ST. LOUIS, Dec. 13 (UPI) -- U.S. scientists say they have a theory for the bizarre appearance of one of Saturn's moons, where a huge ridge around its equator gives it a "walnut" shape.
Researchers at Washington University in St. Louis surmise a giant impact explains the ridge, up to 12 miles high and 60 miles wide, that almost completely encircles Iapetus, the ringed planet's outermost moon, a university release said Monday.
They propose that at one time Iapetus itself had a moon created by a giant impact with another big body. The sub-moon's orbit, they say, would have decayed because of tidal interactions with Iapetus, causing it to spiral towards Iapetus.
At some point, the researchers say, the gravitational forces would have torn the sub-moon apart, forming a ring of debris around Iapetus that would eventually slam into the moon near its equator.
"Imagine all of these particles coming down horizontally across the equatorial surface at about 400 meters per second, the speed of a rifle bullet, one after the other, like frozen baseballs," William B. McKinnon, professor of earth and planetary sciences, says.
"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."
"When you have a debris ring around a body, the collisional interactions steal energy out of the orbit," Andrew Dombard, a former doctoral student of McKinnon, says. "And the lowest energy state that a body can be in is right over the rotational bulge of a planetary body -- the equator. That's why the rings of Jupiter, Saturn, Uranus and Neptune are over the equator."
Other planetary scientists have said they believe the ridge was created from within the moon by activity such as volcanism or mountain-building forces.
"Some people have proposed that the ridge might have been caused by a string of volcanic eruptions, or maybe it's a set of faults," McKinnon says. "But to align it all perfectly like that -- there is just no similar example in the solar system to point to such a thing."
