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ISS013-E-75141 (2 Sept. 2006) --- Erg Oriental, Algeria is featured in this image photographed by an Expedition 13 crewmember onboard the International Space Station. One of the main interests of rainless regions like the Sahara Desert to landscape science is that the work of flowing water--mainly streams and rivers--becomes less important than the work of wind. According to scientists, over millennia, and if enough sand is available, winds can generate dunes of enormous size, arranged in regular patterns. Long, generally north-south trending linear dunes stretch across much of northeast Algeria covering a vast tract (approximately 140,000 kilometers square) of the Sahara Desert known as the Erg Oriental. Erg means dune sea in Arabic, and the term has been adopted into modern geology. Spanning this image diagonally are a series of two kilometer-wide linear dunes, comprised of red sand, from a point on the southwest margin of the erg (center point 28.9N 4.8W). The dune chains are more than 100 meters high. The "streets" between the dunes are grayer areas free of sand. Linear dune chains are usually generated roughly parallel with the dominant winds. It also seems to be true that linear dunes are built by stronger winds. This detailed view shows that smaller dunes, known as star dunes, are built on top of the linear dunes. By contrast, star dunes seem to form in weak wind regimes, with winds from different directions in each season -- resulting in characteristic "arms" snaking away from a central point. Some scientists therefore think the dunes in this image were generated in two earlier climatic phases, different from that of today. (1) During a phase when winds were stronger and dominantly from one direction (the south), major linear sand masses accumulated. (2) Later, when wind strengths declined, the star dunes formed. Modern features--known as wind streaks--on the edge of the present erg (not shown), younger than either the linear or star dunes, show that present-day sand-moving winds blow from the southwest.
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