Back-and-forth blinking of this two-image animation shows movement of ripples covering a sand dune on Mars. The images are part of a study published by Nature on May 9, 2012, reporting movement of Martian sand dunes at about the same flux (volume per time) as movement of dunes in Antarctica on Earth. Image Credit: NASA/JPL-Caltech/Univ. of Arizona/JHU-APL