Columbia space shuttle uses radar as a time machine
It is a hostile wasteland of flat sands and marching dunes, where the temperatureshave soared to 122° F, and the rain falls only once in 30 to 50 years. But for thousands of years, there have been tales that the Sahara’s arid surface concealed a “large river without water.” Last week a team of scientists from the U.S. and Egypt announced that they had definitive evidence that long ago a region of the vast desert in southern Egypt and northern Sudan was a lacy network of major water ways. The proof: radar images of the Sahara taken by the space shuttle Columbia.
The discovery, reported in Science, was a serendipitous success for the flight of Columbia in November 1981. That mission had to be cut from 124 hr. to 54 hr. because of a faulty fuel cell, but before aborting the flight, the astronauts were able to complete an experiment with the ship’s radar equipment. They took a 50-km-wide scan of the Sahara from the shuttle. Radar waves generally penetrate only a few centimeters of the earth, since the beams are dissipated by moisture in the surface of land. But in the dry Sahara, the radar waves were able to pierce to depths of five meters, reflecting from bedrock.
The signals were processed through image-enhancing computer techniques. The resulting X ray-like pictures of the Sahara’s subsurface were then analyzed by scientists from the U.S. Geological Survey, the University of Arizona, the Jet Propulsion Laboratory in Pasadena, Calif., and the Egyptian Geological Survey and Mining Authority. The images of the area, says U.S.G.S. Research Geologist Carol Breed, “showed us a topography that could only have been buried. There was no trace of it on the surface.” Marvels the head of the eight-member group interpreting the pictures, John F. McCauley of the U.S.G.S.: “We were able to look through and use radar as a time machine.”
The images revealed stream channels, broad flood plains and what millenniums ago had been great river valleys, some as wide as those of the Nile. Though a few experts speculate that the ancient and modern water systems were once connected, there is no supporting evidence. In fact, says McCauley, “the trending of the [ancient] rivers is to the south and west, the opposite of the present-day movement. It is possible they all joined up to one large basin of interior drainage as large as the Caspian Sea is now.”
The find confirms that the region was once wet enough to support plants, animals and man, which scientists have long suspected. The Sahara originally dried out 2 million years ago, as the earth slipped into an Ice Age, but brief rainy periods occurred about 200,000, 60,000 and 10,000 years ago. Researchers who have ventured into south-central Egypt have found artifacts of human occupation. The U.S.Egyptian team that visited last September found the area so dry that cardboard boxes, cigarette papers and tracks left by the British army during World War II were perfectly preserved. Seeking signs of earlier inhabitants, the team dug at sites along the banks of hidden riverbeds shown by the new radar maps. Their findings: tools and other artifacts presumably used as long ago as 200,000 years by Homo erectus, one of modern man’s ancestors.
Geologists believe that radar scanning will be valuable in detecting modern waterways lying near the surface in arid areas. “If you want to look for water in the desert,” says Breed, “you would look for that type of site where ground water intersects the surface.” For archaeologists, the technique may help determine sites of early human habitation near former rivers and lakes. And, by indicating telltale subsurface features, it may prove a boon for geologists surveying for oil and minerals.
Space officials are so excited by the radar technique that a follow-up mission for more sophisticated radar imaging has been approved for a shuttle flight in August 1984. Says J.P.L.’s Charles Elachi: “The plan is to concentrate and get much more coverage of the Sahara region.” There is even talk of more radar missions to other planets. Radar pictures, which have already revealed some of the secrets under the clouds of Venus, may help scientists learn how planets developed. First choice would be Mars. The arid red planet’s surface is etched with channels remarkably similar to those found beneath the surface of the Sahara.
—By Anastasia Toufexis.
Reported by Joseph J. Kane/Los Angeles
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