Cracking a Mystery: Space Walnut Created by Moons Crashing

SAN FRANCISCO ? A massive ridge nearly encircling Saturn's moon Iapetus is likely the remains of a mini-moon destroyed by Iapetus' gravity long ago, a new study suggests.

This sub-moon probably formed after a giant object smashed into Iapetus, and the blasted-off pieces coalesced, according to researchers. But over time, Iapetus tore it apart and its bits slammed into the moon along its equator, forming a ridge more than twice as tall as Mount Everest.

"Imagine all of these particles coming down horizontally across the equatorial surface at about 400 meters per second, the speed of a rifle bullet," study co-author William McKinnon of Washington University in St. Louis said in a statement. "Particles would impact one by one, over and over again on the equatorial line. At first the debris would have made holes to form a groove that eventually filled up."

Iapetus: A space walnut

Iapetus' ridge is 62 miles (100 kilometers) wide and 12 miles (20 km) high in places. It neatly tracks the moon's equator and covers nearly 75 percent of Iapetus' surface. [Photo of the ridge on Iapetus]

"It's one of the most astounding features in the solar system," said lead author Andrew Dombard of the University of Illinois-Chicago, who presented the results today (Dec. 15) here at the fall meeting of the American Geophysical Union. "It kind of gives Iapetus the appearance of a giant space walnut."

Other researchers have proposed that volcanism or mountain-building forces inside Iapetus raised the ridge. But Dombard said such theories can't account for the ridge's near-perfect alignment along Iapetus' equator, or why the moon is the only body in the solar system with such a feature.

So Dombard and his team looked beyond Iapetus ? Saturn's third-largest moon, with a diameter of 913 miles (1,470 km) ? for an explanation.

"If it didn't come from below, maybe it came from above," he said.

A lost sub-satellite

Dombard and his colleagues propose that the ridge is made of the shards of a sub-moon that once orbited Iapetus.

This sub-moon, they said, could have been created when a giant body plowed into Iapetus long ago, blasting off material that eventually fused and was captured by the moon's gravity. Such violent collisions likely formed Earth's own moon and Charon, Pluto's largest satellite.

The researchers think Iapetus' sub-satellite then spiraled toward the moon, eventually coming so close that Iapetus' gravity tore it apart.

This lost mini-moon's bits would have formed a debris ring around Iapetus' equator, the researchers added. Over time ? anywhere between 100,000 years and 1 billion years, depending on how close the sub-moon initially was to Iapetus ? this ring slammed into the moon, creating its distinctive ridge.

This theory can explain the ridge's location along Iapetus' equator, as well as why no similar feature is seen on any other solar system body, Dombard said. Iapetus has a far bigger Hill sphere ? the zone around a celestial body where its gravity dominates satellites ? than any other moon in the outer solar system, he said.

So on other moons, the parent planet would've snatched such a sub-moon away relatively quickly.

"Only Iapetus has the orbital space around it to hold onto one of these satellites," Dombard said.

The researchers haven't yet performed any rigorous simulations to show in detail how the ridge-formation process may have proceeded, he added, but they plan to do so soon.

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