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Except for small wind ripples on their surfaces, normal, active sand dunes have very smooth slopes.
However, some dunes found in the Herschel Basin ( map of Mars ) of Terra Cimmeria have very rough, grooved surfaces instead. (Terra means heavily cratered highland)
These grooves indicate that the sand on these dunes are not loose. Here the wind had to scrub the sand to remove it and transport it away from these dunes.
What has caused these dunes to become cemented (not loose) is unknown. Sand dunes like this are extremely rare on Mars.
Picture by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC).
Photo Credit: NASA/JPL/Malin Space Science Systems
This picture shows part of a sand dune field that surrounds the north polar region during early spring in 1998.
Dark spots had appeared on the north polar dunes, and many of them exhibited a radial or semi-radial pattern of dark streaks and streamers.
The dark spots represent the earliest stages of defrosting on the sand dunes. The streaks are the result of wind.
In this picture, the fine, dark streaks show essentially identical orientations from spot to spot - clearly seen from the dark spots in boxes (a) and (b).
Each ray of dark material must result from wind blowing from a particular direction. These dark rays pointing in the different directions must indicate periods when the wind was strong enough to move materials, consistently, in only that one direction.
The sand that makes up the north polar dunes is dark. Each spot and streak is composed of the dune sand. The bright surfaces are all covered with frost.
Photo Credit: NASA/JPL/Malin Space Science Systems
Picture by the Mars Global Surveyor's Mars Orbiter Camera (MOC). It shows a group of sand dunes in Proctor crater. These sand dunes are created by winds.
Picture of several dark sand dunes in the Nili Patera region of Syrtis Major. Map of Mars. The shapes of these dunes indicate that wind has been steadily transporting the dark sand from the right/upper right toward the lower left.
This photo covers an area 2.1 kilometers (1.3 miles) wide.
Photo Credit: NASA/JPL/Malin Space Science Systems
The dark streaks on this photo indicate relatively recent (i.e., in the past few years or less) movement of sand down these slopes. These dunes are in the Rabe Crater.
Sand dunes move forward by two combined actions: 1 - Wind drives sand up the shallow slope on the windward side of the dune. (In this picture this is the slopes that face toward the lower right.) 2 - Avalanching of this sand down the steeper, lee-side slope.
The dark streaks marked by arrows are evidence for sand avalanches that occurred within a few months or years of the time when the picture was taken in March 1999.
Photo Credit: NASA/JPL/Malin Space Science Systems
Photo of sand dunes composed of low albedo (dark) sand grains. These dunes appear near the north polar cap of Mars. This photo was taken during the northern summer in May 1999.
Photo Credit: NASA/JPL/Malin Space Science Systems
