Magnetite morphology and life on Mars

Buseck, Peter R. and Dunin-Borkowski, Rafael E. and Devouard, Bertrand and Frankel, Richard B. and McCartney, Martha R. and Midgley, Paul A. and Pósfai, Mihály and Weyland, Matthew (2001) Magnetite morphology and life on Mars. Proceedings of the National Academy of Sciences, 98 (24). pp. 13490-13495. ISSN 0027-8424

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Abstract

Nanocrystals of magnetite (Fe3O4) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars. The startling report by McKay et al. (1) of evidence for former life on Mars has attracted major interest scientifically and with the general public. The evidence was based on four structural and chemical features in a meteorite, ALH84001, that originated on Mars. No single line of evidence was presented as individually compelling. However, the authors proposed that, when taken collectively, the separate observations provided a credible case for the past existence of life. This assumption has since been vigorously challenged but not disproved. Data amassed since the McKay paper have resulted in the effective elimination of all but one of the original arguments. The case for former Martian life now rests on the identification of a small subset of the magnetite crystals in ALH84001 as biogenic in origin. A recent report by Friedmann et al. (2) interprets scanning electron microscope images of lines of small, bright objects in ALH84001 as magnetite crystals that are aligned in chains. Thomas-Keprta et al. (3–5), on the other hand, address the morphologies of individual magnetite grains and report that a minority (≈27%) of the magnetite crystals in ALH84001 are “indistinguishable” from those produced by a particular strain of terrestrial magnetotactic bacteria. Both of these arguments are flawed. Friedmann et al. interpret contrast features that are at the limit of their available resolution, with no supporting chemical or microstructural evidence that the features they describe are indeed magnetite crystals. Moreover, magnetotactic bacteria are ubiquitous on Earth, and yet intact chains of nanosized magnetite crystals from bacteria are rarely found in terrestrial geological samples (5), suggesting that such chains are unlikely to survive geological processes. Friedmann et al. acknowledge that it is difficult to understand the intact occurrence of their hypothesized magnetite chains within fractures in which it is most unlikely that aquatic magnetotactic bacteria ever lived; the chains from dead bacteria would somehow have had to migrate intact into the fractures and remain there unbroken. In the absence of chemical and structural data, it is difficult to exclude the possibility of other semiperiodic features (such as serrated grain edges, possibly decorated selectively during sample coating). Thomas-Keprta et al. (hereafter collectively referred to as T.-K.) focus on individual magnetite crystals in ALH84001 and conclude that “these Martian magnetites (are) physically and chemically identical to . . . magnetites produced by magnetotactic bacteria strain MV-1” (5). They cite six well known features of bacterial magnetite and conclude that “when taken collectively” these characteristics indicate a biogenic origin for the meteoritic magnetite. By their own reasoning, only a minority of the magnetite crystals in ALH84001 qualify for biogenic status on the basis of their sizes and shapes. It is therefore astonishing, and not widely appreciated, that the entire evidence for the former presence of life on Mars now rests on the shapes of a small fraction of the magnetite nanocrystals in ALH84001 (and their possible alignment in chains). Much has been written about the biogenic Fe minerals in magnetotactic bacteria and about which of their features, if any, provide unambiguous evidence for former life [refs. 5, 6 (and references therein), 7, 8]. Given the importance of the papers and abstracts of T.-K., as well as the confidence with which their conclusions are presented, it is important to examine the reliability of their measurements. These measurements consist of the morphologies (3, 5) and aspect ratios (width/length; refs. 3 and 5) of a selection of magnetite crystals. We consider these issues below, as well as ambiguities in the terminology used to describe the morphologies of magnetite crystals from magnetotactic bacteria and the consequent confusion that results.

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