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Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures

  1. Christopher S. Romanek††
  1. *Lockheed Martin, 2400 NASA Road 1, Mail Code C23, Houston, TX 77058; ‡Iowa State University, Department of Microbiology, 207 Science I, Ames, IA 50011; §California Institute of Technology, Division of Geological and Planetary Sciences, 1200 East California Boulevard, Pasadena, CA 91125; National Aeronautics and Space Administration/Johnson Space Center, Mail Code SN, **Mail Code SN2, ‡‡Mail Code SL, Houston, TX 77058; ‖McGill University, Department of Earth and Planetary Sciences, 3450 University Street, Montreal, PQ H3A 2A7, Canada; and ††Savannah River Ecology Laboratory, Drawer E, University of Georgia, Aiken, SC 29802

  1. Edited by Bruce Watson, Rensselaer Polytechnic Institute, Troy, NY, and approved December 18, 2000 (received for review October 22, 2000)

Abstract

McKay et al. [(1996) Science 273, 924–930] suggested that carbonate globules in the meteorite ALH84001 contained the fossil remains of Martian microbes. We have characterized a subpopulation of magnetite (Fe3O4) crystals present in abundance within the Fe-rich rims of these carbonate globules. We find these Martian magnetites to be both chemically and physically identical to terrestrial, biogenically precipitated, intracellular magnetites produced by magnetotactic bacteria strain MV-1. Specifically, both magnetite populations are single-domain and chemically pure, and exhibit a unique crystal habit we describe as truncated hexa-octahedral. There are no known reports of inorganic processes to explain the observation of truncated hexa-octahedral magnetites in a terrestrial sample. In bacteria strain MV-1 their presence is therefore likely a product of Natural Selection. Unless there is an unknown and unexplained inorganic process on Mars that is conspicuously absent on the Earth and forms truncated hexa-octahedral magnetites, we suggest that 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.

Footnotes

    • ↵† To whom reprint requests should be addressed. E-mail: kthomas{at}ems.jsc.nasa.gov.

    • This paper was submitted directly (Track II) to the PNAS office.

  • Abbreviation

    TEM,
    transmission electron microscope
    • Received October 22, 2000.

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