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File NameFile Size (bytes)WidthHeightAnnotatedCroppedPurposeComments
View ISS017-E-20538.JPG 69409640437 No No
View ISS017-E-20538.JPG 288831540404 Yes Yes NASA's Earth Observatory web site
View ISS017-E-20538.JPG 79844930722098 No No
View ISS017-E-20538.JPG 9197911000748 No Yes NASA's Earth Observatory web site

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Mission: ISS017 Roll: E Frame: 20538 Mission ID on the Film or image: ISS017
Country or Geographic Name: LIBYA
Center Point: Latitude: 22.1 Longitude: 23.7 (Negative numbers indicate south for latitude and west for longitude)

Stereo: (Yes indicates there is an adjacent picture of the same area)


Camera Tilt: 8
Camera Focal Length: 400mm
Camera: N2: Nikon D2Xs
Film: 4288E : 4288 x 2848 pixel CMOS sensor, RGBG imager color filter.


Film Exposure:
Percentage of Cloud Cover: 10 (0-10)


GMT Date: 20081021 (YYYYMMDD) GMT Time: 130556 (HHMMSS)
Nadir Point Latitude: 21.8, Longitude: 23.4 (Negative numbers indicate south for latitude and west for longitude)

Nadir to Photo Center Direction: Northeast
Sun Azimuth: 237 (Clockwise angle in degrees from north to the sun measured at the nadir point)
Spacecraft Altitude: 189 nautical miles (350 km)
Sun Elevation Angle: 36 (Angle in degrees between the horizon and the sun, measured at the nadir point)
Orbit Number: 841


Arkenu Craters 1 and 2, Libya

Geologists often study features on Earth, such as impact craters, to gain insight into processes that occur on other planets. On Earth, more than 150 impact craters have been identified on the continents, but only a few of these are classified as double impact craters. One such pair, the Arkenu Craters in northern Africa, is shown in this image. Arkenu 1 and 2 are double impact structures located in eastern Libya in the Sahara Desert (22 degrees, 4 minutes North; 23 degrees, 45 minutes East). Their respective diameters are approximately 6.8 and 10.3 kilometers (4.2 and 6.4 miles). The craters are unusual in that they both exhibit concentric annular ridge structures (white circles in the image outline the position of the outermost visible ridges). In many terrestrial craters these features are highly eroded and no longer visible.

While the circular structure of these features had been noted, the impact origin hypothesis was strengthened in December 2003 when a field team observed shatter cones—cone-shaped features in rocks created by the shock generated during impact. The field research team also observed large outcrops of impact breccias—a jumble of rock fragments generated at the impact site that are now cemented together into an identifiable rock layer. One theory of the craters' formation proposes they were the result of two impactors, each approximately 500 meters in diameter. The age of the impact event has been dated as having occurred less than 140 million years ago.

While the presence of shatter cones and impact breccias is generally considered to be strong evidence for meteor impact, some scientists now question the interpretation of these features observed at the Arkenu structures. They suggest that the features were caused by erosive and volcanic processes. At present, both craters are being crossed by linear dunes extending northeast-southwest (image upper left to center right). The superposition of the dunes across the annular ridges indicates that they are much younger than the craters.

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