July 30, 2012
Iapetus Ice Avalanches Help Scientists Understand Landslides Elsewhere
Lee Rannals for redOrbit.com - Your Universe Online
Researchers from Washington University in St. Louis, Missouri have found that giant ice avalanches on Saturn's moon Iapetus could provide clues to slippage in other places in the Solar System.
Kelsi Singer, graduate student in Earth and planetary sciences in Arts & Sciences at the university, says that landslides take place everywhere in the Solar System, but Iapetus has more giant landslides than any celestial body in our neighborhood other than Mars.
“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," William McKinnon, PhD, professor of Earth and planetary sciences, said. “So there´s a lot of topography and it´s just sitting around, and then, from time to time, it gives way."
As the landslides fall and reach high speeds, somehow the 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 a rest.
The researchers describe these giant ice avalanches in the July 29th issue of Nature Geoscience.
The Saturn moon's ice avalanches are considered to be larger than they should be given the forces scientists think set them in motion and bring them to a halt.
On Earth, landslides travel at a horizontal distance that is less than twice the distance the rocks have fallen, and it can travel 20 to 30 times farther than it fell. These landslides spill like a fluid rather than tube like rocks.
The debris from these landslides travel 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.
In order to try and explain the long run outs, something must be acting to reduce friction during the runout, according to Singer.
Theories so far include a cushion of air, or lubrication by water, rock flour or a thin melted layer.
“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.”
McKinnon has been studying Iapetus since the Cassini spacecraft flew by it in December 2004 and September 2007, adding weight to the evidence that the Saturn moon is in fact weird.
The moon 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. Iapetus has an extremely tall, razor-straight mountain range that wraps most of the way around its equator.
If the moon's surface locked in place before it could spin down to a sphere, McKinnon then believes there must be stresses in its surface. He put Singer on the case.
She looked through every Cassini image of Iapetus, and didn't find much evidence of fracturing, but did keep finding giant avalanches.
She eventually identified 30 massive ice avalanches in the Cassini images, including 17 that plunged down crater walls and another 13 that swept down the slides of the equatorial mountain range.
The team was unable to find consistent trends with some of the most popular theories for the extraordinary mobility of long-runout landslides. However, the scientists believe data cannot exclude those theories.
“We don´t have the same range of measurements for the Iapetian avalanches that is available for landslides on Earth and Mars,” Singer said.
The researchers have determined that the coefficient of friction of the avalanches is not consistent with the coefficients of friction of very cold ice measured in the laboratory. McKinnon said that really cold ice debris is just as frictional as beach sand.
The scientists set out to determine whether rapid motion would make even super-cold ice slippery.
“If you had some kind of quick movement, whether it was a landslide or the slip along a fault, the same kind of thing could happen,” Singer said.
She said that geologists now realize that major faults are weaker during earthquakes than laboratory measurements of rocks' coefficients of friction suggest they should be.
No one is sure what helps lubricates the faults where they are jolted into motion by an earthquake, but one of the simplest hypothesis is something called flash heating, according to Singer.
Flash heating is when the rocks slide past one another, and tiny contact points on their surfaces are heated by friction. Above a critical speed, the heat would not have time to escape the contact points, which would be flash-heated to temperatures high enough to weaken or even melt the rock.
This weakening might explain high slip rates, and large sliding displacement characteristics of earthquakes.
“You might think friction is trivial,” McKinnon said, “but it´s not. And that goes for friction between ices and friction between rocks. It´s really important not just for landslides, but also for earthquakes and even for the stability of the land. And that´s why these observations on an ice moon are interesting and thought-provoking.”