Phosphates and carbon on Mars: Exobiological implications and sample return considerations

Much of the surface of Mars may preserve chemical information contained in rocks from the Noachian era, with ages that overlap the correspondingly ear liest Archean geological history of the Earth, or from before around 3800 M a (Ma = 1 x 10(6) years). Metabolically sophisticated life, which utilized phosphate and carbon and was capable of fractionating carbon isotopes, was apparently present already on Earth by similar to 3800 Ma, or within 600 Ma after the formation of the planet. An early appearance of life on Earth op ens the strong possibility for a similarly early and rapid emergence of lif e on planet Mars. This hypothesis remains within the realm of plausibility so long as it can be established that liquid water and energy sources were available there for inchoate life, and that the life that emerged reached a level of complexity which could be recognized by its chemical, and perhaps morphological remains. Hypotheses to be used in the search for an ancient Martian biosphere from future sample return missions are testable by examin ing the record of life in ancient terrestrial sedimentary rocks, including those that contain rare and recognizable "physical" microfossils ("morphofo ssils" identified on the basis of their shape alone) and stable, authigenic biominerals which include carbonaceous matter having characteristically fr actionated carbon isotope signatures (here termed "chemofossils"). Prior to sample return, these tests can be applied to the mineral associations of t he SNC meteorites, a group of meteorites believed to have originated on Mar s. Recent claims of a biological origin for secondary minerals and their fe atures as well as for trace organic compounds in the Martian meteorite ALH8 4001, are derived in part from the interpretation of putative "nanofossil" shapes and the nature of the associated mineral assemblage in small carbona te deposits of an igneous rock. Such igneous samples would not normally be the best candidate to search for evidence of past life, even on Earth. Inve stigations of these mineral occurrences in the Martian meteorites and of th e oldest geological records on Earth provide a useful framework for (1) usi ng mineral phase relationships, (2) analytical data of stable carbon isotop ic distributions, and (3) the problematic task of morphofossil interpretati ons, in the search for life via future sample return missions from the anci ent surface of Mars.