Life (?) in Martian Meteorite ALH 84001:
A Preview of Presentations at the Upcoming
30th Lunar and Planetary Science Conference

Magnetite crystals produced by bacteria in order to sense a magnetic field have five characteristic properties: size consistent with magnetic single domains; extraordinary freedom from crystal structural defects (save twin planes); original formation in chains (which do not persist after disintegration of cell membranes); alignment along [111] crystallographic directions; and disequilbrium shapes, where crystallographically equivalent faces are of different sizes. These five criteria are used to identify bacterial magnetotactic magnetites on Earth, and should equally well apply to Mars. On the other hand, bacteria are also known to produce other varieties of magnetite, including elongate needles and grains with screw dislocations.

Magnetite crystals retain memory of their formation conditions in their shapes and chemical compositions. High-temperature (inorganic) magnetites are rarely pure magnetite -- they contain significant substitutions of Ti, Al V, or Cr. High-T magnetites are commonly equant, but elongated and platelet shapes have been reported. Sizes of high-temperature magnetites range from < 2 nm to centimeters across. Inorganic, low-temperature magnetites, like those formed in soils, commonly contain detectable substitutions of many minor and trace elements (e.g., Cu, Zn, Si, Cr). Sizes and morphologies can be quite variable. Biogenic, low-temperature magnetites can form within cells or outside them. Intracellular magnetites are diagnostically: free of trace element substituents, in a restricted size range (~20 -- 150 nm), and with distinctive morphologies (e.g. parallelepipeds). Extracellular biogenic magnetites are similar to inorganic low-temperature magnetites. Magnetites from ALH84001 include a range of sizes and shapes, with ~25% being parallelepipeds. TEM/EDS chemical analyses of these magnetites showed that the parallelepipeds contain no detectable Ti, Cr, Mn, or Ni (i.e. < 0.02% levels). Non parallelepipeds contain detectable Al and/or Cr. These differences suggest that the ALH 84001 magnetites formed by different mechanisms; the authors suggest that the parallelepipeds are intracellular biogenic, and that the others are low-temperature, possibly inorganic.

There has been considerable debate about the smallest possible sizes of biological organisms; the smallest proven viable organism is 100 nm diameter, and the smallest free-living organisms are near 200 nm diameter. But smaller objects can be produced by living organisms, can be preserved in the rock record, and can be characteristic of their existence. These small objects are named here as nanofossils: magnetotactic magnetites are one example; some spheres of hydroxylapatite are another.

Flynn G.J., Keller L.P., Jacobsen C., and Wirick S.
Organic Carbon in Mars Meteorites: A Comparison of ALH 84001 and Nakhla

Continuing their XANES and FTIR studies on new samples of ALH 84001, the authors confirm the presence of organic material throughout the carbonate globules. Organics are much more abundant in the carbonate rims (with magnetite) and are mixed with the carbonate on scales of ~100 nm. These samples showed no aliphatic organics. The authors also examined a fresh fragment of the Nakhla martian meteorite; a few spots in the clay-rich alteration material were rich in organic matter, qualitatively similar to that in ALH 84001 (and also in the C2 carbonaceous chondrites). These spots contained detectable aliphatic hydrocarbons (by FTIR) in addition to the aromatics detectable by XANES. Contamination of this sample seems unlikely.

The authors continue their ongoing work of applying TOF-SIMS methods toward studying PAHs in ALH84001. Here, they report new PAH spectra from the carbonaceous chondrite meteorites Murchison and Orgueil. The PAH mass spectra of these meteorites are similar to that of ALH 84001, meaning that the relative abundances of PAHs in the three meteorites are essentially the same. None of the three show evidence of heterocycles (PAHs containing S or N) nor evidence of extensive alkylation, both of which would be expected in biogenic PAHs. Thus, a non-biological origin can be inferred for the PAHs in all three meteorites.

ALH 84001 and Nakhla appear to contain indigenous (Martian) organic material, based on its wide distribution and the close association of PAHs with aliphatic hydrocarbons. The similarity of the organics in ALH 84001 and Nakhla suggest similar mechanisms of formation. Using a computer-based theoretical model program, the authors show that organics like those in ALH 84001 and Nakhla can arise by metastable Fischer-Tropsch-like processes during rapid cooling. As neither ALH 84001 nor Nakhla cooled fast enough during their igneous processing to yield these hydrocarbons, the authors propose that organics in both meteorites formed during cooling after shock events.

"Infection" of the Murchison (C2) meteorite with terrestrial biota stands as an example of the ubiquity of terrestrial biological contamination of meteorites. After a fungal hypha was noted on a sample of Murchison, that sample was cultured and a fungal species and a Bacillus species grew abundantly. Accompanying the microbiota are abundant spores, a partially mineralized (silicified) biofilm layer, and abundant growth of new minerals (rich in Mg and O) on the biofilm. A separate sample, which had been held in a standard SEM stub holder for 12 years at Johnson Space Center, showed an extensive mat of fungal hyphae.

