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* Lockheed Martin, 2400 NASA Road 1, Mail Code C23, Houston, TX
77058; Edited by Bruce Watson, Rensselaer Polytechnic Institute, Troy,
NY, and approved December 18, 2000 (received for review October 22, 2000)
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.
Special Feature
Evolution
Truncated hexa-octahedral magnetite crystals in ALH84001:
Presumptive biosignatures
,
,
,, and
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
To whom reprint requests should be addressed. E-mail:
kthomas{at}ems.jsc.nasa.gov.
www.pnas.org/cgi/doi/10.1073/pnas.051500898
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