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Iapetus Of Saturn

Saturn's "two-faced" moon Iapetus.
by Bruce Moomaw
Cameron Park CA (SPX) May 08, 2007
During its studies of Saturn and its moons and rings, the Cassini spacecraft has revealed spectacular new mysteries -- most of them involving Titan and Enceladus, both of which have turned out to be even more complex, spectacular and enigmatic than had been expected before the flight. But the purpose of any space probe is to solve existing mysteries as well as revealing new ones; and during its first 2 1/2 years orbiting Saturn, Cassini has already accumulated evidence allowing us to state probable answers to two of the biggest puzzles about the Saturn system.

One I will write more about soon is the puzzle of the longevity of Saturn's rings. Before the flight, there were several different physical reasons to think that they could not have existed at all for more than a few hundred million years -- which would mean that the spectacular event that created them must have occurred, by pure chance, only a few hundred million years ago, despite the fact that Saturn has been around for 4.5 billion years.

This was possible, but the odds were against it. And Cassini has uncovered as many as a half-dozen different lines of physical evidence very strongly suggesting that instead the rings have had their lifetime extended for as much as billions of years by a process called "ring-particle recycling", which has paradoxically greatly prolonged their overall lifetime while producing continuous changes in all the details of their structure.

The other puzzle which Cassini seems likely to have solved already is the mystery of Saturn's "two-faced" moon Iapetus -- despite the fact that Cassini will not be making its first close fly of the 1440-km wide Iapetus until Sept. 10 of this year. Although so far it has not yet come closer than 123,000 km to Iapetus (which is in a distant orbit 3.5 million km from Saturn that is modestly tilted by 7.5 degrees into the bargain, making it hard to arrange flybys of it by Cassini), even its distant observations have given us very strong clues to the solution of the puzzle.

This is a mystery that has endured ever since Jean-Domenique Cassini first discovered the moon in 1671. Cassini noted immediately that Iapetus was far fainter and harder to see on one side of its orbit around Saturn than the other -- and he correctly concluded that this was because the "leading face" of Iapetus (the side of it facing forward along its orbital path, which faces toward the Sun and toward us during half the moon's orbit) was as dark as coal in color, while its "trailing side" (which we see during the other half of its orbit) was a bright whitish shade. In fact, until the 1970s this was the only hint we had about the surface features of any moon in the Solar System except our own.

The obvious question is: what could have turned Iapetus into such a harlequin moon? There were two families of possible explanation: "exogenic" (in which Cassini's color was produced by the fact that one side of it had been hit by something from outside the moon), and "endogenic" (in which the great dark patch had been produced by an geological eruption of dark material from Cassini's subsurface onto its light-colored icy surface -- or, conceivably, the other way round).

When Voyager 1 flew through the Saturn system in November 1980, its distant photos of Iapetus' disk allowed human eyes to directly see the dramatic difference in its sides' colorations for the first time; and Voyager 2 gave us a better, though still distant, view when it flew within a million km of Iapetus nine months later. But these photos provided startling self-contradictory evidence for and against both the exogenic and endogenic explanations.

The Voyager photos showed a quite sharp border between the dark region (now officially named "Cassini Regio") -- which reflects only 4% of the sunlight that hits it -- and the light region, which has an albedo of fully 60%. They also showed that the dark patch was centered almost precisely on the center of Iapetus' leading face -- which pointed very strongly against an exogenic origin, since it would be wildly stretching chance to assume that such an eruption had just happened to occur in that precise place.

This also argued equally strongly against one exogenic explanation in which a great meteoroid had happened to hit Iapetus in just that place and blown away its light-colored surface in one place to expose an underlying dark layer (or that the meteoroid itself had been made of dark material). It pointed, instead, toward an alternate exogenic theory first suggested by Steven Soter in 1974.

In Soter's theory, infalling meteoroids had been blasting material off Saturn's distant little moon Phoebe -- an object obviously captured into orbit around Saturn at some point in its career rather than being originally formed as a moon, since it goes around Saturn in a "retrograde" direction (opposite to both Saturn's rotational direction and the paths of all its other inner moons).

Bigger bits of ejecta blasted off Phoebe by these impacts would tend to take up orbits fairly close to Phoebe, and indeed might often eventually crash back into it. But very fine dust ejecta particles would have their orbits around Saturn radically changed by two other factors.

