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Published February 27, 2001 | Supplemental Material + Published
Journal Article Open

Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures


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.

Additional Information

© 2001 National Academy of Sciences. Edited by Bruce Watson, Rensselaer Polytechnic Institute, Troy, NY, and approved December 18, 2000 (received for review October 22, 2000). We thank J. Hultberg of the National Aeronautics and Space Administration (NASA)/Johnson Space Center Scientific and Technical Information center, R. Christoffersen, T. D. Raub, R. N. Zare, P. R. Buseck, S. Keprta, K. White, R. B. Frankel, and the Honorable T. Campbell. We also thank J. Kulick and A. Jacobson of the Materials Research Science and Engineering Center at the University of Houston. We acknowledge the funding and support of NASA's Astrobiology Institute and the Exobiology Program. D.A.B. was supported by the National Science Foundation.

Attached Files

Published - PNAS-2001-Thomas-Keprta-2164-9.pdf

Supplemental Material - 5008fig4.pdf

Supplemental Material - 5008fig5.pdf


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