Search for evidence of ancient life on Mars

Because environments on Earth and Mars were quite similar 3.4 - 4 Gyr ago, when life was emerging on Earth, and because terrestrial life emerged very rapidly once conditions permitted, a search for evidence of an extinct martian biota is scientifically credible. However, the criteria to be satisfied for general acceptance of such evidence will be severe. The geochemical, mineralogical and morphological observations recently reported by McKay et al. for the martian meteorite ALH84001 fail to meet those criteria. It is unlikely that study of any known martian meteorites will improve upon this situation. A viable search for evidence of ancient life on Mars will require a sequence of robotic-spacecraft missions, culminating in return of carefully selected samples to Earth. By analogy with the record of early terrestrial life, those samples will be rocks which formed in an aqueous environment and which contain organic matter. Key to successful sample return will be a series of precursor missions capable of, first identifying promising landing sites from orbit, and second identifying promising rocks at such a site. A realistic timeframe for an exobiologically optimized Mars sample return mission is about 2009. Presently unanticipated discoveries may permit an earlier mission, but otherwise an accelerated program would be less likely to succeed scientifically.