Rock and roll.

Mars may be devoid of little green men, but it does seem to be home to rocks that migrate against the wind. In simulations reported in the January issue of Geology, researchers attempted to reproduce the evenly spaced arrangements of pebbles on the Martian surface, as photographed by NASA's twin rovers. They discovered that the planet's relatively gentle breezes nudge sand from the front of the pebbles to the back, creating a downward slope. When the wind blows away enough sand--a process that can take thousands of years--the rocks roll downhill with the wind in their face. The team says the same thing probably happens on Earth, but it's difficult to observe because rainfall, plant growth, and animals disturb the patterns.
(Photo: Geological Society of America)