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Dunes on the move.

Scientists studying images from NASA's Mars Reconnaissance Orbiter have caught dunes migrating across the Red Planet's surface. Common wisdom dictates that winds in the planet's thin atmosphere can't deliver the strong, sustained push needed to get sand moving on a large scale, but the new observations add to growing evidence that this assumption is wrong.

The team used two pairs of snapshots from the orbiter's High Resolution Imaging Science Experiment (HiRISE) camera to track dune migration in the equatorial Nili Patera dune field. The measurements, which were taken 105 Earth days apart, showed that dunes moved with an average speed of 0.1 meter per Earth year. Although that migration rate is one-tenth to one-hundredth the speed of terrestrial dunes with comparable heights, the average rate of sand transport--the volume of sand moved over a certain distance in a certain time--is about equal to that of dunes in Victoria Valley, Antarctica, the team reported in the May 17th Nature.

The result shows that Martian winds can easily send sand moving across the surface. What matters is not how hard it is for winds to pick up individual grains, but what happens when the dancing grains fall back down. Sand grains can fly higher and longer in Mars's low surface gravity, gaining more speed than they would have otherwise. When they come bouncing down again, they smack into other grains and get them moving, too. Mars's low gravity makes this easier to do, explains Candice Hansen (Planetary Science Institute), who last year reported evidence for dune changes in the north polar region but was not involved with the current study.

Ultimately, after a few gusts kick up some grains, moderate winds could keep the dunes moving across the landscape. The result implies that Nili Patera could have formed in less than 10,000 years, an interpretation contrary to the long-standing idea that Mars's dune fields formed in a previous climate with a thicker atmosphere.

The new explanation for sand's migration could partially explain the existence of global-scale dust storms (page 26), explains physicist Jasper Kok (Cornell University). Data from NASA's Mars Exploration Rovers also suggest that low-density clumps of dust are easier to lift than sand, but the full answer remains elusive, he says.

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Author:Carlisle, Camille M.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Sep 1, 2012
Words:373
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