The Aorounga impact structure in northern Chad is 12.6 kilometers (7.8 miles) in diameter, large enough to display a central peak and more than one concentric ring. This eroded remnant of a crater (top photo) in Africa’s Sahara Desert is estimated to be less than 345 million years old. Astronauts have photographed it several times over the years, most recently in 2009 and in 2011. But it’s not the crater itself that caught our eyes this time. It is the sandy landscape around it.
Images of the central Sahara Desert taken from the International Space Station (ISS) often include sand dunes. The middle and lower images above feature a series of horn-shaped “barchan” dunes clustered in a narrow corridor between lines of dark-toned hills. (The white box in the top image marks the area depicted in the closeups.) The horns point in the direction of dune migration; that is, from north-northwest to south-southeast under the influence of the prevailing winds. Thick zones of rippled, light-toned dunes are visible at the bottom of the images, with dry river channels in the center and upper parts.
Dunes 3 and 5 have moved furthest, following the well-known phenomenon that smaller dunes move faster. The smallest dunes move so fast that they cannot be tracked over a decade—because they are absorbed by larger dunes; because they move into hilly terrain and break up; or because newer, small dunes are shed from the horns of the larger dunes.
The opportunities for comparisons of the landscape are growing with each new year that we send humans and satellites into space. The imagery is quite valuable to remote-sensing scientists. For example, where large masses of sand move across highways or into farm fields, as is common on the edges of deserts, they cause great environmental damage and cost. With comparative imagery, it is now possible to predict when dunes are likely to cause such damage so that mitigation efforts can be put in place.
Astronaut photograph ISS034-E-70070 was acquired on January 6, 2013, with a Nikon D3X digital camera using an 800 millimeter lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 34 crew. It 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, Jacobs 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.