ISS018-E-014770 (20 Dec. 2008) --- Sand dunes in the Marzuq Sand Sea, southwest Libya are featured in this image photographed by an Expedition 18 crewmember on the International Space Station. This detailed view, taken from low Earth orbit, shows classic large and small sand masses of the Central Sahara where wind is a more powerful land-forming agent than water. "Draa" dunes (from the Arabic for "arm") are very large masses of sand and appear here as the broad network of yellow-orange sand masses (the image covers a region approximately 9.4 kilometers wide), with smooth-floored, almost sand-free basins between them.
These sand masses lie in the western part of Libya's vast Marzuq Sand Sea (greater than 60,000 square kilometers, centered at 24.5N 12W). Geologists think that the draa of the Marzuq have probably been formed by winds different from the dominant north/northeast winds of today. Numerous smaller dunes can be seen developed on the backs of the draa. Three distinct dune types can be identified: longitudinal dunes (formed essentially parallel with formative winds from the north); transverse dunes, usually more curved, formed at right angles to the formative wind; and star dunes, in which several linear arms converge towards a single peak.
The upwind side of the sand masses appears smoother than the more rippled downwind side. Wind is moving sand grains almost all the time. This means that the draa and the dunes are all moving -- as sand is added on the upwind side and blown off the downwind side. It is well known that small sand masses move much faster than large sand masses. This means that the draa are almost stationary, but that the smaller dunes are moving relatively quickly across their backs. When the dunes reach the downwind side of the draa they are obliterated, their sand being blown across the basins as individual grains.