This undated image provided by NASA shows a rippled dune, front, in Herschel Crater on Mars that moved an average of about two yards between March 3, 2007 and December 1, 2010. NASA's Mars Reconnaissance Orbiter (MRO) show sand dunes and ripples moving across the surface of Mars at dozens of locations and shifting up to several yards. These observations reveal the planet's sandy surface is more dynamic than previously thought. (AP Photo/NASA)