When the rimwall of Iapetus' 5-mile-deep Malun crater broken off,… (NASA/JPL/Space Science…)
Poring over images of Saturn's icy moon Iapetus, planetary scientists have discovered massive landslides in which the falling ice travels much farther than should be possible given the coefficient of friction of the falling ice. In one spectacular case in the moon's Malun crater, ice broke off the wall of the 5-mile-deep crater and surged 22 miles across the crater floor -- an unusually long distance. Given that cold ice has a relatively high coefficient of friction, such long distances should not be possible unless there are forces at work that researchers don't yet know about, said planetary scientist William McKinnon of Washington University in St. Louis, who led the team studying the landslides. One possibility is that the falling ice creates friction that melts a thin layer of ice that lubricates the falling ice. A similar mechanism might be at work in earthquakes, he said, and he called on experimental physicists to check out the possibilities.
Iapetus is an unusual moon. It has a pronounced bulge at the equator, possibly because it was spinning faster when it froze solid. It also has a mountain range that is as much as 12 miles high girdling much of the equator, making the body look much like a giant walnut. Impact basins are also very deep.
Studying the Cassini images, McKinnon and his team reported in the journal Nature Geoscience, they identified 30 massive ice avalanches -- 17 that had plunged down crater walls and 13 that had swept down the sides of the mountain range. Careful measurements of the heights from which the ice had fallen and the distance of the runout were not consistent with the most popular theories for the extraordinary mobility of long-runout landslides, but didn't exclude them either, McKinnon said.
Long-runout landslides occur occasionally on Earth and have mystified researchers, he noted. Normally, a landslide travels horizontally less than twice the distance the rocks have fallen. But occasionally, in what is known as a "sturzstrom" or fallstream, they will travel 20 to 30 times farther than they fell, moving more like a fluid than rocks.
McKinnon noted that a similar effect occurs during earthquakes: the apparent friction between sliding rocks is reduced. One possibility, he suggests, is that rocks are heated at points of contact and melt, lubricating the sliding surface. The same thing could happen with ice on Iapetus. It's now up to experimentalists to prove it, he added.
[Updated: 12:05 pm. The original headline of the story wrongly called Iapetus a moon of Jupiter.]