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Summary

Traces of Ancient Martian Life in Meteorite ALH84001:
An Outline of Status in late 2003

Allan H. Treiman, Lunar and Planetary Institute, Houston, TX (2003).

The original hypothesis of McKay et al. (1996) – Four arguments together suggest that formation of carbonate globules in ALH84001 was associated with Martian life [1]. In the globules:

  1. Polycyclic aromatic hydrocarbons, PAHs (organic material) are Martian and characteristic of degraded organic matter.
  2. A mineral assemblage in the carbonate globules is characteristic of biologic influence.
  3. Sub-micron magnetite grains in the carbonate globules have properties indistinguishable from, and unique to, those formed by some Earth bacteria. Therefore, they are biogenic.
  4. Rock surfaces of and near the carbonate globules are decorated with bacteria-shaped objects. These objects are inferred to be mineralized remains of bacteria.

Precondition 1: For this hypothesis to be valid, the carbonate globules must have formed on Mars. This inference is broadly accepted [1,2]. But the carbonate globules may have been affected chemically and biologically during their tenure on Earth (e.g., [3,4]).

Precondition 2: For this hypothesis to be valid, the carbonate globules must have formed at conditions consistent with life-as-we-know-it, or -as-it-might-conceivably-be. Most investigators would agree that this precondition is met.

Temperature estimates have ranged from 0°C to 700°C, and possibly even higher in shock environments [1,5-6]. However, chemical zoning in the carbonates suggests low time-at-temperature [7,8], and carbonate globules like those in ALH84001 have been made in the laboratory at ~150°C [9]. Carbonate globules like those in the meteorite have been found in rocks of an Earth volcano and as crusts in an alkaline lake, both deposited by liquid water at T < 100°C [10,11].

Argument 1 of McKay et al. [1]: Organic matter (PAHs) associated with the carbonate globules is Martian and is biogenic.

  1. Are PAH organics present? Nearly certainly.
    McKay and collaborators [1,12] found organic matter in the carbonate globules and also along fractures distant from the globules, as did other researchers by other methods [13,14]. One research group has consistently found no evidence of organic matter in the carbonates [15].
  2. Are the PAH organics Martian? Possibly.
    A Martian origin for the specific PAHs targeted in [1] was suggested by intimate mixing with Martian carbonate, and decrease in abundance near fusion. [1,12,13]. However, nearly all the organic carbon in the meteorite is terrestrial contamination [16-18]. Recent C isotope data on organics in the carbonate [14] is consistent with a Martian origin, but is also consistent with biological formation from carbon of the carbonates (vis. [2]).
  3. Are the PAHs ultimately of biogenic origin? Unproven and unprovable?
    McKay et al. [1] claimed that the distribution of PAH species they found was similar to that of “degraded” biological matter, but no data have been presented to support that claim. Oxidative alteration of all sorts of PAHs leads to ‘core’ molecules like those observed in ALH84001 [19]. CM chondrite meteorites and interplanetary dust particles contain a similar mix of PAHs [20], leading to the hypothesis that the PAHs are from meteoritic infall onto Mars. Similarly, reactions between carbon and hydrogen-bearing gases (i.e., Fischer-Tropsch reactions) can produce complex mixes of PAH molecules inorganically [21-24].

Argument 2 of McKay et al. [1]. A mineral assemblage in the carbonate globules is characteristic of biologic influence. This argument is incorrect, though the mineral assemblages cited do not preclude Martian life.

McKay et al. [1] claimed that magnetite+“iron sulfide”+siderite in the carbonate globules represented a chemical disequilibrium, and thus was indicative of biogenic processes. In fact, that mineral assemblage is common in low-temperature, unoxidized, aqueous systems [9, 21], and is not diagnostic of biogenic processes [25]. However, the assemblage is not inconsistent with biological processes.

Argument 3 of McKay et al. [1]. Sub-micron magnetite grains from the carbonate globules have properties indistinguishable from, and unique to, those formed by some Earth bacteria. Thomas-Keprta et al. [26,27] modified the claim of [1] to include only approximately ~1/4 of the magnetite grains as being biogenic. This population, called truncated hexa-octahedra (THO), are claimed to be identical to magnetite grains from magnetosomes of some magnetotactic bacteria in size, shape, form, structural perfection, and lack of chemical substituents [1,26,27]. Therefore, they are biogenic.

