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 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 mountain range running around much of its middle, like the cusp where the halves of a walnut shell join. 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. 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.