 Nili Patera is an ancient martian volcano with a 2-kilometer deep caldera. Wind-swept dunes&amp;nbsp;constantly shift across the surface of the caldera floor as seen in this HiRise image from the Mars Reconnaissance Orbiter. It's&amp;nbsp;a detail from a larger image scientists use to monitor the dune dynamics.&amp;nbsp; Credit:  Nasa/JPL-Caltech/University of Arizona   