Saturn's "Walnut" Moon Mystery Cracked?

Saturn's moon Iapetus looks like a walnut because it lies in a "Goldilocks zone" around the giant planet, new research suggests. The moon was once a fast-spinning blob of rock and ice, but its location was just right for locking an unusual feature in place as the spin slowed.

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 University of California, Santa Cruz.

In their new model, Kreslavsky and UCSC colleague Francis Nimmo suggest Iapetus formed in a region where the moon was far enough from the planet to retain a lot of its initial spin even after it was fully grown. However, the moon was close enough that Saturn's gravitational forces eventually slowed things down.

(Find out about a related theory that suggests Iapetus has its overall shape because it was "cryogenically preserved" when it was young.)