Nov. 7, 2008 -- Billowing down the cliffs of the red planet, scientists have spotted a strangely Earth-like phenomenon -- snowy avalanches near Mars' north pole.
Led by Patrick Russell of the University of Berne in Switzerland, a team of researchers first noticed the avalanches in February while combing through images from the Mars Reconnaissance Orbiter's high resolution camera, HiRISE.
"We were looking at frosted sand dunes, and caught the avalanches in action," Russell said of the images, which show large clouds of dust and snow near the base of a 500 meter-high cliff.
During the Martian winter, carbon dioxide frost and snow builds up in the north.
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On the polar ice cap, snows can reach one to two meters (three to six feet) thick, and on the cliffs that border the cap they may be 30 centimeters (11.8 inches) deep.
When the sun rises again on the cliffs in Mars' spring, the carbon dioxide turns from solid straight into a gas. On the steep cliff slope the bubbling gas could be enough to trigger an avalanche 100 meters (328 feet) wide, and roaring down at up to 50 miles per hour.
"If you were standing near the base of the slope, these things would be pretty impressive," Russell said. The team's findings are due to be published in the journal Geophysical Research Letters.
After spotting the first slide, the HiRISE camera took two more images and saw a total of nine avalanches tumbling off the polar cliffs.
"These images show that there's more activity on Mars than we may have thought," Craig Kochel of Bucknell University said. "I don't think anyone thought about this at all too much before HiRISE."
The avalanches look similar to "dry avalanches" he has seen at Alaska's Wrangell mountains, where ice and snow mix with debris and come thundering off a near-vertical rock cliff. And they too can be incredibly active; when Kochel and his research team visited the Wrangells in the summer of 2006, they witnessed almost 300 avalanches in just eight days.
The Wrangell avalanches are similar to the Martian ones in that they are both relatively dry. Unlike many avalanches on Earth that are mostly snow, they are mostly ice.
But there are important differences. For one, Martian slides tend to be mostly fine grains of dust and snow that form large clouds. The ice in the Wrangell avalanches often comes in large, heavy chunks.
Earth's gravity and thick atmosphere also mean that terrestrial avalanches are thunderous, powerful slides that can flatten trees and sturdy buildings. Martian gravity is three times weaker and its atmosphere 100 times thinner, by contrast.
"The avalanches we're seeing wouldn't have nearly as much oomph as ones on Earth," Russell said.
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Mars Reconnaissance Orbiter's high resolution camera, HiRISE
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