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Giant claw marks on Mars dunes? Blocks of dry ice to blame, study says.

Experiments with dry ice in Utah supported the theory that the narrow gullies spotted on some Martian dunes were caused by the annual spring thaw of chunks of frozen carbon dioxide.

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Another member of the team reporting the new results, Candice Hansen of the Planetary Science Institute in Tucson, has been studying the effects that the appearance and disappearance of seasonal CO2 ice have on the Martian landscape, including the spiders. She also proposed that the gullies were formed by something rolling downhill – perhaps blocks of CO2 ice.

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During a visit to Utah, where Dr. Hansen lives, Diniega stopped by to talk about the observations. The two researchers decided to stop at a local grocery store for some dry ice, then visit some small dunes nearby to take the ice for a test drive to see if the idea was plausible.

As suspected, the ice slid easily down the gentle dunes. So Diniega conducted two more sets of tests, making more-rigorous measurements, at dune fields in southern Utah and at Kelso Dunes, the largest dune field in southern California's Mojave Desert. The hypothesis held up. Some ice blocks never stopped until they reached the bottom or hit a bush.

What was happening? The warm dry sand was melting the underside of the ice to generate a cushion of CO2 gas underneath. A bit like air under a hovercraft, the layer of gas beneath the ice in effect greased the skids for the downhill run, even on gentle slopes. As the slabs of ice traveled, they pushed sand aside until they stopped, leaving an abrupt end to the mini-gullies they formed.

Modeling work performed by Planetary Science Institute researcher Jim McElwaine found that the same process would work, even at the much colder temperatures on Mars. [Editor's note: The original version of this story did not include this paragraph.]

"We have seen blocks of ice sitting in the channels in our HiRISE images," said University of Arizona planetary scientist Alfred McEwen, who also is the lead scientist for HiRISE, which took the images, in a prepared statement. "Later, we saw them disappear by sublimation, in a matter of months."

During sublimation, ice morphs from solid to gas without passing through a liquid state.

It's unclear whether the experiment will spawn a new sport – dry-ice races. During the experiment in southern California, a number of Diniega's JPL colleagues were present and found themselves cheering on various blocks of ice as they slid and pirouetted down the dune slopes.

But the team will continue to scan HiRISE images for more evidence of ice carving gullies in dune slopes to continue to build their case.

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