Giant ice avalanches discovered on Saturn’s moon Iapetus provide clue to extreme slippage elsewhere in the Solar System.
“We see landslides everywhere in the Solar System,” said Kelsi Singer, graduate student in earth and planetary sciences at Washington University in St. Louis and lead author of the study published in Nature Geoscience, “but Saturn’s icy moon Iapetus has more giant landslides than any body other than Mars.”
“The reason 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,” said William McKinnon, professor of earth and planetary sciences at Washington University in St. Louis.
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-runout rock landslide, or sturzstrom (German for “fallstream”). Most landslides travel a horizontal distance that is less than twice the distance the rocks have fallen.
On rare occasions, however, a landslide will travel 20 or 30 times farther than it fell, traveling for long distances horizontally or even surging uphill. These extraordinarily mobile landslides, which seem to spill like a fluid rather than tumble like rocks, have long mystified scientists.
The mechanics of a normal runout 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 runouts, some other mechanism must be invoked as well. Something must be acting to reduce friction during the runout,” Singer said.
The trouble is, there is no agreement about what this something might be. 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,” Prof McKinnon joked.
“The landslides on Iapetus are a planet-scale experiment that we cannot do in a laboratory or observe on Earth,” 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.”
“If the Iapetian surface locked in place before it could spin down to a sphere, there must be stresses in its surface,” Prof McKinnon reasoned. So he suggested Singer check the Cassini images for stress fractures in the ice. She looked carefully at every Cassini image and didn’t find much evidence of fracturing. Instead, she kept finding giant avalanches.
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 runout did not find trends consistent with some of the most popular theories for the extraordinary mobility of long-runout landslides.
The scientists said data can’t exclude them, however. “We don’t have the same range of measurements for the Iapetian avalanches that is available for landslides on Earth and Mars,” Singer said.
Bibliographic information: Singer KN et al. 2012. Massive ice avalanches on Iapetus mobilized by friction reduction during flash heating. Nature Geoscience 5, 574–578; doi: 10.1038/ngeo1526