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.