[X] My Assistant

[Guest_BruceMoomaw_*]

While Cassini has opened up new important mysteries about the Saturn system, it looks more and more as though it has already solved two of the biggest preexisting mysteries. One is the puzzle of the longevity of Saturn's rings; Cassini has provided solid evidence on several fronts for Esposito and Canup's "recycling" theory, on which I may have more to say later. The other is the mystery of Iapetus' dichotomy. One new abstract by Denk et al at the upcoming EGU meeting ( http://www.cosis.net/abstracts/EGU06/08352/EGU06-J-08352.pdf ) seems to provide the last piece of that puzzle.

One theory floating around for a long time is that material spiraling in from Phoebe has colored Iapetus' leading face -- not directly (Bonnie Buratti's near-IR spectra of Iapetus for a long time have failed to match Phoebe's, although they do match Hyperion's), but by grains of Phoebean dust smacking into Iapetus' leading-face surface fast enough to vaporize away the ice there and leave a lag deposit of native Iapetan dark carbonaceous rock grit which is somewhat different from that which makes up Phoebe. The trouble with this theory has been that, in that case, the dark Cassini Regio should neatly cover all of Iapetus' leading face and none of its trailing face -- whereas in reality it's saddle-shaped; it fails to extend all the way up to Iapetus' poles, but it DOES extend a short distance around onto the trailing side at low latitudes.

John Spencer came up with a neat and simple theory to explain that last year ( http://www.aas.org/publications/baas/v37n3/dps2005/745.htm ) -- namely, thermal effects. Iapetus' poles are so cold that it's harder for dust impacts to vaporize ice there withput it quickly refreezing -- and, moreover, water vapor from ice vaporized at lower latitudes tends to drift to the poles and refreeze there, leaving them white. On the other hand, at the lower, warmer equatorial latitudes, the dark area absorbs enough sunlight and thus becomes warm enough to start also vaporizing away ice in neighboring light areas -- so that, at low latitudes, the dark patch has grown somewhat around onto the trailing side of Iapetus.

Denk's new abstract uses Cassini's VIMS data to apparently nail down the final proof of this: a reddish color "that almost exactly correlates with the leading/ trailing side orientation", instead of following the borders of the dark patch -- it covers even the bright poles, but does NOT cover those parts of the dark patch that curve onto Iapetus' trailing side. This reddish material presumably IS the actual dust raining inwards from Phoebe onto Iapetus. "Besides accounting for the Iapetus brightness and color dichotomies, such a scenario might explain the lack of bright spots within Cassini Regio (with both processes active, fresh craters might be darkened and reddened over a time interval of only millions of years), the bright, polewards-facing crater walls at mid-latitudes (correlated to the solar incidence angle), the relatively bright, but reddish surface of Hyperion (if the thermal effect is absent or only acts on crater floors, Spencer, privcomm. 2005), the similar crater densities of the bright and dark terrains, the probably rather low thickness of the dark blanket (Porco et al., Science 2005), as well as other detailed properties of Iapetus."

As mentioned, Denk has apparently also solved the mystery of Hyperion's abnormally deep, dark-bottomed craters, using the same mechanism. Hyperion's whole surface has the same reddish tinge, suggesting that it too has been bombarded by infalling Phoebe dust. (One abstract I saw a few years ago -- which I'll have to track down in my records -- concluded from orbital analyses that Iapetus and Hyperion would be sprayed by Phoebean material, but that Titan would field virtually all the rest of the infalling Phoebe dust, preventing it from reaching the inner icy moons.) In Hyperion's case, however, its tumbling rotation has led to it being evenly bombarded on all sides by the dust -- which also means that the ice vaporized by the impacts refreezes evenly all over Hyperion. Except, that is, in the very bottoms of its craters, which are warmed to a greater degree by reflected sunlight -- causing the ice to boil away there, leaving a lag deposit of dark Hyperion rocky debris which in turn is warmed by sunlight and boils away more ice from underneath itself, thus sinking downward and deepening the initially shallow craters. Another new EGU abstract ( http://www.cosis.net/abstracts/EGU06/00057/EGU06-J-00057.pdf ) -- albeit one which has now been withdrawn from the conference for some reason -- reenforces this idea where craters on comets are concerned: "The result of calculation shows that even minimal heating of the side-walls of the crater gives an infrared flux onto its bottom that increases the flux of sublimate from the bottom of the crater." (So much for my initial belief that Hyperion's weird craters might be due to it having a very low-density surface, so that impacting meteoroids plowed a certain distance down through it before exploding to create a deep funnel-shaped crater, as in ground simulations of the Deep Impact impactor hitting Tempel 1.)

As Denk says, there are still some small puzzles regarding Iapetus' surface: "While the above mechanism is straightforward, it is still doubtful that the very complex albedo patterns at equatorial latitudes revealed by Cassini images (Denk et al., LPSC 2005) are also produced entirely by these two rather simple processes. Closeup imaging planned for 10 Sep 2007 will provide additional information that might ultimately solve the Iapetus riddle." The impression left, however, is that this is a mop-up job, and that after over three centuries the main puzzle has finally been solved.

