Earth Observations taken by the Expedition 17 Crew

ISS017-E-020538 (21 Oct. 2008) --- Arkenu Craters 1 and 2 in Libya are featured in this image photographed by an Expedition 17 crewmember on the International Space Station. 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 example, the Arkenu Craters in northern Africa, is shown in this image. Arkenu 1 and 2 are double impact structures located in eastern Libya (22.04 degrees north latitude and 23.45 degrees east longitude) in the Sahara desert, with diameters of approximately 6.8 kilometers and 10.3 kilometers, respectively. The craters are unusual in that they both exhibit concentric annular ridge structures (gray circles in the image indicate the position of the outermost visible ridges). In many terrestrial complex 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 -- conical-shaped features in rocks created by the high shock pressures generated during impact. 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 -- were also observed by the field team. Two impactors, each approximately 500 meters in diameter, are thought to have created the craters. According to scientists, the age of the impact event has been dated as occurring 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 and suggest that they were caused by erosive and volcanic processes. At present, both craters are being crossed by linear dunes extending northeast-southwest -- the superposition of the dunes across the annular ridges indicates that they are much younger than the craters.