Not only did this totally solve the Huygens problem, but it produced to major pieces of pure serendipity -- an additional very close flyby of Enceladus in Feb. 2005 and that Iapetus flyby, both of which produced major scientific finds.

At first, though Cassini's clear new photos of Iapetus just further deepened the puzzlement over what had caused its strange division between a black leading face (albedo 4%) and a white trailing face (albedo 60%). The craters near the border between the dark and bright areas showed some very peculiar albedo patterns.

In the north polar region of Iapetus -- where the dark region is missing (as it is in the south polar region, at the same time that it hooks 200 kilometers onto Iapetus' trailing side in its equatorial zones so that it looks like a saddle) -- many craters in the dark region near its boundary had north-facing inner walls that were white even when the rest of the crater was black; while many craters in the white northern region near the boundary had black south-facing walls even when the rest of the crater was white.

This would seem to be a very nice piece of additional evidence for the theory that Iapetus' leading face has been spray-painted over the eons by a stream of tiny black carbonaceous dust particles blasted off Saturn's tiny, distant moon Phoebe, which (since Phoebe is a "captured" moon that revolves around Saturn in almost exactly the opposite direction to its closer moons) have spiralled gradually closer to the planet and crashed into Iapetus head-on.

Not only would the infalling dust grains be dark themselves, but the heat flashes from their 6 km/second impacts against Iapetus' surface would tend to boil away water ice from those leading spots, leaving behind a "lag residue" of Iapetus' own native black grit.)

Since this dust stream would be moving almost straight toward Iapetus' leading face, near the dark region's edge the particles would be racing over Iapetus' surface almost horizontally, and so crater slopes facing away from the central point of the leading face (in this case, northward) would sometimes be shielded from the blackening dust stream even when the rest of the crater wasn't.

Meanwhile, some craters in the white region near the boundary edge would have slopes pointing toward the leading face (in this case, southward) which would "field" a particularly large amount of Phoebe dust and be darkened by it even when the dust hitting the rest of the crater was much more dispersed.

However, Iapetus' craters on the trailing side at near-equatorial latitudes did not show this pattern. Instead of craters there having dark slopes facing toward the leading face, or light ones facing away from it, those mixed-brightness craters at these latitudes showed much more random patterns.

Very often -- as with the "Moat" (the huge 250-km crater in this zone) -- they would have totally bright inner walls and bright central floors, but a dark ring around their outer floors at the foot of the walls. Smaller craters in this zone would sometimes have totally bright walls but totally dark floors. And while such dark-floored craters seemed also seemed to be commoner near the edge of the dark Cassini Regio, some scattered ones could be found all across the white trailing side -- but only near the equator!

So the mystery remained. Many pieces of evidence pointed toward the Phoebe dust-spray theory, rather than the possibility that the dark material had been erupted from beneath Iapetus' surface by geological activity or splashed onto it by a single giant meteor impact -- the fact that the dark area ("Cassini Regio" is precisely centered around the forward most point on Iapetus' surface; the appearance of the northern boundary-zone craters; the fact that Earth-based radar had shown a few years earlier that the dark layer on the leading side is apparently very thin.

But other pieces pointed just as firmly against it: the fact that the dark region doesn't perfectly cover Iapetus' leading face, but instead is shaped (as mentioned) like a saddle; the different appearances of the dark-floored equatorial craters; the fact that Earth-based spectrometers had indicated that Iapetus' dark material has a faint but definite reddish tint that is missing on the material on Phoebe's surface; the fact that it was hard to see how, even over 4 billion years, meteor impacts could have kicked enough dark material off little Phoebe to utterly blacken almost half the surface of the far bigger Iapetus.

However, during that long-range flyby, Cassini had looked at Iapetus with other instruments besides its cameras. And the data from one of them -- the "CIRS" (Composite InfraRed Spectrometer), which observes long-wavelength infrared radiation to both analyze gases and map the surface temperature of Saturn's airless moons -- had provided what now looks as though it may be the crucial clue to solve the puzzle.

