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Posted by: Palomar Sep 13 2005, 11:19 AM

http://www.saturntoday.com/news/viewpr.html?pid=17761 Yep, Titan's having to share the spotlight.

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

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

-Cindy

Posted by: volcanopele Sep 13 2005, 06:48 PM

I don't buy it. Other meteors have impact Enceladus (and the other satellites) and have left unmistakeable craters. What makes this one special? That it is made of rock? I seriously doubt one would provide enough radioactive material to heat up that one area to not only relax the crater but to form extensive fracturing like we see there? I'd have to vote nay on this idea. Sorry.

Posted by: mike Sep 13 2005, 07:08 PM

It would have to be a rare sort of meteor to contain that much radioactive material, and thus far Enceladus' hot spot is the only one we have found so far, making it rare..

Alternatively a warm object made by sentient beings crashed into Enceladus and sunk into the crust sometime thereafter, though I suppose that is rarer still.

It's an intriguing spot..

Posted by: Bob Shaw Sep 13 2005, 08:38 PM

Regarding the impact-generated hot spot notion, why involve an exotic impactor at all? On Earth, a large impact might result in a heat anomaly for hundred of thousands of years, so why not on an icy moon? The material is unimportant compared to the kinetic energy! On Earth, groundwater would percolate through cracks to create pseudo-geothermal zones, and for the life of me I can't see why such a process would be inapplicable elsewhere. Imagine: Enceladus on the brink of being able to pop ice-eruptions, when along comes a nice impact...

Posted by: Richard Trigaux Sep 14 2005, 08:47 PM

What I think is that what we saw on Miranda and what we see on Enceladus is the same thing: diapirs of warmer ice upwelling from the interior and doing these strange drawings on the surface. On Miranda, it is long ago cooled and craterized, on Enceladus we have the chance to see it active.

Why a hot spot, and not a volcanism all around the planet like on Earth? Likely because these bodies are much smaller than Earth, with lower dimention and lower density: so when a source of heat arises in the interior, it forms an unique diapir (eventually several in history, but only one active at a time). As if the hotter core was popped asides like the pit of a cherry. So there is no need to suppose that there is something special at the place where it appears.

The real mistery is what was heating the core of Enceladus or Miranda, and let it cool, and heat it again, several times as far as we can see. Not radioactive heating, it is a one time process. Not impact, it generates heat only on the surface. The only remaining candidate is tidal heating. And there can even be a delay between the tidal heating and the appearance of the diapir at the surface, or the tidal heating being constant and diapirs appearing at times.

The other lesser mystery if the appearance of large boulders on Enceladus surface.

Another open question is why the lesser icy moons have geological activity, where the larger have not. we should expect the countrary.

Posted by: JRehling Sep 14 2005, 09:03 PM

If two moons have the opportunity to tug on each other's orbits, it is the smaller one that receives the larger eccentricity and therefore greater tidal flexing and heating. This is much like why, among almost all of the planet-satellite pairs in the solar system, it is the small body that has been the tidally-locked one. Jupiter plays havoc with Io, but Io barely affects Jupiter.

Posted by: Richard Trigaux Sep 15 2005, 08:23 AM

yes....

Posted by: ljk4-1 Sep 15 2005, 02:24 PM

Maybe they are - exhaust vents for the heat from the nuclear reactor drive to escape?

Posted by: Jeff7 Sep 15 2005, 07:41 PM

One problem with a radioactive object - would it not have had a LONG time to decay, and thus no longer be radioactive? Or rather, not radioactive enough to heat super-cold ice to its melting point. Uranium samples seem to be capable of causing cancer, but they won't roast a chicken that's sitting adjacent.

Posted by: BruceMoomaw Sep 15 2005, 08:27 PM

Yep -- the traces of radioisotopes (U, Th, K) in ordinary rock greatly heat the interior of a world ONLY because they're deeply buried, so that their extremely slowly-built up heat nevertheless escapes at an even more glacial pace. By contrast, even quite radioctive material near the surface wouldn't significantly raise the temperature of the nearby ice (or other substances) simply because its emitted heat energy could escape to the surface and radiate out into space before raising the surface material's temperature significantly.

I think we are definitely looking at localized tidal heating as the cause of Enceladus' hot spot -- unless we have some kind of very freakish (and unlikely) concentration of radioisotopes in one place in its rock core that's generated a slow convective flow of warm, relatively soft ice from deep inside Enceladus to the surface there.

Posted by: tty Sep 15 2005, 08:54 PM

If (I admit it is a big if) such a small body can sustain a continental-drift-type convective system it would presumably be a single cell/plume system which might actually look much like what we are seeing on Enceladus. The tidal heating wouldn't have to be localized in that case.

tty

Posted by: BruceMoomaw Sep 16 2005, 05:13 AM

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.

Posted by: edstrick Sep 16 2005, 07:15 AM

Bruce Moomaw: "I think we are definitely looking at localized tidal heating as the cause of Enceladus' hot spot -- unless we have some kind of very freakish (and unlikely) concentration of radioisotopes in one place "

Obviouslty the heat from the reactor of the chariot-bearing god's starship <deranged-smirk>

Given that natural levels of radioactivity cant drive activity in Rhea or Callisto, they'd be utterly insignificant in Enceladas

...unless it's the habanero filling, and the proper name is Enchiladas....

Posted by: Richard Trigaux Sep 16 2005, 07:24 AM

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.

Posted by: Richard Trigaux Sep 16 2005, 07:37 AM

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?