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Possible Source Craters For Martian Meteorite Found

Contact: Dr. Nadine G. Barlow
Department of Physics
University of Central Florida
(407) 823-0251


Two possible source craters for the martian meteorite ALH84001
have been identified through an extensive search of impact craters
on Mars. The 1.9-kg (4.2 lb) meteorite, recently identified as
showing possible evidence of past martian life, was formed about
4.5 billion years ago and was blasted off of Mars during a
meteorite impact about 16 million years ago. Dr. Nadine Barlow, a
planetary scientist at the University of Central Florida,
identified the two likely source craters through a search of a
crater catalog she compiled while doing her graduate work at the
University of Arizona in the mid-1980's.

A number of characteristics of the meteorite helped Dr. Barlow
narrow the search for possible source craters. The 4.5 billion
year old age of the meteorite indicated it must have come from the
most ancient terrain on Mars, while the 16 million year old
ejection age indicates that the crater from which the meteorite
was ejected should still show very young features. Evidence of
pre-ejection shock events indicates that one or more large, old
craters should be found near the meteorite ejection site, and the
presence of carbonates in the meteorite suggests that evidence of
water should be present. Previous work by other researchers
indicate that martian meteorites can only be ejected either by a
very large impact (100-km diameter or larger) if the impact is
near-vertical, or by smaller impacts if they strike at an angle
close to the horizon. The low angle impacts will create a
distinctive elliptical-shaped crater.

Dr. Barlow's crater catalog, which contains information on 42,283
martian impact craters, was used to search for fresh, elliptical
impact craters larger than 10-km-diameter and for fresh, circular
craters larger than 100-km-diameter on ancient terrain. The search
produced 23 possible craters. Dr,. Barlow then used images of the
martian surface taken by the Viking Orbiter spacecraft in the
mid-1970's to eliminate those craters which showed evidence of
being older than 16-million years. "16 million years may sound
like a long time to humans, but for geologic processes it is a
very short time period, particularly for a planet like Mars which
has apparently experienced little geologic activity over the past
billion years" said Dr. Barlow.

The two craters which survived the analysis are both of the
smaller, elliptical crater type. Both are located in the heavily
cratered southern highlands of Mars. The first crater, located in
the Sinus Sabaeus region of Mars south of the Schiaparelli impact
basin, is 23 x 14.5 km in diameter, displays a pristine ejecta
blanket and sharp crater rim, and is superposed on the rim of a
much older highly degraded 50-km-diameter crater. Several small
channels which formed early in the planet's history are located
nearby, including one called Evros Vallis. The second possible
source crater for ALH84001 is located east of the Hesperia
Planitia region, is 11 x 9 km in diameter, and also displays a
pristine ejecta blanket and sharp crater rim. It is located less
than 10 km from an older 25-km diameter crater in an area which
also shows some possible evidence of ancient fluvial activity.

The identification of possible source craters for ALH84001 will
allow NASA to focus its efforts on these areas with future lander
missions to Mars.

Dr. Barlow's findings are being prepared for submission to the
Journal of Geophysical Research. She also plans to report on these
results at the October meeting of the Division for Planetary
Sciences of the American Astronomical Society meeting to be held
in Tucson, Arizona.