Like almost all the "true" moons in the Solar System (as opposed to Phoebe and the other tiny "captured" moons that orbit the four giant planets at millions of kilometers distance), Iapetus keeps one face steadily toward its planet -- and so the day-night cycle at any place on its surface is just as long as its period of revolution around Saturn. And Iapetus is much farther from its planet than any other regular moon in the Solar System -- it takes fully 79 days to circle Saturn once. This means that any point on its surface will undergo almost 40 days of continuous daylight before being shrouded in night for the same long period.

And that means that -- despite the great distance of the Sun -- places on Iapetus' equatorial surface can get surprisingly warm, and that the same places can cool off to a really unusual degree during the long night. Now add the fact that the dark region on Iapetus can absorb a lot more sunlight, and so be warmed by it a lot more in the daytime, than the whitish region is. Most of Saturn's moons have a difference of only about 20 degrees C. between their peak noon and midnight temperatures. But Cassini -- which happened to fly by Iapetus while its dark area was aimed sunward -- found a difference of fully 70 degrees between the noon temperature in the dark region and the midnight temperature in the light region: minus 165 degrees C. versus minus 235 degrees.

This led the University of Colorado's John Spencer to start thinking: what if this big temperature difference could have caused a "positive feedback" effect that would amplify an initial difference in albedo between Iapetus' leading and trailing faces that had been caused by the Phoebe dust stream?

Even a moderately dust-darkened leading face, in the low-latitude zones, could get warm enough during Iapetus' daytime for the water ice that made up much of its surface to slowly "sublimate" into water vapor in the vacuum of space and drift to the cooler trailing face, where it would refreeze and actually lighten that region's surface color. And the more this process went on, darkening one part of Iapetus and brightening the other the more it would amplify and feed on itself. It could greatly amplify an initial rather mild difference in brightness between Iapetus' leading and trailing faces into today's dramatic dark/light split.

Moreover, such a thermal effect would perfectly explain the otherwise mysterious "saddle" shape of the dark Cassini Regio. Iapetus' poles -- even on its dust-darkened leading side -- would stay cold enough during the daytime that they would not lose their surface ice. In fact, like the moon's trailing side, they would actually be further whitened as they accumulated more ice that had been transferred from the lower-latitude dark regions.

On the other hand, on the dark/white boundary in Iapetus' equatorial regions, the black terrain would slightly warm the light terrain located immediately next to it -- slowly boiling more ice away from that terrain, so that the black region would slowly grow sideways in the equatorial latitudes, and start hooking around onto the trailing face at both the east and west sides of the leading face.

And it would also perfectly explain the odd black/white patterns in the craters near the boundary line. Moderately high-latitude craters on the leading side would have their poleward-facing inner walls perpetually cooler, and their equator-facing walls perpetually warmer, than the rest of the crater. So craters near the black/white boundary line in the polar bright regions, which were otherwise white, could have their equator-facing walls cooked black -- while near-borderline craters in the dark region at moderately high latitudes could keep their poleward-facing walls cool enough to retain their white surface ice even if all the rest of the crater had lost its ice.

Near-borderline craters in the equatorial region, on the other hand, wouldn't have such oriented permanent differences in the temperatures and thus the coloration of their walls. But some of them on the white side -- if they were near the equator, and thus receiving more intense solar warming than those at higher latitudes -- might serve as concentrating solar reflectors, with their steep walls focusing enough heat onto their floors to set off the same thermal ice-migration process even without the darkening effect of Phoebe dust. Ice would very slowly boil out of their floors and refreeze on their cooler upper walls and elsewhere.

And some larger craters, such as the Moat, would have raised central peaks that would also remain white, making those craters look like dark rings. (This would especially be true because craters' inner wall slopes would tend to reflect more sunlight onto their floors near the foot of the inner walls, making it easier for their outer ring of floor material to darken than it would be for the center of their floor to do so, even if they didn't have a central peak.) Moreover, it would be somewhat easier for equatorial craters in the white areas to be warmed enough for this contrast-amplifying thermal-feedback process to start working if they were also near the edge of the dark Cassini Regio and so receiving a bit more warmth from it.

In short, the otherwise totally baffling features of the dark and light areas on Iapetus all fall into place perfectly once one assumes that an initial mild darkening of the leading half of Iapetus by Phoebe dust has been amplified by this sunlight-produced thermal effect -- but only at low latitudes, with the very same effect actually lightening Iapetus' poles instead on the leading side.

