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Astronaut Photography of Earth - Display Record
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IdentificationMission: ISS018 Roll: E Frame: 14770 Mission ID on the Film or image: ISS018
Country or Geographic Name: LIBYA
Features: MURZUQ LINEAR AND STAR DUNES
Center Point Latitude: 24.5 Center Point Longitude: 12.1 (Negative numbers indicate south for latitude and west for longitude)
Stereo: (Yes indicates there is an adjacent picture of the same area)
ONC Map ID: JNC Map ID:
CameraCamera Tilt: 20
Camera Focal Length: 400mm
Camera: N2: Nikon D2Xs
Film: 4288E : 4288 x 2848 pixel CMOS sensor, RGBG imager color filter.
Percentage of Cloud Cover: 10 (0-10)
NadirGMT Date: 20081220 (YYYYMMDD) GMT Time: 131441 (HHMMSS)
Nadir Point Latitude: 23.7, Longitude: 12.9 (Negative numbers indicate south for latitude and west for longitude)
Nadir to Photo Center Direction: Northwest
Sun Azimuth: 216 (Clockwise angle in degrees from north to the sun measured at the nadir point)
Spacecraft Altitude: 188 nautical miles (348 km)
Sun Elevation Angle: 34 (Angle in degrees between the horizon and the sun, measured at the nadir point)
Orbit Number: 1786
CaptionsSand Dunes, Marzuq Sand Sea, Southwest Libya
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.
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