[X] My Assistant

[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.

[Guest_AlexBlackwell_*]

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]

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?

[Guest_AlexBlackwell_*]

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

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.

[Guest_AlexBlackwell_*]

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:

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.

[Guest_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.

[Guest_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

[Guest_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.

[Guest_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...

-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