This HiRISE image shows dark dunes and light polygonal terrain in Olympia Undae, also known as the North Polar Erg.
Two sets of dunes are obvious. The major set trends ~north-south, indicating winds from the east or west. Between the crests of these dunes is a second set oriented mostly east-west.
Zooming in on the dunes, a rippled texture is apparent, probably due to redistribution of sand at the scale of meters and less. Near the crests of some dunes are channel-like features, with some branching downslope. The origin of these channels is unknown, but they may result from the flow and displacement of sand that was fluidized by sublimating carbon dioxide or water frost.
Bright patches of ground are found in some inter-dune areas, with many having a polygonal texture. Polygons on Earth form from contraction induced by stresses from dehydration, cooling, and other processes, so these features may have a similar origin. The CRISM instrument on MRO and OMEGA on Mars Express indicates that many dunes in Olympia Undae are rich in the mineral gypsum.
Image PSP_001736_2605 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on 09-Dec-2006. The complete image is centered at 80.2 degrees latitude, 191.2 degrees East longitude. The range to the target site was 316.4 km (197.7 miles). At this distance the image scale is 31.6 cm/pixel (with 1 x 1 binning) so objects ~95 cm across are resolved. The image shown here has been map-projected to 25 cm/pixel.
The image was taken at a local Mars time of 02:37 PM and the scene is illuminated from the west with a solar incidence angle of 69 degrees, thus the sun was about 21 degrees above the horizon. At a solar longitude of 147.9 degrees, the season on Mars is Northern Summer.