Cassini spots runaway landslides on Iapetus
Sen—Giant landslides observed on Saturn’s moon Iapetus by NASA's Cassini space probe are causing quite a stir because they travel much further than they should.
The avalanches are the second largest seen in the Solar System - the biggest were observed on Mars. This is due to a combination of extremely deep impact basins, as well as a 20 km high mountain ridge, which is more than twice the height of Mount Everest.
Iapetus is an icy moon shaped like a squashed sphere that most likely froze in place when it was spinning much faster. If it did freeze into its walnut shape before it could become spherical, then there should be a lot of stress on the ice. Cassini images did not show stress fractures, however, but instead showed plentiful landslides. Around 30 landslides were found along crater walls and the giant mountain ridge that surrounds the moon.
The avalanches of ice on Iapetus are larger than predicted as the level of resistance that exists between the ice and the surface it is moving over - something scientists term the "coefficient of friction" but basically how slippery it is - seems mysteriously to drop, so that the ice will travel for several kilometres horizontally after it crashes to the ground.
The ice almost seems to move as if it is flowing rather than tumbling. The coefficient of friction ranges from almost zero to greater than one. Very cold ice should have a high level of resistance, so that it isn’t very slippery. However, the ice in the Iapetus avalanches seems to have much less friction when it is sliding against the ground.
A similar situation can occur on Earth occasionally in what is known as a sturzstrom. While most landslides will travel horizontally for around twice the distance of the falling rocks, until the friction between the debris and the ground causes it to stop, these particular landslides will travel up to 30 times further.
“The landslides on Iapetus are a planet-scale experiment that we cannot do in a laboratory or observe on Earth,” said Kelsi Singer from Washington University. “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.”
It is possible that the high speed of the ice could change the coefficient of friction. For the sturzstrom on Earth it is thought that the high speed causes friction between the rocks within an avalanche, until they are heated in a process known as flash heating. This heat can melt rock and could help explain why large displacements of land occur during earthquakes.
“You might think friction is trivial,” said William McKinnon, also from Washington University, “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.”