How did Saturns walnut-shell-shaped moon, Iapetus, get its giant ridge?
The short answer is that no one has a clue. The ridge around Iapetus is perhaps the greatest out-and-out surprise that Cassini has sprung so far. It girdles the 1400-kilometre-wide moon, running at least half way around the equator, and rising up to 20 km from the plain. There is nothing like this ridge on any of the scores of known planets and moons, and there is so far no adequate theory to explain it. Whatever the explanation turns out to be, it may be connected with the strange black material coating one side of the moon, or with the slightly wonky overall shape of Iapetus.
