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[Palomar]

Tiny Enceladus May Hold Ingredients of Life Yep, Titan's having to share the spotlight.

*A friend wrote the following to me privately yesterday, regarding Enceladus:

In the absence of a better idea from any quarter, I wondered about a large rocky meteor perhaps having impacted Enceladus at that anomalously warm point some time ago, burying itself in the moon's icy crust. The radioactive elements in the meteor, decaying over geological time periods like a lesser version of the ones powering volcanism in Earth's interior today, might be producing just enough heat to melt a deep portion of the crust, causing the outgassing, and elevating the surface temperature by the observed 20 deg.K.  That same relative warmth, softening the frozen crust, might quickly have erased the crater which resulted from the collision and removed the tell-tale evidence of the meteor's existence.
         
Being a comparatively rare event, an impact like this would explain why Enceladus alone has a hotspot while other icy moons of a similar size are uniformly cold and geologically dead, as we would expect.

It's an understatement to say that's an extremely interesting speculation.

-Cindy

[Guest_BruceMoomaw_*]

That is possible. I presume you're thinking of a "one-plume" convective mantle model such as has been hypothesized by some to explain Mars' Tharsis Bulge. It seems to me that it would have to be awfully concentrated to explain a temperature difference as radical as what Cassini saw on Enceladus, though.

[Guest_Richard Trigaux_*]

Hi BruceMoomaw and tty,

I heard in the 1980 of that stories of one-plume conveective model.

It considered the Reynolds number, a figure of turbulence after geometrical and thermodynamical conditions. On Earth, the Reynolds number is very high, leading to a very complex and random movement of the mantle. Earth mantle is "boiling", at its scale of time of course.

But on smaller worlds, the Reynolds number would be much lower, leading to a simpler and ordered convection scheme: one plume, or a small number ordered in a symmetrical manner. Benard celles, transposed from a flat layer to a spherical layer.

So it is very temptative to think at small bodies like Enceladus (and Miranda, and Moon and Mars...) as a one-cell convection pattern, explaining much naturally the appearance of only one hot spot, without requiring anything special to explain why it is here and not elsewhere.

If we assume a 1 density for the ice mantle of Enceladus (a reasonable hypothesis, as no ammonia was detected in the volcanic plumes) and a 2-3 density for the core (maybe composed mostly of black carbonaceous materials with some silicia dust, which never melt) we obtain a relatively large core, half of the radius (a bit smaller if we assume a once molten rocky core). So the core cannot be "popped aside" as I wrote higher.

Where is the heat generated? If the core is formed of black carbonaceous dust, it is likely to be soaked with liquid water, and plenty of friction can generate heat. If the core is a rigid rock (once molten) it does not have the same mechanical properties that the ice mantle, and thus there is friction at the limit. If there is a liquid layer around the core, heat is generated here. In any case, this leads to a situation very similar to Earth mantle: a thin warmer layer at the bottom of the mantle generates hot spots by bulging, and after these hot spots migrate toward the surface like diapirs of hotter rock. (At least this is the situation on Venus, where convection is dominated by hot spots. On Earth there are hot spots too, but there is also plate techtonics, which is rather driven by large layers of cooled rock sinking into the mantle from the top). The difference is that Earth mantle is larger, and contains about 50 hot spots, where on Enceladus there is place for only one.

But in any cases the hot spots are only the consequence of convection being driven in an assymmetrical geometry from an initial symmetrical one.

[Guest_Richard Trigaux_*]

To be noted that water alone cannot explain the steam plumes observed on Enceladus. Liquid water, even very hot, is much denser than ice, so it can simply not raise into a volcanic chimney (oherwise we could observe cryovolcanism in Greenland and Antarctic).

So the steam plumes were necessarily driven by some outgassing. Even deep in the mantle this water must contain enough dissolved gas to be lighter than ice and rise. But this gas, seemingly, was not detected in the plumes. So what?