Saturn's Smashed-Up Moon Mystery Solved

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Iapetus probably had its own natural satellite, the remains of which now form a prominent mountain ridge around its body.

THE GIST

- Saturn's third-largest moon, Iapetus, has a mountain ridge composed of the remains of a busted-up moon.

- Iapetus shows no signs of geologic activity such as volcanism or plate tectonics that could otherwise explain the mountains.

Saturn's bizarre two-tone moon, Iapetus, wears a six-mile high chain of mountains around its equator. Now scientists have a theory that explains why.

It turns out the moon may have had its own moon. A new study shows that after a billion years or so, the moon's moon lost a gravitational tug-of-war with Iapetus and got shredded into a debris ring. Over time, the chunks were pulled to the surface of Iapetus, where they neatly piled up along the equator.

ANALYSIS: Iapetus Fights the Battle of the Bulge

"I don't think you can ever get a volcanic eruption to look like this ridge, perfectly aligned on the equator on a body that has not other evidence for internal activity. Nor does it look like any other tectonic or fault structure I have ever seen in the solar system in its perfect symmetry," planetary scientist William McKinnon with Washington University in St. Louis, told Discovery News.

"I've always felt that its precise position on the equator really demanded an almost astronomical explanation -- it wasn't simply a fault or an ordinary mountain range or a product of volcanism of any sort. There's no sign of geologic activity," McKinnon said.

Scientists have been trying to figure out how the mountain ridge formed ever since they got a close-up view of Iapetus' rugged topography from NASA's Saturn-orbiting Cassini spacecraft.

PHOTOS: Saturn's Moons

"We hadn't seen anything like this," Andrew Dombard, with the University of Illinois' Earth and Environmental Sciences department, told Discovery News. "It's taken a while for our ideas to catch up with what the observations are showing us. The ridge itself is a massive wall that sits on the surface of Iapetus."

"This actually explains more of the observations than the other (computer) models," he added.

Scientists theorize that the 930-mile (1,500-kilometer) wide Iapetus once spun faster than it does today, which kept its small sub-satellite moving outward, much like Earth's moon is moving away from our planet today.

Over time, Saturn's clutch on its large outer moon tightened, causing Iapetus' rotational spin to slow. That in turn shifted the tidal balances between Iapetus and its satellite, which eventually got reeled in like a fish on a line. At some point, the moon's moon experienced so much force that it blasted apart.

ANALYSIS: Super Ring Dwarfs Saturnian System

The pieces gathered into a ring of debris and those eventually landed on Iapetus, forming the ridge, scientists theorize.

In addition to more computer modeling, researchers would like to use a catapult to test their theory that debris raining down on a body could build up enough height to account for Iapetus' mountains.

The research has implications for understanding how satellites form, including Earth's moon and Pluto's large moon Charon.

The study appears in the Journal of Geophysical Research-Planets

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