Image caption Mystery still surrounds the formation of Iapetus' imposing equatorial ridge

Saturn's moon Iapetus frequently plays host to a huge type of landslide or avalanche that is rare elsewhere in the Solar System, scientists report.

Sturzstroms or "long-runout landslides" move faster and farther than geological models predict they should.

They have been seen on Earth and Mars, but there is debate about their causes.

Now, images from the Cassini space mission, reported in Nature Geoscience , suggest that heating of icy surfaces helps the landslides keep going.

On Earth, landslides typically travel a horizontal distance that is less than twice the distance that the material has fallen.

Long-runout landslides, by contrast, can travel as much as 30 times the vertical falling distance.

A great many mechanisms have been proposed to explain this phenomenon, ranging from simple sliding on ice to the sound waves from the slide making rock and debris behave more like a fluid.

But there is little consensus on which of these theories, if any, is correct.

Now, Kelsi Singer of Washington University in St Louis, US, and colleagues report that the geography of Iapetus is a unique setting to test these theories.

Iapetus Fatter at equator than at its poles; encircled by huge equatorial ridge

The ridge - a chain of mountains wrapped around Iapetus - reaches up to 20km high

One side of Iapetus is covered in dark material, while the other side is much brighter

As a result, it has been described as the "yin and yang" of Saturn's moons

Discovered by Giovanni Cassini in 1671; Cassini also notices dark-light difference See Sir Patrick Moore viewing Cassini-Huygens's first images of Saturn's moons

"The landslides on Iapetus are a planet-scale experiment that we cannot do in a laboratory or observe on Earth," Ms Singer said.

"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."

Iapetus is a geologically interesting place to look; it is a squashed sphere, fatter at its equator than its poles, and is mostly encircled by a ridge that reaches peaks some 20km high.

It also has a number of giant impact craters reaching depths of 25km.

The icy satellite has more giant landslides than any Solar System body other than Mars. The reason, says Prof William McKinnon, also from Washington University, is Iapetus' spectacular topography.

"Not only is the moon out-of-round, but the giant impact basins are very deep, and there's this great mountain ridge that's 20km (12 miles) high, far higher than Mount Everest," he explained.

"So there's a lot of topography and it's just sitting around, and then, from time to time, it gives way."

Ms Singer was looking for stress fractures in the moon's ice, but instead found evidence of 30 massive landslides - 17 along crater walls and 13 along the giant equatorial ridge.

Analysis of the images from these events suggests that the "coefficient of friction" - a measure of how much the slip-sliding of material in a landslide tends to slow it down - on Iapetus is far lower than expected for ice.

It appears that this faster-moving ice seen on Iapetus has a lower friction coefficient than that of slow-moving ice measured in Earth-bound laboratories.

The team suggests that the tiny contact points between bits of ice debris in such a landslide may heat up considerably, melting it and forming a more fluid - and thus less friction-limited - mass of material.

They suggest that physicists here on Earth test the idea in the laboratory, giving insight not only into what is happening on Iapetus, but closer to home as well.