Space Walnut! The Strange Ridge on Iapetus

"Cracking a Mystery: Space Walnut Created by Moons Crashing "
Mike Wall, Space.com (December 15, 2010)

"A massive ridge nearly encircling Saturn's moon Iapetus is likely the remains of a mini-moon destroyed by Iapetus' gravity long ago, a new study suggests.

"This sub-moon probably formed after a giant object smashed into Iapetus, and the blasted-off pieces coalesced, according to researchers. But over time, Iapetus tore it apart and its bits slammed into the moon along its equator, forming a ridge more than twice as tall as Mount Everest.

" 'Imagine all of these particles coming down horizontally across the equatorial surface at about 400 meters per second, the speed of a rifle bullet,' study co-author William McKinnon of Washington University in St. Louis said in a statement. 'Particles would impact one by one, over and over again on the equatorial line. At first the debris would have made holes to form a groove that eventually filled up.'..."

Iapetus has a special place for the Lemming, since it's where one of the big black monoliths were in A. C. Clarke's novelization of Kubrick's 2001: a Space Odyssey. A 'making of' book that's - somewhere in the Lemming's attic - explained that Kubrick would have used Clarke's locale: but doing so would have meant special effects to make Jupiter, and then adding rings. Even Kubrick had limits, so that part of the movie was set in orbit around Jupiter.

That self-indulgent stroll down memory lane is to explain why, when the Lemming saw that ridge precisely over the equator of Iapetus, 'artifact' came to mind.

Not as a serious idea: more along the lines of 'that would make a good story.'

The Iapetus feature is unique in the Solar system - the part inside the orbits of Pluto/Charon and Neptune, anyway - but the collision that may have caused it isn't.

Not according to current best-fit models about how planets form, anyway. Right now, it looks like Earth's moon and Charon got started after major collisions kicked up moon-size divots. Debris piles, actually.

One of the reasons no other Saturnian moons have features like this ridge is - probably - that the orbit of Iapetus isn't particularly close to other moons. Other satellites didn't have that kind of elbow room.

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