It appears that THO magnetite grains can be formed inorganically and biologically. This argument has been explored extensively in the years since 1996, without a definitive conclusion. Disputes have arisen over whether these particular sub-micron magnetite grains are actually indistinguishable from intracellular bacterial magnetite grains, and over whether the general shape and size of the magnetite grains is unique to bacteria.

  1. Are these submicron THO magnetite grains really present in the carbonate globules? Yes.
    The presence of these magnetite grains and range of their shapes and sizes have been confirmed by several research groups [26-30].
  2. Did these submicron THO magnetite grains form on Mars? Yes.
    Despite considerable dispute on how the magnetite grains formed there is general consensus that they formed on Mars or during ejection from Mars. For a dissenting voice, see [3].
  3. Are these submicron THO magnetite grains identical to magnetosome magnetite grains from Earth bacteria? Maybe identical to some magnetosome magnetite grains from some bacteria.
    Thomas et al. [26,27] cite the near identity of the THO magnetite grains to magnetosome magnetite grains of the bacterium MV-1 [31] in: shape, sizes, distribution of sizes, structural perfection, and lack of chemical substituents. Other researchers have reported distinct differences between the ALH84001 and MV-1 magnetite grains in shape [30,32] and in distribution of sizes [24].
    It should be noted that few magnetotactic bacteria on Earth contain magnetosome magnetite grains like those in MV-1. Taylor et al. [33] figure many magnetosome magnetite grains that do not possess one or more of the ‘biomarker’ properties of magnetite grains cited above and in [26,27].
  4. Can THO magnetite grains like those in ALH84001 form only by biological processes? No.
    Golden et al. [29,30] have produced similar or identical magnetite grains by thermal decomposition of iron carbonate (siderite) in the laboratory, under reasonable conditions for Mars. Several researchers have shown that formation of these magnetite grains is consistent with thermal decomposition of iron-rich carbonate in the carbonate globules within the known geological history of ALH84001 [24, 28, 34-38].
  5. Does ALH84001 contain chains of submicron magnetite grains like magnetosomes? Probably not.
    Friedmann et al. [39] reported finding chains of electron-dense materials in ALH84001 that they interpreted as magnetosome chains of magnetite grains. At this time, there is no evidence that these electron-dense objects are magnetite, nor that they are in chains rather than in random groupings, nor that the objects are Martian.

Argument 4 of McKay et al. [1]. Rock surfaces of and near the carbonate globules are decorated with bacteria-shaped objects – spherical, ellipsoidal, and cylindrical objects < 400 nm long. All that is known about these objects is shape. These objects are inferred to be mineralized remains of bacteria.
These bacteria-shaped objects (BSOs) remain problematic. They are visually appealing, but scientifically weak (at this time), and the several images available may not represent the same type of structure(s). Little data has been presented to support a biogenic origin, and there is strong doubt that free-living cells can be as small as the BSOs. Several alternative abiotic hypotheses have been offered.

