Water Flows on Mars? Not So Fast

Signs of recent liquid water on Mars may have instead been caused by mini-avalanches of dirt. In December, NASA released photos of Martian gullies that showed enigmatic tracks of material suddenly appearing in the last five years. Researchers interpreted the deposits as having possibly come from trickling water.

The problem with that theory is that such features can be reproduced in the lab by letting small particles slide away, says granular materials researcher Troy Shinbrot of Rutgers University. "You find that every single thing that geologists say, 'Ah, that means there's water,' you can duplicate," says Shinbrot, who described his research on sliding grains at this week's meeting of the American Physical Society here.