Evolution of North Polar Dunes (edm - natural colors; credits: Lunexit)

The circular shape of this Dunefield is interesting because the outer dunes reflect changing winds whereas the central dunes do not.
This could indicate several distinct stages in the formation of the Dunefield. On top of the dark dunes are tiny ripples similar to those seen on top of sand dunes on Earth.
It is likely that these dune ripples are active, as shown by avalanches (grainflow) from the crest of the dunes.