  1. Are these BSOs really present in the meteorite? Probably.
    Certainly, BSOs as figured in [1,40] are observed by scanning electron microscopy on surfaces of ALH84001. However, they may be artifacts of sample preparation – thickened conductive coatings on surface irregularities [41], and McKay et al. have agreed that some are artifacts [42]. Some probably are not artifacts [43,44].
  2. Are these BSOs of Martian origin? Possibly.
    Discounting BSOs that are laboratory artifacts, it is difficult to say whether most of the imaged BSOs are certainly Martian [1,40,43]. As most of the BSOs reside on fracture surfaces in ALH84001, it is extraordinarily difficult to exclude terrestrial effects. A comparison of ALH84001 and an Antarctic meteorite from the Moon showed similar “microfossil” features on both, suggesting that the “microfossils” are terrestrial [45]. One published image appears to show BSOs partially covered by indigenous meteorite material, identified as carbonate in a globule [40].
  3. Are these BSOs consistent with being fossil bacteria? Probably not.
    McKay et al. [1] claimed that the BSOs were fossil bacteria, but their volumes are ~ 1/1000 that of normal bacterial (e.g., E. coli), and 1/10 the volume of the smallest known free-living terrestrial bacteria [46]. A vocal minority of researchers insists that “nannobacteria,” with sizes comparable to those of the ALH84001 objects, can exist as free-living biological entities (e.g., [44, 47-49]).
    With the few images of BSOs available and the absence of chemical or biochemical data or internal structure, it is difficult to evaluate their biogenicity. It is worth noting that < 10 images of BSOs have been published, and nearly each image shows a different BSO morphology. In the available images, there is no sense of community structures, nor of commensalisms among different morphologies.
  4. Could these BSOs be biogenic, but not bacteria? Possibly.
    In the years since McKay et al. [1] was published, several sorts of submicron biogenic objects have been recognized. Thomas-Keprta al. [40] suggested that some of the BSOs were appendages of bacteria (like flagellae) or highly desiccated bacteria. Several other biological processes can yield spheres or rods in this size range [50-52].
  5. Could these BSOs be entirely abiotic? Possibly.
    Several researchers have suggested that the BSOs are entirely inorganic, or inorganic with decoration from sample preparation (see above).
    Bradley et al. [35] suggested that some of the BSOs are magnetite grains exposed on the surfaces of carbonate minerals. They noted that the submicron magnetite grains, ~100nm long, are approximately the same size as most of the BSOs. They also noted that some magnetite grains are found epitaxially arrayed within carbonate mineral grains, in a pattern strikingly similar to one of the most popular images circulated by the McKay group.
    Bradley et al. [41] suggested that some of the other BSOs are ridges on weathered mineral surfaces, decorated or not with conductive coating for SEM investigation.
    Finally, it is possible that the BSOs are merely mineral precipitates from solution [53-55]. In the absence of chemical data or internal structures, this simple alternative cannot be excluded.

Summary

The hypothesis of McKay et al. [1] has not been validated. As originally proposed, the hypothesis could not be accepted until all four of its arguments were proven. Proof has not been established for any one of the four arguments. Lacking additional data, there seems little reason to accept arguments 1, 2, and 4. {1} Although there are PAHs in the carbonate globules in ALH84001, there is no evidence that the PAHs are biogenic or were derived from biogenic precursors. {2} The targeted ‘biogenic’ mineral assemblages are common in abiotic situations, and easily made in sterile laboratory conditions. {4} There is no evidence beyond shape that the BSOs might be biogenic, nor is there evidence that they are Martian. Several reasonable hypotheses can explain the BSOs as inorganic structures.

Argument 3 has become the focus of research related to life in ALH84001 – whether or not a sub-population of the submicron magnetite grains is biogenic. Research in the last years casts more and more doubt on this argument. The shapes of the ALH84001 magnetite grains may not be the same as those in the target MV-1 bacterium [29,30,32], and the distribution of magnetite sizes is significantly different [24]. A credible inorganic hypothesis has been proposed, that explains the formation of the submicron magnetite grains by thermal decomposition of siderite [24,28-30,34-38]. Thus, argument 3 has fallen farther into doubt, although it may be impossible to prove that none of the magnetite grains has a biogenic origin.

In conclusion, no available data compel acceptance of any argument of McKay et al. [1], so there is no reason to accept its hypothesis. At some point, the discussion of [1] involves personal decisions about the nature of acceptable scientific proof. Available data do not disprove the any of the arguments of McKay et al. [1], nor the more general concept of life on ancient Mars. Lack of proof is not disproof. On the other hand, nearly all the data on ALH84001 and on Earth life developed since 1996 is not consistent with the claims, arguments, and hypothesis of McKay et al. [1]. These data are not a formal disproof of the hypothesis, as they do not (and probably cannot) definitively that each of the arguments is wrong – i.e., that all the PAHs in ALH84001 were formed abiotically. This inconclusive state cannot be of much comfort for advocates of McKay’s hypothesis [1]. Lack of disproof is not proof.

Acknowledgments.

I am grateful to M. Race for the opportunity to update this short summary of research on ALH84001. I have been assisted by so many people that it would be impossible to list them all. This work supported by NASA Grants NAGW-12184 (Cosmochemistry) and NAGW-12271 (Mars Fundamental Research). LPI Contribution # 1xxx.

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