When planetary scientist Kelsi Singer studied images of Saturn's icy moon Iapetus, she found something unexpected: huge ice avalanches.

As far as moons go, Iapetus is as eccentric as they come. One half of the planet is light-colored and the other half is dark. It has 12-mile-high mountains – twice the height of Mount Everest. And a mountainous ridge bulges out at its equator, giving it the distinct appearance of a walnut.

The avalanches are "something we never expected to see on Iapetus," said Singer, a graduate student in earth and planetary sciences at Washington University in St. Louis and lead author of a paper published today in Nature Geoscience.

Image: Kerry Sieh / USGS

These icy landslides are similar to long-runout landslides on Earth known as sturzstroms (German for fallstreams), which can travel a distance equal to 20 to 30 times the height they fall from. Normal landslides typically only travel twice the height they fall from.The Iapetus landslides were probably triggered by objects impacting the moon's surface.

A famous example of a sturzstrom is the prehistoric Blackhawk landslide in southern California. This type of landslide can cover plains for tens of miles. "If you've got a house out in the plains, you kind of want to know that," said study co-author Paul Schenk, a planetary geologist at the Lunar and Planetary Institute in Houston.

Scientists don't agree on what mechanism allows them to travel so far, but there are several candidates including riding on a cushion of trapped air, sliding on groundwater or mud, sliding on ice, or slipping caused by strong acoustic vibrations. Singer suspects that on Iapetus, which lacks an atmosphere or groundwater, the landslides occur by frictional heating of the ice. "We're able to do an experiment that we can't do on Earth, because of the conditions," said Singer.

Singer's team analyzed images taken by NASA's Cassini spacecraft as it orbited Saturn in September 2007 and December 2004. By measuring the ratio of the landslide's vertical to horizontal motion, they estimated the friction involved. The height-to-length ratios suggested that friction was "flash heating" the ice until it was slippery enough to slide, without fully melting.

"Everyone knows that ice is slippery," Singer said, but "it's not settled scientifically exactly why." It could involve a phenomenon known as pre-melting, where only a thin layer of ice crystals melts. Because Iapetus is so cold, its ice acts much like rock does on Earth. So a similar flash heating mechanism might explain rocky landslides.

"This kind of landslide has been seen on every single body in the solar system," said geologist Jay Melosh of Purdue University, who was not involved in the study. Melosh favors the sound wave-induced model of long-runout landslides, but he called Singer's study "an important contribution to helping pin down the mechanism."