Workshop on "Martian Meteorites:
Where Do We Stand and Where Are We Going?"
A Review of Presentations Emphasizing Martian
Meteorite ALH 84001

The LPI will convene a workshop on martian meteorite ALH 84001, and the broader issues surrounding its possible evidence of ancient martian life on November 2-4 at the Lunar and Planetary Institute, Houston, Texas.

The talks and anticipated discussion will revolve around five themes: "Allan Hills 84001"; "Signs of Life: Biomarkers and Biogenicity"; "Key Research: Life on Earth"; "Key Research: Meteorites and Planetary Geology"; and "Search for Life on Mars." There will be three invited talks on each theme and much time for learned discussion; yours truly has the lead-off talk. All "non-invited" contributors will bring a poster to share their thoughts.

All the talks and posters at this meeting are relevant in one way or another to ALH 84001 and the controversies it has started. All abstracts of talks and posters are on line. I've summarized the abstracts that focused most tightly on ALH 84001. My apologies if your abstract (or your favorite abstract) is not summarized. To read a full abstract from the conference, double-click on the highlighted title, which will connect you to the online abstract in PDF format. To view the abstracts, you need the Adobe Acrobat Reader, which can be downloaded from Adobe.

Abstracts are summarized in the order they appear in the program. References are listed after the summaries. Summaries and a few comments (in italics) are by Allan H. Treiman, Lunar and Planetary Institute.

Invited Talks

Treiman A. H. Ancient martian life in Allan Hills 84001? Status of some current controversies.

There is no agreement about the formation temperature of the ALH 84001 carbonates, hosts to possible signs of martian biota. Estimates range from ~700°C to ~0°C, with identical data commonly interpreted to mean both high and low T. Treiman feels that the bulk of evidence suggests T < 400°C, with little evidence of whether T was cool enough for life as we know it. McKay et al. (1996) reported organic matter, PAHs, in ALH 84001; that finding has been confirmed. It is not clear, however, if these PAHs are martian or are terrestrial contaminants; most or all of the other organic matter in ALH 84001 is terrestrial. If the PAHs are martian, they could have formed without biology. Bacteria-shaped objects (BSOs) were identified as possible fossilized martian bacteria (McKay et al., 1996), and that claim is disputed. Various BSOs have been claimed to be magnetite crystals, mineral surface irregularities, and artifacts of sample preparation. In addition, Earth bacteria and fungi are now known to inhabit martian meteorites (Steele et al., 1998). The hypothesis that ALH 84001 contains traces of ancient martian biota has not been proved or disproved at this time.

Harvey R. P. Formation of carbonates in Allan Hills 84001 by impact metasomatism: Cooking with gas.

The carbonate globules in ALH 84001, hosts to possible evidence of martian biota, actually formed at ~675°C during the aftermath of an asteroid impact onto Mars (Harvey & McSween, 1996). This impact event produced fluids rich in CO2, which then permeated ALH 84001, deposited the carbonate globules, and cooled within a few minutes or seconds. On entry, the CO2-rich fluid stripped Fe and Mg from olivine and pyroxene in ALH 84001, leaving pyroxene and silica in their stead. Iron and Mg in the fluid then produced the carbonate globules. The carbonates have complex textures at submicrometer scales and show thermal and mechanical effects of shock. Magnetite grains in the carbonates also formed at high temperatures, possibly in the same event that formed the carbonates. The carbonate-forming event was "nearly instantaneous" and does not represent a process at chemical equilibrium.

McKay D. S. Evidence for ancient life in Mars meteorites: Lessons learned.

The controversy and continuing research about ALH 84001 teach important lessons for Mars sample return and the search for life in the solar system. (1) Martian meteorites can be extraordinarily complicated, and there is no reason to suspect that returned martian samples will be less so. Robotic instruments landed on Mars could not have found or collected any of the evidence presented on either side of the controversy. (2) Our understanding of biomarkers is inadequate. (3) Our ability to determine ages for secondary events in rocks is inadequate. (4) It is difficult to prove or disprove a hypothesis built on circumstantial evidence. Proposers of a hypothesis may have the moral obligation to prove (or refute it), but not necessarily the responsibility to do so.

Allan Hills 84001 Posters

Bodnar R. J. Fluid inclusions in Allan Hills 84001 and other martian meteorites: Evidence for volatiles on Mars.

One of the critical issues in formation of the carbonates in ALH 84001 (and their putative signs of martian life) is the composition of the fluids that formed them. In ALH 84001, Dr. Bodnar has found inclusions of fluid that appear to have formed early in the rock's history. The inclusions consist of clear liquid and vapor bubbles, which are in rapid Brownian motion. By optical examination the liquid and vapor appear to both be CO2. This inference is consistent with scenarios in which the ALH 84001 carbonates form at high temperatures from fluid rich in CO2 (Harvey and McSween, 1996).

Borg L. E., Nyquist L. E., Shih C.-Y., Wiesmann H., Reese Y., and Connelly J. N. Rubidium-strontium formation age of Allan Hills carbonates.

