two members who have pointed out two important potential objections. The first is about the possibility that any substance can be a liquid and sublimate slowly enough in the near vacuum of Enceladus to last on the surface for some meaningful period of time. The second is about the observable flow features connected with this process. Let me have a day or two to prepare responses to both.

Of course, please continue to suggest other potential objections. I appreciate your helping me focus on these key issues. martin
remcook
If anyone's around Oxford in two weeks time...

ASTOR LECTURE IN PLANETARY SCIENCE

'Exploring the Outer Planets: Extending the Boundaries of the Habitable Zone'

to be given by

Professor Andrew P. Ingersoll
Earle C. Anthony Professor of Planetary Science, California Institute
of Technology

Martin Wood Lecture Theatre, Clarendon Laboratory

4.30 p.m. Friday, 16 March 2007

Abstract: The words "habitable zone" used to mean a band around the
sun at the Earth's orbital distance, where water exists as liquid.
New observations force us to re-define this zone as an archipelago
that includes many of the icy moons of the outer solar system. The
latest entry is Enceladus, a small satellite of Saturn. Observations
by the Cassini spacecraft reveal a surprising amount of activity on
this small object - a geologically young surface, warm fissures that
emit plumes of water vapour and water ice particles, and organic
molecules coating the surface around the vents. The talk will cover
the observations, the attempts of scientists to infer the conditions
below the vents, and the possibility of sub-surface life.

This lecture is open to members of the University and the general public.
martin peters
QUOTE (dvandorn @ Mar 1 2007, 03:52 PM) *
...if liquid was flowing on Enceladus in any significant amount, I'd have to think we'd see fluid flow features on the surface.


The 6 GW of heat radiating from the tiger stripes might be provided by a total fluid flow rate (for all tributaries flowing toward the tiger stripes) of as little as 0.4 m^3 per second. Knowing neither the chemical system carrying the energy nor the particular energy density of the system, of course, the best we can do is look around for some representative values. The 0.4 m^3 s^-1 is for a system of liquid ozone and liquid propane. If they could be made to react completely to form H2O and CO2, 0.2 m^3 of propane and 0.2 m^3 of ozone would produce roughtly 6 GW. I am not suggesting the system of ozone and propane to be a candidate to run this process on Enceladus, but it serves as a real example of energy density. A second example might be the decomposition of ozone to O2. If this decomposition were to proceed so that all O3 was converted to O2, a fluid flow rate of 1 m^3 s^-1 would provide the needed 6 GW. Naturally, systems with less energy density would require proportionally greater fluid flow rates.

If all the tiger stripes are hot, then the total fluid flow would have to be divided up into at least four branches. Thus, even if the total flow is 10 m^3 s^-1, there would still be only 2-3 m^3 s^-1 in each. Furthermore, these flows are not necessarily in wide, shallow, easily observable flow channels, but more probably, in my view, in the bottoms of cracks or valleys. Therefore, I suggest that the likely flow rate combined with the fact that the flows might be confined into narrow channels is not inconsistent with the lack of identifiable flow features in existing images of the surface of Enceladus.

martin
martin peters
Several days ago I proposed a cold-interior, solar model to explain why the tiger stripes of Enceladus are hot. One component of that process was for a liquid to condense on the surface and flow to the tiger stripes. It was, however, overly restrictive of me to require a flowing liquid. What the model requires is that material move (by any process) toward the tiger stripes. Glacier-like flow of an 'ice-field,' would suffice, if the total chemical energy passing any arbitrary point amounted to 6 gigajoules per sec. The cross-section of the moving mass would have to be appropriately large, and the slower it moved, the larger the cross-section would have to be. If there were to be a material on Enceladus containing 1 gigajoule per m^-3 (chemical potential energy), then the total flow volume would have to be 6 m^3 s^-1. In that case, average flow velocities of 10, 1, 0.1 mm/s would require cross-sections of 600, 6000, and 60,000 m^2, respectively. Needless to say, nothing about this model prevents there being a combination of glacier-like (nearer to 55 deg S, for example) and stream-like (nearer to the tiger stripes, for example) flows.

martin
ugordan
Press release: A Hot Start Might Explain Geysers on Enceladus.
Rob Pinnegar
Hmmm. This "hot start" theory is almost certainly going to be tied in with the notion that Iapetus had a lot of Al-26 at the outset.
remcook
Another enceladus modelling paper in press at Icarus:


Tidal Heating in Enceladus • SHORT COMMUNICATION
In Press, Accepted Manuscript, Available online 19 March 2007,
Jennifer Meyer and Jack Wisdom

the abstract is short and clear:

"The heating in Enceladus in an equilibrium resonant configuration with other Saturnian satellites can be estimated independently of the physical properties of Enceladus. We find that equilibrium tidal heating cannot account for the heat that is observed to be coming from Enceladus. Equilibrium heating in possible past resonances likewise cannot explain prior resurfacing events."
belleraphon1
All.....

Emily Lakdawalla posted a TPS piece a while back that was most comprehensive
http://www.planetary.org/news/2007/0322_Ch...est_a_Soup.html

This weeks (04/01/07) TPS Radio piece elaborates on the South Polar Sea piece with an interview of one of the authors: http://www.planetary.org/radio/show/00000230/

This all reminds me of ..... Jules Verne..... ahhhhhhhhhhhh the Central Sea, the Saknussemn Sea...
http://jv.gilead.org.il/vt/c_earth/27.html

Craig
AlexBlackwell
A couple of new entries in the Enceladus-related literature, courtesy of Geophysical Research Letters:

Juhász, Antal; Horányi, Mihály; Morfill, Gregor E.
Signatures of Enceladus in Saturn's E ring
Geophys. Res. Lett., Vol. 34, No. 9, L09104
10.1029/2006GL029120
05 May 2007
Abstract

The second paper will be published online in GRL, I believe, on Monday, May 7, 2007. Here's the title and abstract:

Convection in Enceladus' ice shell: Conditions for initiation
By Amy C. Barr and William B. McKinnon
QUOTE
Abstract
Observations of Enceladus by the Cassini spacecraft indicate that this tiny Saturnian moon is geologically active, with plumes of water vapor and ice particles erupting from its southern polar region. This activity suggests that tidal dissipation has become spatially localized, perhaps due to a compositional, rheological, and/or thermal anomaly in its ice shell. Here we examine the role that solid-state convection may have played in Enceladus' prolific activity by creating a suitable rheological and thermal anomaly. We find convection can only initiate in the pure water ice I shell of a differentiated Enceladus if the ice grain size is less than 0.3 mm, which is quite small, but may be realistic if non-water-ice impurities (and/or tidal stresses) keep grains from growing. This grain-size restriction becomes more severe for lower basal ice temperatures, which implies that any ammonia present has not become strongly concentrated in a thin basal ocean (while convection occurs). For a maximally thick pure ice shell and underlying ocean, convective heat flows are ~7–11 mW m^−2 for ice grain sizes of 0.1–0.3 mm, compared with the ~100 mW m^−2 measured for Enceladus' south polar terrain from Cassini CIRS observations. Thus whereas solid-state convection may be a prerequisite for Enceladus' geological activity, the observed heat flow requires strong tidal dissipation within the convecting region, and possibly, that convection reaches the surface.
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