August 12, 1996
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.
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