geolinde

Star Dunes - http://earthobservatory.nasa.gov/IOTD/view.php?id=81996

In some areas, winds tend to blow in roughly the same general direction all year. The Grand Erg Oriental, a sprawling sea of sand dunes in the Saharan Desert, is not one of them.

The winds in northeastern Algeria tend to be complex and changing. Easterly summer winds shift in the winter, becoming westerly. Meanwhile, passing storms and local geographical features further muddle the picture. If winds came consistently from one direction, crescent-shaped barchan dunes would reign. But the dominant dune type along the southern edge of Grand Erg Oriental (shown above) are large, pyramid-shaped star dunes, which only form in areas where winds blow from multiple directions.

The image was acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite on October 27, 2012. It was made from a combination of near-infrared and visible light. In this type of false-color image, sand is tan and shadows are black or gray. The blue-tinted areas are likely mineral-rich evaporites. The image is centered at 29.8°north latitude, 7.9°east longitude, near the town of Gadamis. As is common with star dunes, some of the dunes have long interlacing arms connecting to nearby dunes.

Star dunes comprise about 8.5 percent of the world’s sand dunes. Other areas they can be found include the Badain Jaran in China, the Gran Desierto de Altar in Mexico, and the eastern Rub’ al Khali in the Arabian peninsula.

Clément Narteau, a geophysicist at the Paris Institute of Earth Physics and author of a 2012 study about star dunes, noted that all of the dunes in the image have the same approximate height. “Star dunes tend to grow upward until they reach a maximum size, constant over the entire dune field,” Narteau said. “Then, they exchange sedimentary material through their radiating arms.”

There are very practical reasons for earth scientists to study how dunes form and evolve. “Understanding the dynamics of dunes is critical for developing infrastructure—such as oil and gas fields and the roads and pipelines that link them, as well as for controlling sand movement in areas where they are common,” noted Desert Research Institute researcher and dune specialist Nicholas Lancaster.

• References and Further Reading

• Goddard Earth Sciences Data and Information Services Center Geomorphology From Space: Grand Erg Oriental. Accessed September 6, 2013.
• Lancaster N. et al (1989) Star Dunes. Progress in Physical Geography, (5), 463–467. Accessed September 6, 2013.
• National Park Service Dune Types. Accessed September 6, 2013.
• USGS (1997, October 27) Types of Dunes. Accessed September 6, 2013.
• Zhang, D. et al (2012, June 24) Morphology and dynamics of star dunes from numerical modelling. Nature Geoscience, (5), 463–467. Accessed September 6, 2013.

NASA image courtesy NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. Caption by Adam Voiland.

Instrument(s): 

Terra - ASTER

This detailed astronaut photograph, taken from low earth orbit, shows classic large and small sand masses of the central Sahara Desert, where wind is a more powerful land-shaping agent than water. “Draa” dunes (from the Arabic for “arm”) are very large masses of sand, and they appear here as the broad network of yellow-orange sand masses, with smooth-floored, almost sand-free basins between them. These sand masses lie in the western part of Libya’s vast Marzuq Sand Sea (centered at 24.5 degrees north, 12 degrees east). Geologists think that the draa of the Marzuq were probably formed by winds different from the prevailing north-northeast winds of today.

Numerous smaller dunes have developed on the backs of the draa. Three distinct dune types are visible: longitudinal dunes, which are more or less parallel with the north winds; transverse dunes, which are usually more curved and formed at right angles to the wind; and star dunes, in which several linear arms converge towards a single peak.

The upwind sides of the sand masses appear smoother than the 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. Small sand masses move much faster than large sand masses. The draa are almost stationary, but the smaller dunes move relatively quickly across their backs. When the smaller dunes reach the downwind side of the draa, they are obliterated; their sand is blown across the basins as individual grains.

Astronaut photograph ISS018-E-14770 was acquired on December 20, 2008, with a Nikon D2Xs digital camera fitted with a 400 mm 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 18 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program supports the laboratory 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, NASA-JSC.

Instrument(s): 

ISS - Digital Camera