The Search For Life on Mars

Two possible source craters for the Martian meteorite ALH84001 have been identified through an extensive search of impact craters on Mars. 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. Some 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. Plus the presence of carbonates in the meteorite suggests that evidence of water should be present. 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. 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. It displays a pristine ejecta blanket, a 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 fluid 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.