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.)

Saturn Moon Went From Saucer to Globe

Kreslavsky and colleagues based their new model of Iapetus on a small, rapidly spinning space rock known as asteroid 66391. The asteroid is the only other known body in the solar system that boasts a similar equatorial bulge.

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," Kreslavsky said. That means the spin made the moon's surface material slide toward the equator and bunch up—but not get flung away.

This is what appears to be happening now on asteroid 66391, the models show. But the space rock is just 0.9 mile (1.5 kilometers) wide and is spinning on its axis every 2.8 hours.

For an object the size of Iapetus—930 miles (1,500 kilometers) wide—to have formed a similar ridge via spin, the moon would have had to have been rotating on its axis once every four to six hours.

(Related: "New Saturn Ring Is Largest Known; May Solve Moon Puzzle.")

Currently Iapetus is tidally locked with Saturn—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.

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

The spin theory "explains why this [ridge] happens on Iapetus and doesn't on other satellites," Kreslavsky said last week during a meeting of the American Astronomical Society's Division of Planetary Sciences in Pasadena, California.

While plausible, the gravitationally-driven geologic forces that pinched the ridge into its present shape must have been titanic, planetary scientist William McKinnon of Washington University in St. Louis commented during the presentation.

"It's the most colossal tectonics imaginable," McKinnon said. "It's mind-boggling."