Those were the pressure of sunlight on the grains (which would speed them up on one side of their orbits and slow them down on the other, making their orbits gradually more elliptical so that they would tend to swoop in closer to Saturn), and the "Poynting-Robertson effect" -- a subtler result of the fact that any object moving perpendicularly to the direction of sunlight has the photons hitting it at a very slight forward angle and so slowing it down -- makes them gradually spiral in closer and closer to Saturn in general.

The result of these two effects is that much of Phoebe's dust ejecta eventually hit Iapetus' leading face at high speed. The fact that Phoebe's orbit is in the reverse direction to Iapetus' means that such dust grains hit Iapetus at fully 6 km per second -- a fact which, as we'll later see, is important in itself -- but if they were orbiting Saturn in the same direction as Iapetus, their impacts would also be spread out much more evenly across its whole surface instead of being concentrated on its leading face. And so, by this theory, Iapetus' leading face has been "sandblasted" by Phoebe dust over the eons.

But while this theory nicely explains the perfect symmetry of Iapetus' dark patch around its leading face, there are other features of that dark Cassini Regio that showed up in the Voyager photos that it does not explain.

For one thing, if this simple version of the theory was right, then the entire leading hemisphere of Iapetus should be dark and the entire trailing hemisphere should be much lighter.

But the Voyager photos showed that the dark Cassini Regio is actually shaped instead like a saddle -- it doesn't stretch up to Iapetus' polar regions at all; while in the low-latitude equatorial regions, its edges actually hook around fully 200 km onto Iapetus' trailing hemisphere on both sides. And there are quite a few isolated craters on the trailing side -- located near the hemispheres' boundary line -- that have dark floors, as though the meteors that had created them had blasted after all through a thick ice crust on Iapetus to expose an underlying dark subsurface stratum.

There is also the "Moat" -- a huge 250-km crater located near the hemispheres' boundary -- that complicates matters further by having a light-colored center surrounded by a dark ring running completely around the crater at the foot of its inner walls, so that it looks like a huge dark circle.

Moreover, it was hard to see how enough dirt could have been blasted off Phoebe over the eons (even given its low gravity) to spray-paint half of the much larger moon Iapetus so darkly.

Finally, Earth-based near-infrared spectra in the 1990s showed that Iapetus' dark region had a distinctly more "reddish" tint than the surface material of Phoebe -- in fact, colorwise it was a better match to Saturn's next moon inwards, the little hamburger-shaped Hyperion that orbits just a short distance beyond Titan (and tumbles rotationally in every direction as a result of Titan's tidal tugs on its irregular shape, instead of keeping one face pointed loyally toward its home planet like all other close-in moons in the Solar System.

Could Hyperion possibly have spray-painted Iapetus in some way, instead of Phoebe? Or did those dark-floored craters on Iapetus' bright trailing side indicate that Cassini Regio really had been the result of a huge eruption of dark material from inside Iapetus -- an eruption that, by fantastic chance, just happened to be centered on Cassini's most forward-facing point?

In short, Iapetus -- at the time of Cassini's arrival in Saturn orbit -- was a classic scientific detective story in which the clues pointed confusingly in all directions and contradicted each other: a perfect "impossible crime".

Anyone who reads mystery stories, however, knows perfectly well that in an impossible crime, the Great Detective eventually points out that everyone has been looking at the clues in the wrong way, and that -- looked at from another point of view -- they point firmly toward the real criminal.

In Part 2 of this story, we'll see how Cassini's observations of Iapetus -- even from a long distance -- have together provided the clues that scientists needed to put together a single, nicely unified explanation of that moon's bizarre coloration.

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

Editor's Note: The first edition of this article described the moon Iapetus as Lapetus - with a capital L rather than a capital I. It was a font typo mistake where a correctly cased capital I in Arial font appeared to be a lower case L. It was then transferred all the way through the production processed - including the imaging stage where several "Lapetus moon" search results came back with many good looking shots of the moon 'Lapetus'. An unfortunate mistake - but one that is entirely explainable within the context of text production processes. Typos and misspellings are a regular feature of all publishing operations, and to be perfect to the point where there is a zero error rate is something all publisher's might aspire to, but accept will never actually be achieved in the real world - only in the blogsphere - which was where this article was meant to have been published as a Bruce Moomaw SpaceBlogger report - see the corrected version and a place to comment further here.

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