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The power of the wind

Last but not least, dark dune fields in the left half of the picture (color image) testify to the power of the wind and its ability to transport grains of sand over long distances and around obstacles. The grayish-blackish dune sands, which are very common on Mars, are of volcanic origin. This means that they consist mainly of old, temporarily buried volcanic ash, which was often brought to the surface by impacts from the Martian subsurface. In addition, these sands often contain fragments of crushed lava rock and volcanic glass. All these materials consist of dark, volcanically formed minerals, which were not converted to lighter minerals by the temporary covering in the Martian underground as elsewhere by water, which explains the blackish color of the dune sands.

 The types of dunes also make it easy to determine the direction of the wind that has shaped the dunes: In this example you can clearly see isolated sickle dunes (so-called barchane), which have grown together at the end of the valley to form a barchanoidal dune field. In this case, the wind came from the southeast (bottom left of the picture color image), blew into the valley, drove the sands in front of it and deposited most of the material at the foot of the mountains.