This animation flips back and forth between views taken in 2010 and 2014 of a Martian sand dune at the edge of Mount Sharp, documenting dune activity.
The images are from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. They cover an area about 740 feet (about 225 meters) wide, showing a site called "Dune 2" in the "Bagnold Dunes" dune field. NASA's Curiosity Mars rover will observe this dune up close on the rover's route up Mount Sharp.
North is toward the top. The edge of the dune at the crescent-shaped slip face on the south edge advances slightly during the four-year period between the dates of the images. Figure A is an annotated version with an arrow indicating the location of this change. The lighting angle is different in the two images, resulting in numerous changes in shadows.
The "before" image is part of HiRISE observation ESP_018854_1755, taken Aug. 4, 2010. The "after" image is part of HiRISE observation ESP_039280_1750, taken Dec. 13, 2014.
An earlier example of dune activity elsewhere on Mars is documented at: http://photojournal.jpl.nasa.gov/catalog/PIA15295.
The University of Arizona, Tucson, operates HiRISE, which was built by Ball Aerospace & Technologies Corp., Boulder, Colorado. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter Project and Mars Science Laboratory Project for NASA's Science Mission Directorate, Washington.