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Small "bolide" (Nasa press release)

Since extinctions and bolides are being discussed here I thought
I should throw this in... 

Dalmiro Maia

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>From NASANews@mercury.hq.nasa.gov Wed Mar 20 23:23 MET 1996
Date: Wed, 20 Mar 1996 14:12:08 -0500
From: NASANews@luna.osf.hq.nasa.gov (NASA HQ Public Affairs Office)
To: press-release-other3@mercury.hq.nasa.gov
Subject: Chain of Impact Craters Suggested by Spaceborne Radar Images
Sender: NASANews@hq.nasa.gov
Content-Type: text
Content-Length: 3897

Douglas Isbell
Headquarters, Washington, DC                    March 20, 1996
(Phone:  202/358-1753)

Mary Hardin
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-5011)

RELEASE:  96-55


        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, Pasadena, CA. "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, CA.
"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 the
University of Arizona, 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
-- less than a mile in diameter -- and the craters are all
similar in size -- about seven to ten miles 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.


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