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Contact: Mary A. Hardin
FOR IMMEDIATE RELEASEMarch 20, 1996
CHAIN OF IMPACT CRATERS SUSPECTED IN SPACEBORNE RADAR IMAGES
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
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
"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
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
"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.