Iapetus is the third largest moon of Saturn, and eleventh-largest in the Solar System. it is an odd one due to its steep topography. Giant ice avalanches on Iapetus provide clues to similar extreme slippage elsewhere in the solar system. "We see landslides everywhere in the solar system," says Kelsi Singer, graduate student in earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, "but Saturn’s icy moon Iapetus has more giant landslides than any body other than Mars."
The reason for the landslides, says William McKinnon, PhD, professor of earth and planetary sciences, 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 20 kilometers (12 miles) high, far higher than Mount Everest. So there’s a lot of topography and it’s just sitting around, and then, from time to time, it gives way,"
Falling from such heights, the ice reaches high speeds — and then something odd happens in terms of how the slide material fans out.
Somehow, its coefficient of friction drops, and it begins to flow rather than tumble, traveling many miles before it dissipates the energy of the fall and finally comes to rest.
In the July 29 issue of Nature Geoscience, Singer, McKinnon and colleagues Paul M. Schenk of the Lunar and Planetary Institute and Jeffrey M. Moore of the NASA Ames Research Center, describe these giant ice avalanches.
The ice avalanches on Iapetus aren’t just large; they’re larger than they should be given the forces scientists think set them in motion and bring them to a halt.
The counterpart to the Iapetian ice avalanche on Earth is a long-run out rock landslide, or sturzstrom (German for fallstream). Most landslides travel a horizontal distance that is less than twice the distance the rocks have fallen.
When the rimwall of Iapetus’s Malun crater broke off and plunged more than five miles to the crater floor, it surged an astonishing 22 miles out from the base of the wall before finally coming to rest.
The mechanics of a normal run out are straightforward. The debris travels outward until friction within the debris mass and with the ground dissipates the energy the rock gained by falling, and the rock mass comes to rest.
But to explain the exceptionally long run outs, some other mechanism must be invoked as well. Something must be acting to reduce friction during the run out, Singer says.
Proposals have included a cushion of air, lubrication by water or by rock flour or a thin melted layer. "There are more mechanisms proposed for fiction reduction than I can put on a PowerPoint slide," McKinnon noted.
Almost everything about Iapetus is odd. It should be spherical, but it’s fatter at the equator than at the poles, probably because it froze in place when it was spinning faster than it is now. And it has an extremely tall, razor-straight mountain range of mysterious origin that wraps most of the way around its equator. Because of its stoutness and giant ridge, the moon looks like an oversized walnut.
Singer eventually identified 30 massive ice avalanches in the Cassini images — 17 that had plunged down crater walls and another 13 that had swept down the slides of the equatorial mountain range.
Careful measurements of the heights from which the ice had fallen and the avalanche run out did not find trends consistent with some of the most popular theories for the extraordinary mobility of long-runout landslides.
It is nonetheless clear that the coefficient of friction of the avalanches (as measured by a proxy, the ratio between the drop height and the run out) is not consistent with the coefficients of friction of very cold ice measured in the laboratory.
Coefficients of friction can range from near zero to greater than one. Laboratory measurements of the coefficients for really cold ice lie between 0.55 and 0.7.
"Really cold ice debris is as frictional as beach sand," McKinnon says.
The coefficients for the Iapetus avalanches, however, scatter between 0.1 and 0.3.
For further information see Landslides.
Craters image via NASA.