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The Dark Side of Iapetus
Hugh Pickens writes "The difference in coloring between Iapetus' leading and trailing hemispheres is striking. NASA's Jet Propulsion Labs has just released a report on a bizarre 'runaway' process that may explain the strange and dramatically two-toned appearance recently revealed in images collected during a close flyby by the Cassini spacecraft. Scientists believe that initially dark material on one side of Iapetus may have come from other moons orbiting Saturn in the opposite direction. Since Iapetus is locked in synchronous rotation about Saturn, as dusty material from the outer moons spiraled in and hit Iapetus head-on, the forward-facing side began to darken. As it absorbed more sunlight, its surface water evaporated, and vapor was transported from the dark side to the white side of Iapetus. Thermal segregation then proceeded in a runaway process as the dark side lost its surface ice and got darker still. Now the leading hemisphere is as dark as a tarred street and the trailing hemisphere resembles freshly fallen snow."
As a matter of fact it's all dark.
There are also moons which are considered to have been once independent objects caught by Saturns gravity. E.g. in this list the ones with a negative orbital period are retrograde. Saturn seems to have quite some of those.
Wow, you're right to highlight the fact that they indicate retrograde satellites might be the cause. Iapetus itself is in an unusually inclined and distant prograde orbit... I hadn't heard any retrograde satellite theories for the dark region.
Who do you get to be an expert to tell you something's not obvious? The least insightful person you can find? -J Roberts
Many of Saturn's moons are probably captured asteroids, and have highly eccentric orbits. For various reasons, it's a lot easier for a body to be captured into a retrograde orbit, going "the wrong way."
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It's a fairly substantial theory; you'd be hard-pressed to find any planetary scientist who thinks the moon formed any other way. The makeup of the moon (as it's currently understood) doesn't really accommodate alternate theories. As for its direction: when the Mars-sized planet whacked the nascent Earth, it most likely sent up an accretion disk of its own rather than a sending a huge chunk of proto-moon into orbit; this disk gradually formed the moon. Given that the disk's movement would be directed by the Earth, which would in turn be directed by the rest of the solar system, the moon's direction would, indirectly, be dictated by the solar system's original accretion disk. Pretty much the only reason (that I can think of, anyway) for a moon to have a retrograde orbit would be its capture as a more-or-less intact body.
However, IANA[A-Z], so I'm willing to be contradicted on all this.
Some other mysteries are coming together. There are more data on the signature mountain ridge that gives Iapetus its "walnut" appearance. In some places it appears subdued. One big question that remains is why it does not go all the way around...And the ridge looks too solid and competent to be the result of an equatorial ring around the moon collapsing onto its surface. The ring theory cannot explain features that look like tectonic structures in the new high resolution images.
/.) is falling out of favor? So the walnut mystery remains. Giant impacts can sometimes mess up a moon's shape, but usually the odd damage is on the opposite side of the impact, not a circumference ring ridge. Whoever can pose a physical scenario that can cause such a feature (outside of orbiting ring collapse) may somebody have it named after them.
So the collapsed ring theory (posted earlier on
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No references to the book 2001, A Space Odyssey yet? You guys are slipping. In the mid '60s, when A.C. Clarke wrote the book, he asked asked astronomers (mainstream scientists, not UFO nuts) "if you had to pick one object in the solar system that appeared artificial, what would it be?" They all picked Iapetus. At the time, the blurry photos we had from ground-based telescopes could tell us that it was 50% light and 50% dark, but nothing else. It was a big mystery, even after the Voyager flybys. For that reason, Clarke used Iapetus as the sight of the monolith stargate (the movie version used Jupiter).
We're really lucky to live in a time when all these mysteries are solved.