THEMIS Image of the Day, March 19, 2019. This VIS image shows part of the floor of an unnamed crater in Noachis Terra. Individual dunes and a large sand sheet with surface dune morphology fill a large area of the crater floor.
The amount of sand controls the formation of dunes. Regions with only a limited amount of sand will contain small individual dunes. As more sand enters a region the dunes will grow larger. When large amounts of sand fill a region, individual dunes will coalesce into larger and larger dunes, eventually completely covering the surface as a sand sheet with dune crest features on the top. Dunes often occur in craters on Mars.