Rubidium-strontium age dating of carbonate minerals in ALH 84001 shows that they formed 3.90 or 3.85 + 0.04 billion years ago (depending on which half-life one uses for radioactive Rb). The authors derived this age by acid leaching a sample of ALH 84001 in successively stronger acids, dissolving the carbonates in order of their susceptibility to acid attack (dolomite, siderite, magnesite). Analyses of the purest of these leaches gave the age date. The carbonates formed ~600 million years after ALH 84001 crystallized from magma, and at about the same time as the impact event dated by K-Ar and some Rb-Sr analyses. The similarity in the timing of impact and carbonate formation suggests they may be related, but not necessarily that the carbonates formed at high temperature during the impact.

Brearley A. J. Rare K-bearing mica in Allan Hills 84001: Additional constraints on carbonate formation.

This is the first confirmed sighting of a martian, water-bearing mineral in ALH 84001; earlier reports of clay minerals did not show that they were martian (Thomas-Keprta et al., 1997). Using transmission electron microscopy, Dr. Brearley found small grains of a layer silicate mineral (water bearing) inside a fragment of a carbonate globule. The grains are martian because they are cut off by, and decomposed against, the silicate glass that encloses the carbonate fragment. The silicate glass formed on Mars, so the water-bearing silicate must have also. Dr. Brearley identifies the mineral as illite, a mica mineral, and suggests that it grew at the same time as the surrounding carbonates. Illite forms only at temperatures below ~300°C, and the structural perfection of the ALH 84001 illite grains suggests that they formed above ~100°C.

Flynn G. J., Keller L. P., Jacobsen C., and Wirick S. Organic carbon in carbonate and rim from Allan Hills 84001.

To supplement their previous observations of organic carbon in the ALH 84001 carbonate globules, the authors prepared another sample of globule for XANES analysis (X-ray Absorption Near-Edge Structure). XANES allows estimation of the abundance of carbon-bearing and organic matter, and determination of how the carbon atoms are bonded. As before, X-ray absorption features can be ascribed to organic carbon (confirmed by infrared spectroscopy). Globule rims have more organic carbon (compared to carbonate) than do the globule cores -- the rims' organic carbon is concentrated in hot spots but never without carbonate. So, the organic carbon and carbonate are intimately mixed on scales of ~100 nm in the rim. The same XANES absorption feature are seen in the rims and cores of globules, indicating that both contain the same type(s) of organic matter. A different type of organic matter is seen in areas rich in feldspathic glass and chromite.

Ksanfomality L. V. The structure of findings in the Allan Hills 84001 may hint at their inorganic origin.

Dr. Ksanfomality suggests that some of the bacteria-shaped objects (BSO) cited by McKay et al. (1996) in ALH 84001 may have inorganic origins. The swarm of BSOs in McKay's Figure 6b [I think this is the figure they refer to] are all the same size, which terrestrial bacteria are not. The fossil "worm" in another McKay et al. image is similar to abiogenic mineral structures found in granitic pegmatites -- a purely igneous, high-temperature environment.

Scott E. R. D. Biogenic or abiogenic origin of carbonate-magnetite-sulfide assemblages in martian meteorite Allan Hills 84001.

Dr. Scott compares the carbonate globules of ALH 84001 with terrestrial analogs, and concludes that "Arguments for a biogenic origin . . . are absent or weak." Of the submicron magnetite grains in the ALH carbonates, all but the bullet-shaped grains are known to form inorganically. Whisker magnetites (with central screw dislocations) and ribbon magnetites are unlikely to be bacterial. Dr. Scott sees no reasonable explanation for finding magnetite from magnetotactic bacteria in an igneous rock like ALH 84001, and concentrated uniformly into double shells at the edges of carbonate globules. Sulfide grains found in the ALH 84001 carbonates cannot be distinguished from inorganic precipitates, and are not found in magnetotactic bacteria. Nor are the ALH 84001 carbonate globules comparable to bacterially precipitated (or induced) carbonate aggregates.

However, abiogenic low-temperature precipitates of carbonate-magnetite-sulfide in carbonaceous chondrite meteorites are not comparable to those in ALH 84001. Dr. Scott favors a high-temperature origin, with globules crystallizing rapidly from carbonate-rich melts.

Scott E. R. D. and Krot A. N. Origin of carbonate in martian meteorite Allan Hills 84001.

The authors argue that the ALH 84001 carbonate globules formed as crystallized carbonate-rich magma, originally produced in a high-temperature impact event. (1) The carbonates and feldspathic glasses in fractures have essentially the same shapes and spatial distributions, and so they likely formed in the same way. As the glasses reflect melting at T > 1200°C, so were the carbonates melted. (2) The carbonates cannot have formed by replacement of earlier minerals, involving either gas or water, because replacement cannot explain the lack of aluminosilicate replacement minerals and the association of carbonate and feldspathic glass. (3) In addition, carbonate masses in cracks are actually lenticular, not pancake-shaped, which can be explained as the fractures closing during carbonate growth. Before being melted, the original carbonates in ALH 84001 were probably formed at low temperature, perhaps as evaporite deposits. This model does not explain the chemical zoning of the globules, but experimental data on relevant systems are lacking.

