A dune is a hill of sand built by the wind. Dunes occur in different forms and sizes. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter slip face in the lee of the wind. Dunes can be found in any environment where there is a substantial atmosphere, winds, and dust to be blown. Dunes are common on Mars, and they have also been observed in the equatorial regions of Titan. Sand dunes in a vast area of northern Mars long thought to be frozen in time are changing with both sudden and gradual motions, according to research using images from a NASA orbiter. These dune fields cover an area the size of Texas in a band around the planet at the edge of Mars’ north polar cap. The new findings suggest they are among the most active landscapes on Mars. However, few changes in these dark-toned dunes had been previously detected before a campaign of repeated imaging by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter.