Iapetus's ridge explained

Perhaps.

Have you ever seen Iapetus? It's a strange moon owned by Saturn. It's got a pale side and a dark side. It's got a strange orbit. And it's got an incredibly bizarre equatorial ridge, its most distinctive feature.





That's right. That moon has a mountain range that reaches over twelve miles high that runs in a straight line right along the equator! Almost all the way around the moon!





The Cassini spacecraft (such a cool, useful little ship) discovered that ridge in 2004, and it has puzzled astronomers since. How could such a feature -- unique in our solar system -- have formed on such a small moon? What forces came into play to form that ridge?

Well, now they think they're on to an explanation.

It might have been a sub-moon.

The giant ridge around the middle of Saturn moon's Iapetus that makes it resemble an oversize walnut may have essentially formed as a "hug" from a dead moon, researchers say.

...

Scientists had been at a loss to explain how this mountain range might have formed. Of all the planets and moons in our solar system, apparently only Iapetus has this kind of ridge — any process that researchers previously suggested to explain its formation should also have led to similar features on other bodies.

Now investigators suggest this ridge could be the remains of a dead moon. Their model proposes that a giant impact blasted chunks of debris off Iapetus at the tail end of the planetary growth period more than 4.5 billion years ago. This rubble could have coalesced around Iapetus, making it a "sub-satellite," a moon of a moon.

Under this scenario, the gravitational pull Iapetus exerted on this sub-satellite eventually tore it back into pieces, forming an orbiting ring of debris around the moon. Matter from this debris ring then rained down, building the ridge Iapetus now sports along its equator fairly quickly, "probably on a scale of centuries," [planetary scientist at the University of Illinois at Chicago Andrew] Dombard said.

(from Space.com)

Did I mention that Cassini was absolutely cool?