Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures

McKay et al. [(1996) Science 273, 924-930] suggested that carbonate globule s in the meteorite ALH84001 contained the fossil remains of Martian microbe s. We have characterized a subpopulation of magnetite (Fe3O4) crystals pres ent in abundance within the Fe-rich rims of these carbonate globules. We fi nd these Martian magnetites to be both chemically and physically identical to terrestrial, biogenically precipitated, intracellular magnetites produce d by magnetotactic bacteria strain MV-1. Specifically, both magnetite popul ations are single-domain and chemically pure, and exhibit a unique crystal habit we describe as truncated hexaoctahedral. There are no known reports o f inorganic processes to explain the observation of truncated hexa-octahedr al magnetites in a terrestrial sample. In bacteria strain MV-1 their presen ce is therefore likely a product of Natural Selection. Unless there is an u nknown and unexplained inorganic process on Mars that is conspicuously abse nt on the Earth and forms truncated hexaoctahedral magnetites, we suggest t hat these magnetite crystals in the Martian meteorite ALH84001 were likely produced by a biogenic process. As such, these crystals are interpreted as Martian magnetofossils and constitute evidence of the oldest life yet found .