Monday, August 6, 2012

Meteor-Triggered Avalanches

On a moon of Saturn -- quick, someone report these to the Iapetus Avalanche Center!

Giant Icy Avalanches Seen on Saturn Moon

Iapetus landslides traveled unusually long distances, study says.

The moon Iapetus.
Saturn's moon Iapetus is seen in a 2007 Cassini image.
Image courtesy SSI/NASA

Andrew Fazekas
Published July 30, 2012
A landslide on Iapetus.
A landslide from Iapetus's Malun crater surged an astonishing 22 miles from the crater's base. Image courtesy SSI/NASA

Thirty giant icy avalanches onSaturn's moon Iapetus have been spied by NASA's Cassini spacecraft, a new study says.The events, likely triggered by large meteors, may offer a unique insight into the mechanics of landslides on Earth.
With steep crater walls and a 12-mile-high (19-kilometer-high) mountain ridge more than twice the height of Mount Everest, Iapetus has nearly a perfect setup for avalanches, according to study leader Kelsi Singer, a Ph.D. candidate in geology and geophysics at Washington University in St. Louis.
"When you look at Iapetus from space, you can clearly see the equatorial ridge sticking out, and it makes the icy moon look somewhat like a walnut." (Related: "Saturn's 'Walnut' Moon Mystery Cracked?")
The moon "has some of highest topography for its size of any major body in the solar system, and has the most landslides other than Mars," Singer said.
Analyzing the Cassini landslide images, Singer and her team noticed that icy debris falling down the crater walls and mountain ridges would travel surprisingly long distances horizontally across the terrain—sometimes 50 miles (80 kilometers), which is 20 to 30 times the height from which they fell.
"The scale is just enormous—if you were standing on the ground, you wouldn't be able to see" all of the cliff.
Flash Heating Spurs Long Landslides?
Most landslides on Earth spill out to twice the height from which they fall.
However, a less understood type of landslide called a long-runout rock landslide, or sturzstorm, does fall longer distances, behaving like those seen on Iapteus. (Watch video: Landslides 101.)
Long-runout landslides on Earth have long stumped scientists, since there should be enough friction to stop the tumbling rock or ice.
On Iapetus, scientists suspect, an unknown factor is reducing the friction of the ice avalanches.
The culprit may be a phenomenon called flash heating, during which friction from the landslide heats up the ice, making it slippery enough to speed along the rocks and debris as they fall.
"Because the material is moving very quickly, [the heat] doesn't have much time to dissipate into the surrounding material, and so the heat is concentrated in a small area—just enough heat to help make the cold, hard ice more slippery," said Singer, whose study appeared this week in the journal Nature Geoscience.
Unlocking Earth's Landslides
Singer and her team are most excited about how the Iapetus landslides will aid our understanding of similar natural events on Earth.
"Long-runout landslides are a natural hazard here on Earth that can seriously affect people if they were to occur in a populated area," she said.
"So of course we want to know more about the mechanisms that allow them to happen, and the landslides on Iapetus help narrow down the possible mechanisms."
For instance, the new study gives "extra evidence that flash heating may be at play for Earth's long-runout landslides," she said.


  1. I have suspected that the friction created in snow avalanches is the reason that the debris is so hard. Friction heats up and melts a small amount of snow and upon coming to rest, the free-water in the snow combined with the now higher density snow crystals and refreezes. Do you know of any reading that has described this process?

  2. Here's a summary as presented by McClung, p. 124:

    "At the sliding surface (and in the core), the friction is determined by particles (snowballs and crystals) interacting (colliding) with each other and the sliding surface, including the hardness of the sliding surface. This interaction takes the form of collisions and frictional rubbing between snow particles. The collision and rubbing friction causes heat to be generated on the exterior of particles, which produces small amounts of water on their surfaces. Once the deposit comes to rest, the water on article surfaces can freeze to fuse the particles together, producing a very hard deposit."