LIFE ON MARS - CHEMICAL ARGUMENTS AND CLUES FROM MARTIAN METEORITES

Primitive terrestrial life - defined as a chemical system able to tran sfer its molecular information via self-replication and to evolve - pr obably originated from the evolution of reduced organic molecules in l iquid water. Several sources have been proposed for the prebiotic orga nic molecules: terrestrial primitive atmosphere (methane or carbon dio xide), deep-sea hydrothermal systems, and extraterrestrial meteoritic and cometary dust grains. The study of carbonaceous chondrites, which contain up to 5% by weight of organic matter, has allowed close examin ation of the delivery of extraterrestrial organic material. Eight prot einaceous amino acids have been identified in the Murchison meteorite among more than 70 amino acids. Engel reported that L-alanine was surp risingly more abundant than D-alanine in the Murchison meteorite. Cron in also found excesses of L-enantiomers for nonprotein amino acids. A large collection of micrometeorites has been recently extracted from A ntarctic old blue ice. In the 50- to 100-mu m size range, carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon , on average. They might have brought more carbon than that involved i n the present surficial biomass. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surfac e of Mars, attesting the presence of an atmosphere capable of decceler ating C-rich micrometeorites. Therefore, primitive life may have devel oped on Mars as well and fossilized microorganisms may still be presen t in the near subsurface. The Viking missions to Mars in 1976 did not find evidence of either contemporary or past life, but the mass spectr ometer on the lander aeroshell determined the atmospheric composition, which has allowed a family of meteorites to be identified as Martian. Although these samples are essentially volcanic in origin, it has bee n recognized that some of them contain carbonate inclusions and even v eins that have a carbon isotopic composition indicative of an origin f rom Martian atmospheric carbon dioxide. The oxygen isotopic compositio n of these carbonate deposits allows calculation of the temperature re gime existing during formation from a fluid that dissolved the carbon dioxide. As the composition of the fluid is unknown, only a temperatur e range can be estimated, but this falls between 0 degrees and 90 degr ees C, which would seem entirely appropriate for life processes. It wa s such carbonate veins that were found to host putative microfossils. Irrespective of the existence of features that could be considered to be fossils, carbonate-rich portions of Martian meteorites tend to have material, at more than 1000ppm, that combusts at a low temperature; i .e., it is an organic form of carbon. Unfortunately, this organic matt er does not have a diagnostic isotopic signature so it cannot be unamb iguously said to be indigenous to the samples. However, many circumsta ntial arguments can be made to the effect that it is cogenetic with th e carbonate and hence Martian. If it could be proved that the organic matter was preterrestrial, then the isotopic fractionation between it and the carbon is in the right sense for a biological origin.