Dune gullies in the Matara Crater on Mars change year after year: Between 2008 and 2010 (top to bottom), the alcoves at the dune's crest and the channel beds widened during the Martian winter as material moved down the slope. New deposits are visible in the bottom image as the darker material extending from the channels. Material can be seen filling the down-slope portion of the upper channel, only to be carved out the following year. Credit: NASA/JPL-Caltech/The University of Arizona