Both Earth and Mars have atmospheres that can mobilize particles to form sand dunes. This image is from the caldera of an inactive Volcano (Nili Patera) on Mars. The steep avalanche face on the downwind side of the dunes indicates wind direction (see arrow). There are several types of sand dunes in this image, some of which have not been previously recognized on Mars.

(a) Lateral dune.
(b) Climbing dune.
(c) Linear extension from a transverse dune.
(d) Linear dune supplies sediment to barchanoid ridge (d') on the downwind surface.
(e) Transverse aeolian ridges (they may be mega ripples) are being buried by larger dunes.
(f) Barchan dunes.

Despite the presence of a 100 m deep double-trough, the sand transport pathway is not significantly interrupted. The march of the dunes across the trough may be aided by the tendency for wind speed to accelerate as it meets vertical obstacles. PSI's Mary Bourke and collaborators at John Hopkins University and University of Loughborough, UK, have modeled wind as it interacts with troughs on Mars and demonstrated that, under Martian conditions, wind speeds up by 30% at the downwind trough walls. (Bourke, M.C., J. Bullard, and O. Barnouin-Jha, in press.)

The Mars Orbital Camera Image is from the Malin Space Science Systems site. Image number: M17-00435, (8.99°N, 293.29°W, 5.84 m/pixel).