January 8, 2009

Study: Pebbles can move against wind

Pebbles that become part of clastic rocks in places like Arizona's Lower Antelope Canyon don't move with the wind but against it, a geosciences professor said.

Rather than being pushed into formations, the pebbles, or clasts, have the loose sand around them removed by the wind, Andrew Leier of the University of Calgary said in the journal Geology.

The sand removal causes scour-pits to form on the wind side of larger clasts, and the pebbles eventually fall toward the wind into the scours, he said. The process then repeats, he said.

Behind the larger grains, the sand is protected from the wind erosion and so sand shadows develop, Leier said.

These shadows prevent the clasts from being pushed downwind and from bunching up with one another.

The same can be said for sediment transport and surface processes on Mars's sandy surface, he said.

The U.S. National Aeronautics and Space Administration proposed that wind was moving the rocks around. But Leier, who co-authored the study with the University of Arizona's Jon Pelletier and the University of Wyoming's James Steidtmann, said that would be impossible.

The wind is less effective at moving clasts on Mars because the atmosphere is less dense, he said. And for the wind to move the rocks downwind, it would have to be moving on the order of 8,000 kilometers an hour.

That's about 5,000 mph.

The researchers found that the pebbles spread out from one another, and almost always moved into the direction of the wind, regardless of their initial configuration.