In general, moons that form around planets—rather than those believed to be captured objects—spin due to the motion of debris as it consolidates into a larger orbiting body.

Unlike Saturn's other spherical or ellipsoid moons, Iapetus has a unique, slightly squashed shape with an 8-mile-high (13-kilometer-high) mountain range running around much of its middle, like the cusp where the halves of a walnut shell join.

(Related: "Saturn's Largest Moon Has Ingredients for Life?")

Previous theories had suggested this odd ridge formed via plate tectonics or volcanoes. Those models tended to produce a broader "ridge zone" rather than a single narrow feature, noted co-author Mikhail Kreslavsky of the