Titan's periodic strong gravitational tugs on its irregular shape, as that big moon keeps passing it at the same three points in its orbit, throw it into what seems to be a totally "chaotic" rotation -- in which its rotation continually speeds up, slows down, and slews into new tilts in a pattern that would be extremely difficult for even a high-powered computer to predict in advance. (Although there are limits to this -- for instance, Hyperion never switches direction and rotates backwards).

And Earth-based spectrometers had indicated that Hyperion's overall surface -- although a good deal brighter than Iapetus' dark region, reflecting about 30% of the sunlight that hits it -- has the same slight reddish tinge, the one that is lacking on Phoebe. This led to theories that Iapetus' dark material might somehow have been originally sprayed out by Hyperion rather than Phoebe, although it was hard to suggest a convincing way this could have happened.

When Cassini finally made its one planned close flyby of Hyperion in September 2005, however, what it revealed was one of the most bizarre-looking objects in the Solar System -- something that might be called "the Bath Sponge of the Gods". It is densely peppered with large craters which -- instead of looking like shallow or even flat-bottomed bowls -- look instead like deep funnels penetrating down into Hyperion's interior.

Most of this, however, is due to an optical-illusion effect -- many of Hyperion's larger craters have light-colored upper slopes sharply separated from bottoms coated with black material, which looks like the shadows cast by the walls of a deep hole. But there are a smaller number of craters that don't have such darkened bottoms.

Moreover, a few tiny craters spotted on the darkened bottoms of bigger craters have clearly punched through the dark layer to expose more light-colored ice immediately underneath -- so the black stuff can't be Hyperion's interior.

What could possibly cause this utterly unique appearance for Hyperion's craters? The possibility suggested by John Spencer is that it is exactly the same thermal-runaway process -- or, as he calls it, "thermal segregation" -- that has so sharply divided the albedos of Iapetus' light and dark halves, but operating in a different way.

The orbit of Phoebe, by pure chance, is almost lined up with the orbital planes of Iapetus and Saturn's other moons, except that Phoebe happens to go around Saturn in the opposite direction. The result is that -- as the fine dust kicked off Phoebe's surface by meteoroid impacts spirals gradually closer to Saturn under the influence of sunlight's pressure -- most of it eventually runs head-on into Iapetus.

The calculations of Cornell University's Joseph Burns indicate that Iapetus, in fact, fields fully 70% of Phoebe's in-spiralling dust. But the rest gets past it and continues to spiral in closer to Saturn -- and Burns' calculations also indicate that Hyperion ends up intercepting about 18% of Phoebe's dust (with the huge moon Titan then intercepting virtually all the rest, so that none reaches Saturn's inner moons).

This raises the possibility that Phoebe's dust eventually darkened Hyperion's surface just enough to set off the same process of water ice sublimating out of Hyperion's surface in warm spots and refreezing on colder ones -- thus further darkening one, lightening the other, and so further amplifying the thermal process. But because Hyperion, unlike Iapetus, rolls around constantly in its orientation relative to Saturn, it would be evenly coated all over with Phoebe's dust -- and so the only spots on its surface that would be warmer are the bowls of its craters, which act as concentrating reflectors to focus more sunlight on their own sides and bottoms.

If the deposited dust finally darkened Hyperion's overall surface to a certain extent, then at some point the self-amplifying migration of water ice could finally get under way -- but it would only occur in the bottoms of Hyperion's craters, whose ice would get up and move itself to the cooler surface on all the rest of the moon, mildly brightening the albedo of the rest of Hyperion, and leaving the moon with a strange Dalmatian-spotted appearance.

That is, the bottoms of craters all over Hyperion would darken in just the same way that some equatorial craters all across Iapetus' bright side do so (except that Hyperion's craters, because of their smaller size on that little moon, lack central peaks and also aren't big enough for any of them to have cool light-colored central floors surrounded by dark outer floors that form dark "rings", as is the case with a few of Iapetus' bigger craters).

Finally -- on both Hyperion and Iapetus -- the darkening would occur only in those craters steep-walled enough to focus enough sunlight onto their floors. Shallower craters would never get their floors warm enough to kick off the thermal feedback process. But in the case of Hyperion, dark-floored craters would still be much commoner than on the white trailing face of Iapetus, because Hyperion's whole surface has been somewhat darkened by Phoebe dust.

(By contrast, on Iapetus' trailing side they only turn up occasionally near the equator, where they can sometimes accumulate enough solar warmth during Iapetus' 40-day-long "day" to set off that contrast-amplifying thermal feedback even without any help at all from Phoebe dust.)

One would also expect the majority of Hyperion's surface -- which is still fairly light-colored -- to have the same salmon-pink tinge from accumulated Phoebe dust that one sees in the light-colored polar regions on Iapetus' leading face; and that is exactly what Cassini's color views and spectra do show.

