Life on Mars: evaluation of the evidence within Martian meteorites ALH84001, Nakhla, and Shergotty

Analyses both support and are in opposition to the hypothesis that the Mart ian meteorite ALH84001 contains evidence for possible biogenic activity on Mars. New observations in two additional Martian meteorites, Nakhla (1.3 Ga old) and Shergotty (300-165 Ma old) indicate possible biogenic features. F eatures in the three Martian meteorites compare favorably with the accepted criteria for terrestrial microfossils and evidence for early life on the E arth. There is strong evidence for the presence of indigenous reduced carbo n, biogenic magnetite, and the low-temperature formation of carbonate globu les. The morphological similarities between terrestrial microfossils, biofi lms, and the features found in the three Martian meteorites are intriguing but have not been conclusively proven. Every investigation must recognize t he possibility of terrestrial contamination of the meteorites, whether or n ot the meteorites are Martian. The search for evidence of ancient life in M artian meteorites has emphasized the difficulties confronting the scientifi c community with the respect to the positive identification of evidence of past biogenic activity. (C) 2001 Elsevier Science B.V. All rights reserved.