Mystery of Saturn’s walnut moon’ solved

Saturn`s moon Iapetus looks like a walnut because it lies in a ‘Goldilocks zone’ around the giant planet.

Washington: Research from the University of California, Santa Cruz, suggests that Saturn`s moon Iapetus looks like a walnut because it lies in a ‘Goldilocks zone’ around the giant planet.

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.

Previous theories had suggested this odd ridge formed via plate tectonics or volcanoes. But Mikhail Kreslavsky and his colleagues 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.

According to the new model, Iapetus was once spinning so rapidly that centrifugal force at the moon``s equator was nearly strong enough to throw material off into space.

"Gravity forces at the equator are approximately equal to centrifugal force," National Geographic News quoted Kreslavsky as saying.

This means the spin made the moon``s surface material slide toward the equator and bunch up—but not get flung away. For an object the size of Iapetus to form a similar ridge via spin, the moon would have had to be rotating on its axis once every four to six hours.

Currently, the moon makes a complete rotation only once during its 79-day orbit around the planet, so that one side of the moon always faces Saturn. The gravitational forces that created this lock would also have slowed the moon``s spin over time, allowing it to settle from a flying-saucer shape into a rounder body.

Lapetus` thin ridge remained, though, because the moon``s crust was thick enough to support the weight of the mountain range, the models show.