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 earliest Archean geological history of the Earth, or from before around 3800 Ma (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 opens the strong possibility for a similarly early and rapid emergence of life 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 examining the record of life in ancient terrestrial sedimentary rocks, including those that contain rare and recognizable "physical" microfossils ("morphofossils" identified on the basis of their shape alone) and stable, authigenic biominerals which include carbonaceous matter having characteristically fractionated carbon isotope signatures (here termed "chemofossils"). Prior to sample return, these tests can be applied to the mineral associations of the SNC meteorites, a group of meteorites believed to have originated on Mars. Recent claims of a biological origin for secondary minerals and their features as well as for trace organic compounds in the Martian meteorite ALH84001, are derived in part from the interpretation of putative "nanofossil" shapes and the nature of the associated mineral assemblage in small carbonate 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. Investigations of these mineral occurrences in the Martian meteorites and of the oldest geological records on Earth provide a useful framework for (1) using mineral phase relationships, (2) analytical data of stable carbon isotopic distributions, and (3) the problematic task of morphofossil interpretations, in the search for life via future sample return missions from the ancient surface of Mars.