Life on Mars: Chemical Arguments and Clues from Martian Meteorites
Colin PILLINGERa and Andr6 BRACK*b
a Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom
b Centre de Biophysique Moleculaire, CNRS, Rue Charles Sadron, 45071 Orleans cedex 2, France
Primitive terrestrial life-defined as a chemical system able to transfer its molecular information via self-replication and to evolve - probably originated from the evolution of reduced organic molecules in liquid water. Several sources have been proposed for the prebiotic organic molecules: terrestrial primitive atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems and extraterrestrial meteoritic and cometary dust grains. The study of the carbonaceous chondrites that contain up to 5% by weight of organic matter, has allowed close examination of the delivery of extraterrestrial organic material. Eight proteinaceous amino acids have been identified in the Murchison meteorite among more than 70 amino acids. Engel reported that L-alanine was surprisingly more abundant than D-alanine in the Murchison meteorite (1). Cronin found also excesses of L-enantiomers for non- proteinaceous amino acids (2). A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50 to 100 μm size range, the carbonaceous micrometeorites represent 80% of the samples and contain 7 % of carbon. They might have brought more carbon than that involved in the present surficial biomass (3).
The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars attesting the presence of an atmosphere capable of deaccelerating C-rich micrometeorites. Therefore, primitive life may have developed on Mars as well and fossilized microorganisms may still be present in the near sub-surface (4). The Viking missions to Mars in 1976 did not find evidence of either contemporary or past life but the mass spectrometer on the lander aeroshell made measurements of the atmospheric composition which has allowed a family of meteorites to be identified as martian (5). Although these samples are essentially volcanic in origin, it has been recognized that some of them contain carbonate inclusions and even veins that have a carbon isotopic composition indicative of an origin from martian atmospheric carbon dioxide (6). The oxygen isotopic composition of these carbonate deposits allows calculation of the temperature regime existing during formation from a fluid which dissolved the carbon dioxide. As the
