The Search for Biosignatures in Martian Meteorite Allan Hills 84001

Abstract

Proposed biosignatures in the ancient Allan Hills 84001 martian meteorite are most plausibly explained as abiotic features. The purported evidence of biological activity on Mars included biogenic minerals (magnetite and sulphide formed by magnetotactic and sulphate-respiring microorganisms), organic matter resulting from the decay of such organisms, microfossils, and biofilms, all physically associated with biologically mediated carbonates. The zoned carbonate globules formed by inorganic precipitation from an aqueous fluid or evaporative brine circulating within fractures in this igneous rock. A subsequent shock event partially volatilized Fe-carbonate, and its decomposition produced nanophase magnetite crystals with unusual morphologies, structures, and compositions consistent with vapour condensation. Sulphur isotopes in sulphide are unlike those in terrestrial biogenic sulphides. The organic compounds identified in ALH 84001 include polycyclic aromatic hydrocarbons, complex macromolecules, graphite, and amino acids, most of which are terrestrial, based on their carbon isotopes and stereochemistry. A small amount of the organic matter may be martian, but even that likely had an exogenic (chondritic) source. The putative microfossils were identified only by morphology, without any other supporting observations. These forms are apparently too small to represent viable organisms, which has engendered controversy about the plausibility of nanobacteria. Observations of possible fossilized biofilms are compromised by infiltration of the meteorite by terrestrial microorganisms in the Antarctic environment from which the meteorite was recovered. The controversial hypothesis that ALH 84001 contains evidence of extraterrestrial biology has mostly subsided, but it has fuelled a Mars exploration program focused on the search for life and has helped refine the criteria for the recognition of biosignatures.