Iapetus, one of dozens of moons that orbit the planet Saturn, is known for having one hemisphere that is much darker than the other and for the equatorial bulge that makes it resemble a walnut more than a sphere.
But it also has some of the largest and most numerous landslides that have been found anywhere in the solar system — with the exception of Mars — in part because it has such rugged terrain.
The moon, which is estimated to be about three-quarters ice and one-quarter rock, has impact craters as deep as 25 kilometres and a mountain ridge that reaches as high as 20 kilometres in parts circling almost its entire equator.
"There's a lot of topography, and it's just sitting around, and then, from time to time, it gives way," says William McKinnon, a professor in the department of earth and planetary sciences at Washington University in St. Louis, Mo., in a news release.
Icy avalanches display little friction
McKinnon and graduate student Kelsi Singer studied about 30 avalanches that occurred within Iapetus's craters and along its equatorial ridge by examining images of the moon's surface captured by the Cassini spacecraft, which was sent to Saturn in 2004 to study the planet and its satellites.
They found that the sliding ice wasn't behaving as expected. As it fell from large heights, it sped up and began acting more like a fluid than a solid, flowing — rather than tumbling — down the ridge or crater and travelling a much longer distance than expected before coming to a stop.
When the researchers measured the heights and lengths of the landslides in order to get an idea of the frictional forces acting on the avalanche debris, they found the friction coefficient, a ratio of the relative friction between two surfaces in contact with each other, to be lower than that usually seen for static or sliding rock and ice.
Normally, very cold ice is "as frictional as beach sand," said McKinnon.
When measured in the lab, ice at temperatures that fall within the range seen on Iapetus (–198 C to –173 C), and subjected to the same pressures that a landslide would impart, has a friction coefficient of between 0.55 and 0.7. But the Iapetus avalanches showed coefficients of only 0.1 to 0.3.
This suggested to the researchers that something was acting to reduce the friction of the avalanche ice.
Unusual landslides travel long distances horizontally
The slippery avalanches the researchers saw on Iapetus resemble a type of rock slide seen on Earth called a sturzstrom, or long-runout landslide. These unusual landslides travel 20 to 30 times farther horizontally than they fall vertically and sometimes even surge uphill.
Normally, a landslide will travel a horizontal distance that is no more than twice the distance it has fallen.
Scientists don't agree on what causes long runouts and have differing opinions on what, if anything, reduces the friction between the materials in these landslides. Some have speculated that a cushion of trapped air reduces the friction; others have suggested a lubricating effect caused by groundwater, wet debris, mud, fine particles called rock flour or a layer of melted ice.
The avalanches on Iapetus presented the researchers with a chance to test some of these theories on landslides that behave similarly to those on Earth but in a starkly different environment. .
"The landslides on Iapetus are a planet-scale experiment that we cannot do in a laboratory or observe on Earth," Singer said in a news release. "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."
Friction key to understanding landslides, earthquakes
On Iapetus, scientists could rule out water and air as friction-reducing influences since the moon has no atmosphere and is very cold.
They instead focused on the speed of the sliding ice and an effect known as shear heating.
Generally, if ice is moving quickly enough and temperatures are above –30 C, the friction of an icy surface is reduced and it becomes slippery.
Although temperatures on Iapetus are well below –30 C, McKinnon and his colleague suggest that the ice avalanches they observed, which ranged from a height of one to 12.5 kilometres and seven to 80 km in length, released enough energy as they fell to heat up the ice near the base of the landslides. That heated ice made the route slippery and spread the landslide farther than normal.
The researchers say that a similar phenomenon could be at work when faults in rocks are set in motion during an earthquake.
If the rocks slide past each other at a high enough speed, the heat generated at small contact points on the rock surfaces can't escape in time and will instead "flash heat" the rock enough to weaken or melt it, causing the large sliding displacements that occur during earthquakes.
The rocks end up having a friction coefficient that is in the same range as those measured in Iapetus's ice avalanches.
"You might think friction is trivial, but it's not," McKinnon said. "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."
The findings were published this week in the journal Nature Geoscience.