[Guest_BruceMoomaw_*]

One footnote: the walls of the craters on Hyperion might also serve as a "focusing mirror" not just for sunlight, but for dust particles striking that moon. That is, the impacts would be spread in a more dispersed way over the inclined walls of craters, but a lot of the particles might survive their initial impacts and then ricochet onto the crater floors, which would thus be bombarded by a particularly dense concentration of dust particles -- which in turn might contribute to the ice apparently being scoured away from the floors of Hyperion's craters, while remaining in large amounts both on the crater walls and on the moon's surface outside the craters. Without some such additional contributing effect, it's rather hard to visualize Hyperion being sunlit to just the right temperature so that the ice was removed from its mildly warmer crater bottoms but remained in place, despite the dust bombardment, everywhere else on Hyperion. (It's possible that the positive-feedback self-amplifying nature of the thermal darkening -- the darker the surface became because of sublimating ice, the more sunlight it would absorb to thus become still warmer -- might be adequate to explain the dramatic crater-bottom albedo difference by itself; but in that case one would expect similar effects on the crater bottoms of the other icy moons just from sunlight alone. It seems likely that a combined effect from crater-wall focusing of both reflected sunlight AND infalling dust is necessary to explain Hyperion's extraordinary pattern of albedo differences.)

[JRehling]

The mechanism put forth in this pair of papers doesn't address, as far as I can see, the assymetry of CR's eastern and western boundaries, the "feathered" nature of the northern and eastern boundaries, the white mountains in western CR, or the fact that the darkness seems to have hit the Snowman craters from the *other* direction. It's also suspicious that The Ridge runs down the middle of CR, and surely it didn't actively align with Phoebe-dust. I think it's fair to say that on all the surface area of all the solid worlds in the solar system, CR and The Ridge are among the very most salient equator-centered features, and it would be very odd if those happened to occur on the same world for unrelated reasons.

I think the big problem with the interpretation is going to prove to be ground patterns of the dark/light boundary that show that the angle of incidence of the dark stuff on its ballistic fall to Iapetus does not support an interpretation that it came "from space, above the leading side". Certainly the saturnshine images of the Snowman craters don't gibe with that. So we know that either the stuff was originally emplaced/had-triggering-matter-emplaced in a different way on one part of Iapetus or else the melt pattern "creeped" in a curiously selective way (here, I picture the Grinch oozing snakelike along the floor of the home he invaded).

And how to explain the white mountains? They're new? They are volcanic cones that simply never had any of this dark stuff inside of them?

Of course, it's always possible that two or more weird phenomena have stumbled over each other to make Iapetus bizarrely uninterpretable. In a big enough universe, it's bound to happen. I used to study commercial airline disasters, and they usually consisted of a bizarre combination of orthogonal irregularities. But then, there are (luckily!) millions of nominal airline flights for every crash. With only a few dozen large-ish rocky worlds in the solar system, what are the odds that one would be marked by a major phenomenon and countermarked by another one or two, when that world's siblings (eg, Rhea) have not even one such phenomenon?

[Guest_BruceMoomaw_*]

The mechanism put forth in this pair of papers doesn't address, as far as I can see, the assymetry of CR's eastern and western boundaries, the "feathered" nature of the northern and eastern boundaries, the white mountains in western CR, or the fact that the darkness seems to have hit the Snowman craters from the *other* direction. It's also suspicious that The Ridge runs down the middle of CR, and surely it didn't actively align with Phoebe-dust. I think it's fair to say that on all the surface area of all the solid worlds in the solar system, CR and The Ridge are among the very most salient equator-centered features, and it would be very odd if those happened to occur on the same world for unrelated reasons.

Use Occam's Razor too often and you eventually cut yourself on it. Matson and Lunine think that the Belly Band is due to solid-state ice convection resulting from the combination of its Iapetus' initial Al-26-warmed soft ice and Saturn's tidal influences ( http://www.agu.org/cgi-bin/SFgate/SFgate?&...t;P14A-03" ) -- and of course the pattern of the dust spiralling in from Phoebe to impact Iapetus would tend to be centered around Iapetus' equatorial latitude.

Your other objections are harder to answer, as Denk himself said. But it seems likely on theoretical grounds that a phenomenon like Cassini Regio HAD to exist, and had to assume the shape we're actually seeing -- which means that it's likely to be the primary phenomenon even if other factors may have been mixed with it.

In this connection, another HBS (Half-Baked Speculation): is it possible that Iapetus' original ice was not only lighter than Cassini Regio but DARKER than its current light side, since most of the ice boiled out of the dark area is likely to have refrozen as a relatively pure surface crust on the light region? (And that Iapetus' original ice was also darker than the ice of Saturn's inner icy moons, because it was richer in carbonaceous material?) If so, then some of the larger craters in the light region may have exposed the original surface -- or boiled some of the ice out of their location through impact heat, thus starting up the same albedo-connected positive-feedback thermal processes that have further darkened and enlarged Cassini Regio. By contrast, this wouldn't have happened with individual craters on the inner icy moons because their surfaces were much more scarce in carbonaceous grit to begin with, and so they wouldn't have been darkened enough by the heat from crater-forming impacts to start up the self-amplifying contrast-enhancing process.