Spencer has yet to totally work out the quantitative details of his theory, but his preliminary calculations show that "in only about 10,000 years (after the thermal process starts working on an initial mildly dust-darkened leading side], you start to burn off frost on the leading side." And over a much longer period -- hundreds of millions of years -- the black region can indeed slowly grow itself from the leading side onto the trailing side at equatorial latitudes. (Over Iapetus' remaining lifetime of billions of years, those trailing-side dark regions may continue to very slowly hook around farther onto the trailing side.)

Once Spencer had proposed his theory, Cassini's observations during that flyby immediately provided what looks like other strong support for it. For one thing, there's the matter of the subtle color differences on Iapetus along with its dramatic albedo differences. Casssini's color-sensitive cameras and its even more sensitive "VIMS" visual and near-infrared spectrometer showed clearly that, in the polar regions on Iapetus' leading side, its light-colored ice was slightly darker -- and had a distinctive pinkish tint -- compared to the ice on all of the trailing side.

And the near-black surface in the dark regions on the trailing side was slightly less reddish in tint than the near-black surface on the leading side. That is, the dividing line for the red-tinged part of Iapetus' surface perfectly followed the dividing line between its leading and trailing sides. This is exactly what one would expect if a stream of slightly red-tinged dark Phoebe dust was indeed being sprayed onto Iapetus' leading side. (The polar regions on its leading side are actually a good deal more whitish than they would be if they were simply being moderately darkened by Phoebe's dust, since they are also still slowly accumulating more ice that's being transferred from the growing equatorial black regions on the trailing side.)

Cassini's "UVIS" ultraviolet spectrometer turned up still more evidence from its surface observations. During Cassini's very close flyby of Phoebe in June 2004 as it approached Saturn (another remarkable stroke of pure luck for the mission), its spectrometers had shown that -- despite the fact that Phoebe's surface is almost as black as Iapetus' dark region, reflecting only 6% of the visible sunlight that hits it -- it still has quite a lot of water ice mixed with its dark carbonaceous grit. (Even a small amount of mixed-in carbonaceous dust can tremendously blacken water ice.)

But during its longer-range observations of Iapetus, the UVIS showed that there was much less water ice mixed into Iapetus' forward-facing black region, which was exactly what one would expect if thermal processes had driven all the surface ice out of that region onto Iapetus' lighter regions. (Phoebe, like all the other tiny distant "captured" moons, still rotates quite fast -- it spins, in fact, once every 9 hours -- and so no place on its surface ever has time to warm up or cool down enough to start up the same ice-moving process that apparently happens on Iapetus; Phoebe's poles look as black as the rest of it. The thermal positive-feedback effect can't get a foothold there to initiate its runaway contrast-heightening.)

But there is still a problem with Spencer's theory: the fact that the dark material on Phoebe's surface has distinctly less of a reddish tint than the dark material on Iapetus' leading face. This is hardly an unbreakable roadblock, though. It could very well be that the burst of heat that Phoebe dust particles feel as they slam into Iapetus' surface at 6 km/second modifies some of the complex organic compounds in them and so redden them.

VIMS investigator Roger Clark of the University of Colorado proposes another possibility: his lab tests show that Phoebe's surface may have just enough water ice mixed with its black grit for "Rayleigh scattering" (the same effect that makes our sky look blue) to impose a mild bluish tinge on it which does not exist if the the grit is mixed with either less ice than on Phoebe (as in Iapetus' leading-side dark region) or more ice (as in Iapetus' leading-side light polar areas and its trailing-side dark ones).

So he thinks that the color differences between Phoebe and Iapetus have a physical, rather than a chemical, cause. So there are at least two possible explanations for the one fact that so far has shown any sign at all of throwing a monkey wrench into Spencer's theory, which beautifully explains all the other puzzles about Iapetus' strange coloration that have tormented astronomers for so long.

Finally, there's a sideshow: the totally different but equally bizarre appearance of Saturn's next moon inward, Hyperion -- and Spencer's thermal-feedback theory also happen to do a good job of explaining that puzzle as well, as I'll explain in the last part of this series.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

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