Gaps between magnetite minerals and alignment of long-axis with the orientation of the chain are features associated with production of magnetite via magnetosomes.

Magnetite oriented by a magnetic field should, if in chains at all, be in the more energetically favorable configuration of stacked magnetites with short axes along the chain, which is not what is identified in ALH84001. Therefore, the origin of these structures may be biological.

 

Gaps between magnetite crystals in chains are likely due to the membrane surrounding the magnetosomes and due to any matrix of organic material between them; however abiotic aligned magnetite (i.e., magnetite aligned by a magnetic field) would lack gaps. There are gaps evident in the magnetite chains in ALH84001, which indicates these may be of biotic origin.

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Title

Chains of Magnetite Crystals in the Meteorite ALH84001: Evidence of Biological Origin

Authors

Friedmann, E. Imre, Jacek Wierzchos, Carmen Ascaso, and Michael Winklhofer.

Abstract

The presence of magnetite crystal chains, considered missing evidence for the biological origin of magnetite in ALH84001 [Thomas-Keprta, K. L., Bazylinski, D. A., Kirschvink, J. L., Clemett, S. J., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., Jr., & Romanek, C. S. (2000) Geochim. Cosmochim. Acta 64, 4049–4081], is demonstrated by high-power stereo backscattered scanning electron microscopy. Five characteristics of such chains (uniform crystal size and shape within chains, gaps between crystals, orientation of elongated crystals along the chain axis, flexibility of chains, and a halo that is a possible remnant of a membrane around chains), observed or inferred to be present in magnetotactic bacteria but incompatible with a nonbiological origin, are shown to be present. Although it is unlikely that magnetotactic bacteria were ever alive in ALH84001, decomposed remains of such organisms could have been deposited in cracks in the rock while it was still on the surface on Mars.

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Linear alignment of magnetite shows biogenicity.

There is an observation of defect free magnetite within meteorite ALH84001. This alignment of defect free magnetite crystals is considered an optimal configuration. Under this linearity we note a tendency towards a maximal magnetic dipole moment. Maximizing a magnetic dipole via aligned crystals is achieved by organisms optimizing their own magnetic moments. Thus, we consider this crystal alignment to be pressured in the form of survival of the fittest and thus a biological marker.

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Title

Truncated hexa-octahedral magnetite crystals in ALH84001: Presumptive biosignatures

Authors

Thomas-Keprta, K. L., Clemett, S. J., Bazylinski, D. A., Kirschvink, J. L., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., McKay, M. F., & Romanek, C. S.

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.

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Biogenically derived magnetite tends to form in near perfectly linear chains.

Magnetospirillum- a terrestrial analog to perceived exobiological markers of magnetite producing bacteria- tend to generate chains of aligned organelles known as magnetosomes. These organelles are aligned in an almost perfectly linear fashion. Magnetosomes, aligned organelle chains produced by magnetospirillum bacteria, match the perfect alignment seen in aligned magnetite nanocrystals. Jimenez-Lopez finds these magnetosomes as a marker for order and optimality within biotics. Even if considering the chance that these magnetosomes do not remain perfectly aligned upon decomposition of the originating organism the authors still look to the uncanny linear alignments of magnetosomes as key to discerning life.

Evidence is Sourced from:

Title

Magnetite as a prokaryotic biomarker: A Review

Authors

Jimenez-Lopez, C., Romanek, C. S., & Bazylinski, D. A.

Abstract

[1] Over the years, nanometer-sized magnetite (Fe3O4) crystals have been recovered from many modern and ancient environments including sediments and soils and even meteorites. In some cases these crystals have been used as “magnetofossils” for evidence of the past presence of specific microbes. Magnetite nanocrystals can be formed by a number of different biological and inorganic mechanisms resulting in crystals with different physical and magnetic characteristics. Prokaryotes (bacteria) biomineralize magnetite through two methods that differ mechanistically, including: biologically induced mineralization (BIM) and biologically controlled mineralization (BCM). Magnetite nanocrystals produced by BIM are known to be synthesized by the dissimilatory iron-reducing bacteria, are deposited external to the cell, and generally are physically indistinguishable from magnetite particles formed inorganically. BCM magnetites, in contrast, are synthesized by the magnetotactic bacteria and some higher organisms and are precipitated intracellularly as membrane-bounded structures called magnetosomes. These magnetites appear to have unique crystal morphologies and a narrow size range leading to their original use as magnetofossils. Because of the discovery of nanometer-sized crystals of magnetite in the Martian meteorite ALH84001, the use of these criteria for the determination of whether magnetite crystals could constitute a prokaryotic biomarker was questioned. Thus, there is currently great debate over what criteria to use in the determination of whether specific magnetite crystals are biogenic or not. In the last decade, additional criteria have been established (e.g., the Magnetite Assay for Biogenicity), and new tools and technologies have been developed to determine the origin of specific types of magnetite crystals.

