Clues from Saturn moon avalanches

Giant avalanches on an icy moon of Saturn may provide clues about devastating landslides on Earth, say scientists.

Images from the American space agency Nasa's Cassini spacecraft revealed 30 massive ice falls on Iapetus, a walnut-shaped moon girdled by steep 12-mile high mountains.

In 17 cases the avalanches plunged down crater walls, while another 13 swept down the sides of the equatorial mountain range.

Scientists identified a strange feature of the avalanches. At high speeds, the falling ice began to behave like a liquid, travelling many miles before finally coming to rest. The research is published in the journal Nature Geoscience.

Experts are still trying to explain a similar phenomenon seen in landslides and earthquakes on Earth.

"Long runout" rock landslides sometimes travel 20 or 30 times further than they fall, covering long distances horizontally or even surging uphill. In the same way as the Iapetus avalanches, they appear to spill like a fluid rather than tumble.

US researcher Kelsi Singer, from Washington University at St Louis, who studied the Cassini images, said: "We see landslides everywhere in the Solar System, but Saturn's icy moon Iapetus has more giant landslides than any body other than Mars.

"The landslides on Iapetus are a planetary-scale experiment that we cannot do in a laboratory or observe on Earth. They give us examples of giant landslides in ice, instead of rock, with a different gravity, and no atmosphere. So any theory of long runout landslides on Earth must also work for avalanches on Iapetus."

Laboratory tests suggest that "flash heating" caused by friction may make rocks, or cold ice, slippery. Very cold ice, as occurs on Iapetus, behaves like beach sand.

Professor William McKinnon, also from Washington University, said: "You might think friction is trivial, but it's not. And that goes for friction between ices and friction between rocks. It's really important not just for landslides, but also for earthquakes and even for the stability of the land. And that's why these observations on an ice moon are interesting and thought-provoking."