Magnetofossils from Ancient Mars: a Robust Biosignature in the Martian Meteorite ALH84001
Kathie L. Thomas-Keprta,1* Simon J. Clemett,1 Dennis A. Bazylinski,2 Joseph L. Kirschvink,3 David S. McKay,4 Susan J. Wentworth,1 Hojatollah Vali,5 Everett K. Gibson, Jr.,4 and Christopher S. Romanek6
Lockheed Martin,1 National Aeronautics and Space Adminstration/Johnson Space Center, Houston, Texas 77058,4 Department of Microbiology, Iowa State University, Ames, Iowa 50011,2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125,3 Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada,5 Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina 298026
Evidence of biogenic activity on Mars has profound scientific implications for our understanding of the origin of life on Earth and the presence and diversity of life within the Cosmos. Analysis of the Martian meteorite Allan Hills 84001 (ALH84001) revealed several lines of evidence that has led some investigators to suggest that microbial life existed on Mars approximately 4 billion years ago (45). One of the strongest lines of evidence is the presence of tens-of-nanometer-size magnetite (Fe3O4) crystals found within carbonate globules and their associated rims in the meteorite (57, 58). Approximately one-quarter of these magnetites have remarkable morphological and chemical similarities to magnetite particles produced by magnetotactic bacteria, which occur in aquatic habitats on Earth. Moreover, these types of magnetite particles are not known or expected to be produced by abiotic means either through geological processes or synthetically in the laboratory. We have therefore argued that these Martian magnetite crystals are in fact magnetofossils (57, 58). If this is true, such magnetofossils would constitute evidence of the oldest life forms known. In this respect, we note there is now considerable uncertainty concerning when the earliest terrestrial life forms existed. Until recently, results from the 3.5-billion-year-old Apex cherts of the Warrawoona group in western Australia held this record (52), although this work is now in question (12).
The remaining three-quarters of the ALH84001 magnetites are likely products of processes including, but not limited to, precipitation from a hydrothermal fluid, thermal decomposition of the carbonate matrix in which they are embedded, and/or extracellular formation by dissimilatory Fe-reducing bacteria. No single process, either biogenic or inorganic, can explain the full distribution of magnetite crystal morphologies observed in the ALH84001 carbonates. Unlike most meteorites which experienced only one brief period of aqueous and thermal activity during their formation in the early Solar System, Martian meteorite ALH84001 represents part of a dynamic and evolving planetary surface. From its formation approximately 4.5 billion years ago (49; L. E. Nyquist, B. M. Bansal, H. Wiesmann, and C. Y. Shih, Abstr. Pap. Submit. Lunar Planet. Sci. Conf. 26:1065-1066, 1995) to its landing in Antarctica 13,000 years ago (35), it experienced multiple shock events (59) and at least one, if not more, episodes of aqueous alteration (51, 61). We propose that the magnetite crystals in ALH84001 can be best explained as the products of biogenic and inorganic processes that operated on early Mars.
The report of McKay et al. (45) and subsequent papers on ALH84001 magnetite (57, 58) have not been without controversy. The identification of a population of ALH84001 magnetite identical to magnetite produced by terrestrial magnetotactic bacteria is not in contention. Rather, the debate and surrounding controversy is over whether this "biogenic" magnetite is an exclusive product of biology. To date, the most credible alternative inorganic hypothesis is centered on whether partial thermal decomposition of the ALH84001 carbonates could produce a population of biogenic-like magnetite (27, 28; D. C. Golden, D. W. Ming, H. V. Lauer, Jr., C. S. Schwandt, R. A. Morris, G. E. Lofgren, and G. A. McKay, Lunar Planet. Sci. Conf. 33, CD-ROM no. 1839, 2002). However, there are serious inconsistencies with this hypothesis. First, a preliminary report suggesting the inorganic formation of biogenic-like magnetite (D. C. Golden et al., Lunar Planet. Sci. Conf. 33, CD-ROM no. 1839, 2002) has been invalidated. Second, decomposition of a solid solution [(Fex,Mg1-x)CO3] carbonate, as observed in ALH84001, forms a mixed Fe-Mg spinel (2, 15, 23) and not pure magnetite, as observed for the biogenic fraction of the ALH84001 crystals (57, 58). Therefore, although controversial, the biogenic interpretation for one-quarter of the ALH84001 magnetites remains the most plausible hypothesis yet advanced. Additionally, such a biogenic interpretation is consistent with the broader scenario of how the carbonates themselves formed (45, 51, 60, 61).
Source: Applied and Environmental Microbiology, August 2002, p. 3663-3672, Vol. 68, No. 8