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. WUSTL planetary scientists speculate that steep topography on the ice moon allows the ice to pick up enough speed to become slippery, even though temperatures on Iapetus are in liquid-nitrogen territory.
Iapetus’ spectacular topography not only is surprising, 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. 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.
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. Scientists 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 runout did not find trends consistent with some of the most popular theories for the extraordinary mobility of long-runout landslides.