So far -- unlike on Hyperion -- we haven't spotted any craters on Iapetus' dark Cassini Regio that have punched through the upper "lag layer" of dark grit there to expose the light-colored ice that must still exist underneath. This isn't suprising, however, since Iapetus has a much longer period of rotation than Hyperion, and so its dark regions accumulate much more solar heat during their 40-day-long "day", so that the ice has been baked out of a much thicker upper layer on them. (Sure enough, Cassini's IR and UV spectrometers confirm that even Hyperion's dark crater floors still have a fair amount of ice mixed in with their dark grit, which isn't the case for the dark regions on Iapetus.)

Why doesn't Phoebe itself show any such thermal effects that would lighten some parts of its surface and darken others? Apparently because Phoebe -- being an icy object from the outer Solar System -- had its ice mixed from its formation with a large amount of dark carbonaceous-rock grit, as is the case with most such objects.

Its density, as determined by its gravitational effects on Cassini's flyby path, is fully 1.6 grams/cc -- it must be about a 50-50 mixture of water ice and rock. Now, Roger Clark's lab studies show that water ice can be made thoroughly black just by mixing an amazingly small amount of carbonaceous grit into it -- as little as 1/2 %!

Adding any more does very little to darken it further. So Phoebe's surface was uniformly dark from the very beginning, and there were simply never enough differences in the amount of solar warming on its surface -- at the bottoms of its craters, or anywhere else -- to move enough water ice from there onto its cooler areas to brighten their surfaces

Iapetus' overall density, on the other hand, is only 1.1 grams/cc -- it had very little rock in it from the beginning. (The regular moons of the giant planets seem to consistently have much more ice and less rock than the regular "icy planetesimals" that made up the cores of the giant planets and still comprise the comets and Kuiper Belt Objects. This may be because a lot of the material that went to form the giant planets' moons was captured water vapor from Sun-orbiting icy planetesimals that brushed through the giant planets' nebular formation disks and were vaporized by the friction, whereas the rocky parts of those planetesimals tended to simply fly through the disks without being stopped.)

Also, Iapetus is a bigger object, and so the heat from the trace radioisotopes that were initially mixed into what rock it did have warmed its interior more than it warmed Phoebe's interior -- so it likely "differentiated" (that is, its ice melted early on and most of its rock settled to its center as a core, before the radioisotopes decayed and cooled down).

As for Hyperion, its density turns out to be a mere 0.6 grams/cc -- much less than solid water ice -- showing that (like Saturn's tiny innermost moons) it must be a "rubble clump" of chunks of material only loosely packed together by gravity. There's a lot of puzzlement over how Hyperion formed; but there's a good chance that it is made of small chunks of material orbiting Saturn that happened to drift into that 3:4 orbital-period resonance with Titan and so got herded together so that they ended up lightly clumping together under their own gravity. And most of those chunks might have been ice blasted off the outer surfaces of Saturn's other moons by the giant impacts that were so common in the Solar System's earliest days, rather than rock.

Thus -- unlike Phoebe -- neither Iapetus nor Hyperion had much dark rocky grit on their surface after their formation, and so they started out light-colored enough that their later modest darkening by infalling Phoebe grit could darken their surfaces enough to set off the thermal feedback that made their surface ice migrate, and so darkened some places on them and lightened others.

Has this same thermal process happened to any moons of the other giant planets? After all, we now know that all four giant planets are surrounded by distant swarms of tiny captured satellites with a lot of black carbonaceous grit in them. But we haven't seen any one-sided darkening on any moons besides Iapetus, and there's more than one explanation for this.

In the case of Uranus' and Neptune's moons, they're so far from the Sun and so cold that -- even when their surfaces are pitch-black -- they can never get warm enough for water ice to even begin to sublimate into traces of water vapor in the vacuum of space, and so their ice stays squarely where it is. (Uranus' bigger moons have been considerably darkened, but this has a different cause -- they're so cold that their ices include a good deal of frozen methane, which is turned into dark organic substances by the radiation of space.)

But in the case of Jupiter's two huge moons Ganymede and Callisto, Spencer concluded that their surface ices were also undergoing thermal segregation a good two decades before he developed the same theory about Iapetus. (The subject made up a good part of his Ph.D. thesis.) Although they revolve around their planet -- and so rotate -- far faster than Iapetus does, Jupiter is close enough to the Sun that their own noontime equatorial temperatures are about 17 degrees C. warmer than that on Iapetus' darkened side.

Ganymede is largely covered, and Callisto mostly covered, by a dark carbonaceous-rock grit layer strongly resembling the dark stuff on Iapetus, with the rest of their surfaces consisting of very light-colored water ice -- but it isn't concentrated on one side of either moon. Instead, while the dark and light areas on both moons are as dramatically and sharply separated from each other as on Iapetus -- indeed, the black/white boundaries are extremely sharp even in the Galileo craft's best closeup photos of both moons -- the white patches are scattered evenly all over both worlds wherever there are fairly steep slopes on them (which, in the case of the geologically wrinkled "grooved terrain" that covers 60% of Ganymede, is most everywhere).

