Science 3 March 2006: Vol. 311. no. 5765, p. 1223 DOI: 10.1126/science.311.5765.1223c
In December 1932, scientists surveying the southern Egyptian desert came upon pieces of a translucent, pale yellow-green, glassy substance, from tiny fragments to football-sized chunks, scattered over a huge area at the Libyan border. Known as Libyan desert glass, this almost pure silica contained isotopes showing it to be of extraterrestrial origin. But scientists haven't been able to figure out where it came from.
Now Farouk El-Baz, director of the Boston University Center for Remote Sensing, believes the mystery has been solved. This month, poring over satellite images of the Sahara Desert, he found a gigantic impact crater in the area. At a diameter of 30 kilometers, it's "the largest crater yet found in the Sahara," El-Baz says, and big enough to be the source of the glass, which covers a 60- by 100-kilometer area. He believes the crater hadn't been recognized before because it is so big; also, parts of its rims were eroded by two ancient river systems. El-Baz has named the crater, located on the Gilf Kebir plateau, the Kebira. "This is a large crater and well worth scientific investigation," says Friedrich Horz, a crater expert at the Johnson Space Center in Houston, Texas.
CREDIT: LANDSAT
http://www.sciencemag.org/cgi/content/summary/311/5765/1223c