The authors have found terrestrial bacteria and fungi living in several meteorite samples: two Antarctic chondrites, Murchison (C2), Nakhla (martian) and ALH 84001. The idea that meteorites might be commonly and significantly contaminated (inhabited) by terrestrial organisms is quite new. Appreciation of terrestrial biota in meteorites has been delayed by many factors: lack of impetus, ignorance of the diversity of terrestrial biota; limited study on mechanisms of contamination; reliance on thin sections for study of meteorites; instrumental methods (e.g., electron accelerating voltage in SEM studies), and heterogeneous distribution of the biota itself. Only by characterizing terrestrial biota in meteorites can one hope to isolate pre-terrestrial biota.

The authors have found, in a chip on ALH 84001, a complex structures that appears to be colonies of Antarctic Actinomycetes fungi. The structures appear as mats of 200 nm diameter fibers, found along cracks within 1 mm of the fusion crust. The fibers, with characteristic Y and V shapes places and proper sizes, are interpreted as fungal hyphae. The hyphae show structures which could possibly be spore-forming bodies. Associated with the hyphae is a biofilm, which (because it is not disturbed by the SEM electron beam) may be partially mineralized. Organisms like this are known in the cryptoendolithic microbiotic communities in Antarctica, and so the organism is almost certainly terrestrial. However, it may have utilized martian organics in ALH84001, and so may have a "martian" isotopic signature despite its terrestrial origin.

Samples of the Nakhla martian meteorite, from a stone completely covered with fusion crust, recently became available. SEM imagery of the fusion crust shows the hyphae of fungi growing from spherical spores. SEM imagery of interior surfaces appear to show hyphae which have invaded the stone, and surfaces coated by thin carbon-rich coatings. These carbon-rich coatings do not have the stranded appearance of desiccated biofilms, and may be insoluble organics indigenous to the meteorite. Further studies are in progress.

The martian meteorite Nakhla contains clay rich veinlets which formed on Mars. In these veinlets, the authors have found objects that may be bacteria or mineralized bacteria. In optical microscopy, clay-rich veinlets contain many dark objects at the limit of optical resolution ~ 0.25 -- 0.5 micrometers. By scanning electron microscopy, the authors describe rounded or spherical particles 0.2 to 1 micrometer in diameter associated with the clay-rich alteration material. The authors interpret these bacteria shaped objects as mineralized bacteria. The objects have similar chemical compositions to the surrounding clays, but some may be richer in iron. Some of the spherical objects are beneath and within the martian clays, and so are martian themselves (others are only on surfaces and may be terrestrial). Similar objects have also been observed in the Shergotty martian meteorite.

1436

The authors have investigated fossils and remnants of bacteria in deposits from carbonate hot springs. Hot carbonate springs support diverse of biota, ranging from 15µm microbes to 100 nm "nannobacteria." Organic matter of bacteria degrades rapidly in springs hotter than 30°C. Even so, some biochemical tracers of cell walls are preserved and can be assayed. Biofilm, on the other hand, is preserved intact in water up to 72°C, and can be recognized (even though desiccated) years afterwards. Biofilm can be readily mineralized, commonly by spheres of silica, 50 -- 300 nm across. Biofilms and other structures can be mineralized and be apparent in the petrofabrics of ancient spring deposits. These results enforce the notion that thermal spring deposits should be a target in the exobiological exploration of Mars.

1661

On Earth, iodine is strongly concentrated in biological organisms, leading to a distinctly high I/Cl abundance ratios in biogenic materials, and distinctly low I/Cl ratios in the environment (e.g. seawater). This variable distribution of I may serve as a biomarker for Mars. Carbonate in ALH84001 are relatively rich in iodine (to 0.5 ppm), but this enrichment could equally well reflect inorganic as biogenic processes. Iodine is also fairly abundant in the Nakhla martian meteorite, where it appears to be held on grain surfaces (as is xenon). It is not clear if biological activity is needed to explain the iodine abundances in these meteorites.

1683

The authors reported earlier that carbonate globules from Spitsbergen are comparable in many respects to those in ALH 84001. Like the ALH 84001 globules, those from Spitsbergen: form as open space fillings; consist of carbonate crystals radiating from a core; are zoned from calcian cores to magnesian rims; preserve fine-scale zoning; and are associated with clays and silica. The presence of globules in the basalts at Spitsbergen implies that they are not mantle products nor immiscible melts. Globules have also been found in fluid inclusions with liquid and vapor bubbles along healed fractures in olivine. Additional field work is needed to clarify the origin of the Spitsbergen carbonate globules.

1855

Several varieties of bacteria precipitate siderite (FeCO₃) as an extracellular product of their iron-reducing metabolisms. The authors did experiments on two strains of bacteria, one mesophilic (T=20-35°C) and one thermophilic (T=45-65°C). As the mesophile grew, the solution became more basic and more reducing, and siderite discs (to 3 µm diam. and <0.4 µm thick) were deposited. As the thermophile grew, starting ferric oxide-hydroxide became black and magnetic -- diffraction data showed both siderite and magnetite. The siderite formed globules of 3 -- 5 µm diameter, and the magnetite formed octahedra of <0.3 µm on edge. Other bacteria (not studied here) produce siderite crystals in typical rhombohedra.