This is apparently because Jupiter's captured satellites are scattered in a wide variety of greatly tilted orbits -- indeed, there are about as many of them orbiting in "prograde" orbits (that is, the same direction as the regular moons and the planet's rotation) as in "retrograde" (backwards) orbits. And so Callisto and Ganymede have the in-spiralling dust from those little moons spread pretty evenly all over their surfaces; it hits them not only from in front but in back, and from their north and south polar directions.

By contrast, while Saturn is now known to have a similar swarm of tiny captured satellites orbiting it in every direction, by far its biggest captured moonlet is Phoebe -- which happens, by pure chance, to be perfectly lined up to intensely bombard Iapetus' leading face, and only its leading face, with dust. The amount of dust sprinkled across the rest of Iapetus by Saturn's tinier captured moons is small by comparison, and so any ice-migration tendencies that they produce are totally overpowered by the one-sided ice migration effect from Phoebe.

We don't see frequent patches of exposed white water ice scattered across Iapetus' dark area as we do on Ganymede and Callisto (except near its borders). The reason for this seems to be that the bath of very fine, sunlight-pushed dark dust hitting those two moons is less concentrated than for the front side of Iapetus -- Jupiter's moons are smaller and in more spread-out and tilted orbits around that giant world, and so a lot of their infalling dust misses Callisto and Ganymede altogether and proceeds further inwards until it eventually hits Jupiter itself.

And so occasional landslides and the dust-dislodging effect of meteor impacts keep exposing new small patches of clean white ice on the sloped terrain features of Gaymede and Callisto faster than the modestly-paced rain of black satellite dust can re-darken them -- whereas, on Iapetus, any such freshly exposed patch of water ice undergoes its initial moderate re-darkening by Phoebe's dust pretty quickly and so then re-blackens itself by warming itself up and boiling away its remaining surface ice, with the molecules of water vapor that made up that ice moving hundreds of kilometers to Iapetus' trailing side or its poles.

Finally, there's one more nagging question about John Spencer's theory of the strange coloration of Iapetus and Hyperion. Since a few of Iapetus' equatorial craters on its white side have focused enough sunlight to darken their floors even without any help from Phoebe dust, why aren't we seeing a few similar dark-floored craters near the equators of Saturn's five light-colored major inner moons -- Mimas, Enceladus, Tethys, Dione and Rhea -- despite the fact that Titan prevents any Phoebe dust from reaching them?

There seem to be two reasons for this. First, they revolve around Saturn -- and so rotate relative to the Sun -- much more rapidly than Iapetus, and so even their peak noontime temperatures at their equators can't rise to the same warmth as can happen on Iapetus' white side near its equator.

Second -- as pointed out by Anne Verbiscer in an article in the Feb. 9 "Science" -- there is pretty good evidence that still another Saturnian moon is complicating matters still more by spray-painting the inner moons white instead of dark. Enceladus' spectacular and still puzzling south polar "geysers" are spewing enough fine ice particles and water vapor into space to form the doughnut-shaped, rarified but vast "E Ring" -- and, sure enough, Saturn's other inner moons are light in color in precise proportion to the density of the E Ring at their distances from Saturn.

They're being continually repainted not with dark dust from Phoebe but with water frost from Enceladus, which in fact makes them collectively the lightest-colored moons in the Solar System -- and puts an end to any slim chance that they might ever develop solar-heated dark-floored craters.

Even after everything I've said, it should be emphasized at this point that Spencer's theory is very far from proven. But -- with the possible exception of the fact that Iapetus' and Hyperion's surfaces are redder than Phoebe's surface -- almost all the evidence seen by Cassini seems to point strongly toward it as the solution, at long last, to the three-century-old mystery of Iapetus' harlequin appearance (and also as the cause for Hyperion's very different strange appearance). And we haven't even made that close Iapetus flyby set for this September!

The very detailed observations Cassini will make during its 1600-km flyby over Iapetus' dark and light sides, and their transition region, will probably be enough to either prove or disprove Spencer's theory once and for all: Cassini will obtain not only very high-resolution photos of Iapetus' surface features, but high-resolution CIRS temperature maps of its surface -- which will enable us to determine whether all its dark patches (large and small) do indeed match up well with a considerable temperature difference, and whether they do all face in the right directions to back up Spencer's theory.

We will also get more sensitive and sharper-detailed spectral maps of Iapetus' surface composition, which should confirm whether its reddish-tinted regions do indeed precisely match up with its leading face.

This same flyby will also try to probe an entirely separate new puzzle about Iapetus that was revealed, totally unexpectedly, by Cassini's first photos of it. That is its "belly band" -- an astonishing 20-km high, 1300-km long narrow mountain ridge that runs precisely along its equator, making Iapetus look a bit like a walnut from certain angles.

Initial theories of the formation of this outlandish feature naturally involve not iapetus' exterior, but its interio geological activities during its initial formation -- and some of those theories, remarkably, may also help explain the even more astonishing polar geysers of Enceladus and the mysterious heat source that drives them. But that is a story for another time.

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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