Iapetus is one of Saturn’s largest moons. Thanks to the Cassini Mission, we’ve known for the past five years that Iapetus has a unique equatorial ridge that’s up to 20 kilometers high and 100 kilometers wide. William McKinnon of the University of Washington and Andrew Dombard of the University of Illinois have proposed one way that ridge could have been created: impact with a moon of its own.
Caption: A ridge that follows the equator of Saturn's moon Iapetus gives it the appearance of a giant walnut. The ridge, photographed in 2004 by the Cassini spacecraft, is 100 kilometers (62 miles) wide and at times 20 kilometers (12 miles) high. (The peak of Mount Everest, by comparison, is 5.5 miles above sea level.)
The astronomers propose that Iapetus once had its own satellite. Over time, this satellite’s orbit deteriorated to the point where it was torn apart by Iapetus’ tidal forces. The resulting chunks formed an unstable ring that eventually slammed into the equatorial surface of Iapetus, creating the ridge.
Caption: In 2007 Cassini flew within a few thousand kilometers of Iapetus's surface to take this dramatic picture of the ridge.
Other scientists disagree, arguing that internal activity like volcanoes was responsible for the formation of the ridge. While McKinnon and Dombard haven’t been able to rule out this possibility, they claim that the crashing satellite hypothesis is the better one. Most compellingly, for reasons of energy conservation, a ring of debris would have to form around a stellar body’s equator. In contrast, volcanoes and earthquakes can occur anywhere on a planetary body, making it much less likely that they could be responsible for a ridge encircling the equator.