2032

Carbonate globules in basalts and xenoliths from Spitsbergen are a possible analogy to the globules in ALH 84001. There has been some debate about the formation conditions of the Spitsbergen globules. Here, the authors analyzed O and C isotope ratios in dolomite and magnesian calcite from the globules using SIMS. In the magnesian calcite, oxygen has d ¹⁸O~+22� and d ¹³C=-4.1--+0.5�, consistent with limestones or possibly hydrothermal carbonates. In the dolomite, oxygen has d ¹⁸O~+27� and d ¹³C=-4.8--+7.8�, consistent with diagenetic cements and concretions. These data seem inconsistent with a mantle origin, and suggest that the carbonate globules formed from hydrothermal fluids that remobilized carbonate from crustal sources.

1174

The authors hearken back to J.W. Schopf's criteria for recognizing signs of life in the fossil record, and apply them to their data on ALH 84001. 1) The authors find that the geologic context of ALH 84001, ancient Mars, is compatible with past life. 2) The sample and its stratigraphic location are compatible with life. The authors find that ancient warm, wet Mars is compatible with life. 3) The authors find that ALH 84001 contains evidence of cellular morphologies, although some are too small for accepted constraints on life as we know it. The authors also point to possible biofilms and possible bacterial filaments as evidence. 4) The authors find that evidence for colonies of microorganisms are provided by the presence of putative biofilms. 5) The authors find that the parallelepiped magnetite crystals in the carbonates constitute evidence of biomineralization. 6) The authors suggest that some signs of isotopically light carbon in ALH 84001 point in the direction of isotopic patterns unique to biology. 7) The authors find that the presence of PAHs in the sample constitutes an example of an organic biomarker. 8) The authors find that all these characteristics are indigenous to the ALH 84001. Further the authors believe that their case for ancient martian life in ALH 84001 is stronger now than it was when originally presented in 1996.

1324

Similar to N. Boctor's work of last year, the authors have analyzed the isotopic composition of hydrogen in carbonate minerals in ALH 84001. The hydrogen is quite enriched in deuterium, with d D values to +1344. Larger grains gave higher values, suggesting the presence of a surface-correlated component with a relatively low d D. These data suggest that the Martian atmosphere was already significantly fractionated in hydrogen isotopes by the crystallization time of the carbonates, ~ 4.0 Ga.

1389

Rao M.N., Schwandt C., and McKay D.S.

Trapped Argon and Xenon in EET79001 and ALH 84001: Clues to Low-Temperature Aqueous Activity on Mars

The authors compare how much of Ar and Xe trapped in these Martian meteorites derives from atmospheric vs. other sources. A larger fraction of the Xe is in these meteorites came from the martian atmosphere than did the Ar in the meteorites. This fractionation of noble gases, Xe retained more than the lighter Ar, is consistent with solution of the noble gases in water. Thus, the authors propose that the atmospheric components of Xe and Ar in ALH 84001 were emplaced via groundwater that had equilibrated with the martian atmosphere. The high concentrations of atmospheric noble gases in ALH 84001 can be ascribed to then entrapment on the huge surface areas of carbonaceous particulates, carbonate globules, and granulated pyroxenes.

1397

The authors have measured the ratio of deuterium to hydrogen (D/H) in shock glasses from two martian meteorites. Both show extreme enrichments in D consistent with hydrogen isotopic exchange with the martian atmosphere. Less extreme enrichments are attributed to terrestrial water contamination. The most extreme value in feldspathic glass from ALH 84001 is d D = +1755�. These results also indicate that significant proportions of water, to 0.7% by weight, can be shock glasses.

1539

The authors demonstrate that computed tomography using X-ray absorption can be used to investigate the internal three-dimensional structures of meteorites. In ALH 84001, chromite seemed to be randomly distributed in part of the samples, and was aligned in planar zones in another portion. Comparison with optical and SEM imagery will be require to calibrate these X-ray CT scans.

1637

Earlier, the authors recognized "net-veinlets" of silica in ALH 84001, and had interpreted the silica as having been deposited from water solution. Here, they compare the silica in ALH 84001 with silica in other martian meteorites: Shergotty and Zagami. In both of these meteorites, silica is present in interstitial areas and mesostases, and preserves original igneous textures (e.g., graphic granite). As Shergotty and Zagami were also heavily shocked, it seems that silica can preserve its original textures through shock events. Thus, it seems reasonable that the net-veining silica in ALH 84001 is an original, pre-shock, texture. The silica is held to have been deposited by aqueous solutions on Mars.

1951

An earlier abstract from this group reported the presence of pyroxene-composition glasses in ALH 84001. Here, the authors expand on that observation, finding common films of pyroxene composition glass between solid pyroxene and carbonate globules, along fractures in pyroxenes, and in fractures across chromites. These pyroxene composition glasses must post-date formation of the carbonate globules.

2038

Reflection spectroscopic analyses of fragments of ALH 84001 have correctly identified low-Ca pyroxene as its dominant mineral; carbonates, magnetite, and organic matter have also been detected.