Looking almost straight down onto the Sahara Desert, a crew member aboard the International Space Station took this late afternoon photograph of the Grand Erg Oriental. Astronauts have a unique vantage point from which to view the large areas of Algeria and Libya covered by seas of sand.
Winds have organized vast quantities of sand into straight lines in what geologists call “compound linear chains.” The chains are about 1.5 kilometers (0.9 miles) wide in this view and rise 150 meters (500 feet) above the smooth, intervening flats. The compound chains are made up of numerous smaller linear dunes with sharply defined crests (also known as seif dunes, after the Arabic word for sword). Linear dunes sometimes converge to a point with long tentacle-like arms called star dunes. An astronaut snapped a photo of individual, well-developed star dunes just 20 seconds after this photo was taken. (Click here to view it.)
Geologists now know that different wind patterns are responsible for different dune shapes. Winds that blow from one direction build linear dunes and, ultimately, chains like those in this image. The slight variation in wind direction pushes sand from one side of the dune and then from the other, making the sharp crests of the small linear dunes. These winds also stretch out the dune in the average direction of the winds (southward in this part of Algeria).
By contrast, winds that blow with roughly equal strength from several directions make the star dunes. This suggests that the wind regime changed with time, first building the chains over a long period of time, and then becoming more multidirectional, so that the star dunes formed on top of the chains.
Transverse dunes forms at right angles to the wind direction. Relatively small transverse dunes can be seen at many points in this image. They were made by north winds channeled by the chains, especially along the outer flanks and in hollows within the chains.
Astronaut photograph ISS046-E-48626 was acquired on February 26, 2016, with a Nikon D4 digital camera using an 1150 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by a member of the Expedition 46 crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, Texas State University, Jacobs Contract at NASA-JSC.
One of the main reasons that rainless regions like the Sahara Desert are interesting from the perspective of landscape science is that the work of flowing water—mainly streams and rivers—becomes less important than the work of wind. Over millennia, if enough sand is available, winds can generate dunes of enormous size, arranged in regular patterns. Long, linear dunes stretch generally north to south across much of northeast Algeria, covering a vast tract (~140,000 square kilometers) of the Sahara Desert known as the Erg Oriental. Erg means “dune sea” in Arabic, and the term has been adopted by modern geologists. Spanning this image from a point on the southwest margin of the erg (image center point: 28.9°N 4.8°W) are a series of 2-kilometer-wide linear dunes, comprised of red sand. The dune chains are more than 100 meters high. The “streets” between the dunes are grayer areas free of sand.