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Contact: Mary A. Hardin



       A team of scientists believes they have discovered a chain of impact craters in the central African country of Chad that suggests ancient Earth may have been hit by a large, fragmented comet or asteroid similar to the Shoemaker-Levy 9 comet that slammed into Jupiter in 1994.

       The craters were discovered in radar images of the Earth taken by the Spaceborne Imaging Radar C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) that flew on the Space Shuttle Endeavour in April and October of 1994. The images reveal two new craters adjacent to a previously known impact site, called Aorounga, in northern Chad. The two new craters still need to be confirmed by fieldwork on the ground.

       "The Aorounga craters are only the second chain of large craters known on Earth, and were apparently formed by the break-up of a large comet or asteroid prior to impact," said Adriana Ocampo, a geologist at NASA's Jet Propulsion Laboratory. "With ground confirmation, this second chain will provide valuable data on the nature and origin of small bodies that cross Earth's orbit."

       Ocampo is presenting her findings today at the annual Lunar and Planetary Science Conference in Houston, TX.

       "The two new craters are the first impact craters discovered in SIR-C data," said Dr. Kevin Pope, a SIR-C team member from Geo Eco Arc Research in La Canada-Flintridge. "That shows the power of the SIR-C instrument, because these craters are highly eroded and buried by wind-blown sand. They are hard to see even if you are standing on the ground."

       The most prominent of the craters, called Aorounga South, has been observed in Landsat satellite-based images and space shuttle hand-held photos, and has been verified by ground work. The other two craters, Aorounga Central and Aorounga North, have not been scientifically confirmed through fieldwork and that has caused other scientists to view this discovery with some skepticism.

       "These could very well be impact structures, but we don't have the kind of evidence we need to catalogue them yet," said Dr. John McHone, a SIR-C science team member from Arizona State University, who has studied impact craters for more than 20 years.

       Ocampo and Pope theorize that the object that created these impact sites was either a comet or asteroid that broke up before it hit the Earth. "The pieces were all similar in size -- about 1/2 of a kilometer to 1 kilometer in diameter -- and the craters are all similar in size, about 12 kilometers to 17 kilometers wide," Ocampo said.

       Similar chains of equal size craters have also been seen on Jupiter's moon Callisto.

       The scientists estimate the Chad impact craters date back about 360 million years, to a time when the Earth was undergoing a period of mass biological extinction. By way of comparison, the impact that scientists believed wiped out the dinosaurs 65 million years ago involved an asteroid or comet 10 times larger than the one that broke up to form the craters in Chad.

       "These impacts in Chad weren't big enough to cause the extinction, but they may have contributed to it," Ocampo said. "Could these impacts be part of a larger event? Were they, perhaps, part of comet showers that could have added to the extinction? Little by little, we are putting the puzzle together to understand how Earth has evolved."

       The Spaceborne Imaging Radar project is managed by the Jet Propulsion Laboratory for NASA's Office of Mission to Planet Earth, Washington, DC. SIR-C/X-SAR is a joint mission of the United States, German and Italian space agencies.

3-20-96 MAH