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Precise alignment of magnetite is biologically relevant as a feature of motality for life.

Magnetite crystals born of biological processing tend to align in chains. These chains are a product of the interaction of planetary magnetic field lines and the organism. Further, these crystal arrangements allow for the organism in question to move through its environment. This is a widely observed behavior in magnetotactic bacteria on Earth. Perfectly aligned magnetite crystals are the result of biological processes, as their precise alignment is necessary for the movement of magnetotactic bacteria through their environment. Thus, aligned magnetic structures like those found in modern day gram negative, magnetotactic bacteria point to past life when considering well aligned magnetite crystals.

Evidence is Sourced from:

Title

Magnetofossils from ancient Mars: A robust biosignature in the martian meteorite ALH84001

Authors

Thomas-Keprta, K. L., Clemett, S. J., Bazylinski, D. A., Kirschvink, J. L., McKay, D. S., Wentworth, S. J., Vali, H., Gibson,, E. K., & Romanek, C. S.

Abstract

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).

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Collapse of these structures into the more energetically favorable cluster form is very common, and much more so than the preservation of aligned chains.

The chains identified by Friedmann et al. are at the “limit of their available resolution” and therefore suspicious evidence for life, according to the authors. These chains would have had to, while intact, move into the fractures of the meteorite and remain unbroken for billions of years, which is very uncommon given the favorability of lower energy configurations.

Evidence is Sourced from:

Title

Magnetite Morphology and Life on Mars

Authors

Buseck, Peter R., Rafal E. Dunin-Borkowski, Bertrand Devouard, Richard B. Frankel, Martha R. McCartney, Paul A. Midgley, Mihály Pósfai, and Matthew Weyland.

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.

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The alignment of nanocrystalline magnetite in chains would indicate production by magnetosomes; however, most if not all of the crystals in ALH84001, after a series of analyses, are demonstrated to not be aligned in chains at all.

Chains of equant magnetite crystals have a higher energy than rings of crystals or clumped and non-aligned crystals, and so are commonly not observed as formed through abiotic methods. Here, three tests—low-temperature cycling, the Moskowitz test, and ferromagnetic resonance (FMR)—are applied to ALH84001 to discern magnetic properties. Low-temperature cycling relies on the Curie points of magnetite (i.e., when magnetite will lose magnetic properties based on temperature). This method can discern the composition and crystal size of magnetite. The Moskowitz test can test for crystal size and for the presence of bacteria via the sensitivity to magnetosome chains structures. The FMR test senses the anisotropy originating from crystal shape, composition and mineralogy. Chains were tested for via the Moskowitz test and then independently tested for with the FMR. Results indicate that no more than 10% of the magnetite crystals can be in isolated chains, making it much more difficult to definitively say the origin is biogenic.

Evidence is Sourced from:

Title

Magnetic Tests Magnetosome Chains in Martian Meteorite ALH84001

Authors

Weiss, Benjamin P., Soon Sam Kim, Joseph L. Kirschvink, Robert E. Kopp, Mohan Sankaran, Atsuko Kobayashi, and Arash Komeili.

Abstract

Transmission electron microscopy studies have been used to argue that magnetite crystals in carbonate from Martian meteorite ALH84001 have a composition and morphology indistinguishable from that of magnetotactic bacteria. It has even been claimed from scanning electron microscopy imaging that some ALH84001 magnetite crystals are aligned in chains. Alignment of magnetosomes in chains is perhaps the most distinctive of the six crystallographic properties thought to be collectively unique to magnetofossils. Here we use three rock magnetic techniques, low-temperature cycling, the Moskowitz test, and ferromagnetic resonance, to sense the bulk composition and crystallography of millions of ALH84001 magnetite crystals. The magnetic data demonstrate that although the magnetite is unusually pure and fine-grained in a manner similar to terrestrial magnetofossils, most or all of the crystals are not arranged in chains.

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Abiotic serrated grain edges, meaning irregular edges of grains which can suggest periodic or semi-periodic structures, can be mistaken for magnetite chains.

These chains therefore may be misinterpreted serrated grain edges, potentially decorated selectively during sample coating, than actual grains.

Evidence is Sourced from:

Title

Magnetite Morphology and Life on Mars.

Authors

Buseck, Peter R., Rafal E. Dunin-Borkowski, Bertrand Devouard, Richard B. Frankel, Martha R. McCartney, Paul A. Midgley, Mihály Pósfai, and Matthew Weyland.

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.

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