Shearer C. K. and Adcock C. A comparison between sulfide assemblages in martian meteorites Allan Hills 84001 and Governador Valadares.

The authors are studying the forms and S-isotopic compositions of the sulfide minerals pyrite and pyrrhotite in martian meteorites ALH 84001 and Governador Valadares (GV; a nakhlite). Pyrite is dominant in ALH 84001 and occurs as inclusions in pyroxene (also pyrrhotite), large aggregates (with pyrrhotite), small irregular grains with carbonates, very irregular grains in shock glass, and submicrometer grains in carbonate. This wide range of textures and equally wide range of S-isotopic compositions suggests that the ALH sulfides formed in a number of separate events, modified by post-magmatic heating/impacts. In contrast, sulfides in GV are present only as minute grains in and among pyroxene crystals; the pyrrhotite is probably magmatic and has restricted range of S-isotopic compositions.

Shearer C. K. and Brearley A. Evidence for a late-stage thermal overprint in Allan Hills 84001 and implications for biomarkers.

Allan Hills 84001 has had a complex history that must be properly decoded to evaluate putative evidence for possible martian life. The authors document evidence for a high-temperature thermal pulse after formation of the carbonate masses. (1) The globules are disrupted and veined by feldspathic shock glass. Carbonate globules are commonly broken along their cleavage planes and transported in the feldspathic melt. (2) Olivine grains in the meteorite's orthopyroxene are only found near carbonate globules and fractures that contain disrupted carbonate globules. This suggests that the olivine is related to the carbonates, yet it preserves chemical and isotopic compositions indicating high-temperature equilibrium with the surrounding orthopyroxene. (3) Sheet silicates in the carbonate globules are truncated and decomposed at their boundaries with feldspathic glass (Brearley, this meeting). The authors suggest that orthopyroxene, olivine, and chromite achieved chemical equilibrium at 850°-900°C in this post-carbonate event. Injection of feldspathic melts into carbonates requires even higher temperatures, >1000°C. This high-temperature history explains the presence of elongate "whisker" magnetites in the carbonates, and may explain the range of O-isotopic compositions of the carbonates through progressive decarbonation. [Much of this evidence for a late shock has been given elsewhere, and the proposed timing of the high-temperature equilibration is at variance with earlier theories. I suspect this poster will be controversial.]

Stephan T., Rost D., Heiss C. H., Jessberger E. K., and Greshake A. The lateral distribution of polycyclic aromatic hydrocarbons in Allan Hills 84001: Implications for their origin.

Among the lines of evidence cited by McKay et al. (1996) for martian biologic activity in ALH 84001 was the presence of organic molecules, polycyclic aromatic hydrocarbons (PAHs), associated with the meteorite's carbonate globules. The authors continue their analyses for PAHs in ALH 84001 by time-of-flight secondary ion mass spectrometry (TOF-SIMS), which has much tighter spatial resolution than McKay's method. The authors find that the PAHs are nearly everywhere in ALH 84001 at low abundance levels, but they are less abundant in the carbonate globules! Polished sections (as they use) are inherently more susceptible to contamination, but the authors found that no detectable PAHs on a polished surface of a different meteorite prepared with the same methods. It seems likely that the authors are seeing the same mix of PAH molecules as did McKay et al. (1996), but not the concentration in carbonate globules. The organic material detected by Flynn et al. in the carbonate globule rims may not be PAHs.

van der Bogert C. H. and Schultz P. H. High strain-rate deformation and friction melting as a possible origin for "shock" features in Allan Hills 84001.

Allan Hills 84001 experienced a complex history of deformation and thermal events, all of which have been ascribed to shock metamorphism from asteroid impacts on Mars. The authors make a distinction between the high pressure of shock metamorphism and intense deformation, which can occur without high pressure. ALH 84001 contains few direct signs of the high pressure of shock metamorphism (> 20 GPa). ALH 84001 contains no high-pressure minerals (e.g., ringwoodite, stishovite); its feldspathic glass is not the high-pressure maskelynite but a "normal" glass. Lacking firm signs of high shock pressures, the authors suggest that rapid ("high-strain-rate") deformation caused many of the textural features in ALH 84001, like signs of shearing and folding and finely broken mineral grains (i.e., cataclasis). Identical textures are found on Earth in fault zones and around impact craters. [I think the point here is that asteroid impacts can cause intense deformation without extremely high pressures, and the intense deformation is not "shock metamorphism."]

Warren P. H. The common ion effect in deposition of martian (e.g., Allan Hills 84001) carbonates.

"The most plausible model for origin of the carbonates in ALH 84001 involves deposition from a playa lake or zone of groundwater . . ." (Warren, 1998). A problem for this type of model is the absence of sulfate minerals in ALH 84001. In solutions dominated by cations Mg+2 and Fe2+, carbonates are less soluble than sulfates, and common ion effects would delay the precipitation of sulfate minerals even further. Thus, only carbonate minerals should precipitate from moderately concentrated solutions (e.g., groundwater in pores in rock), and sulfates would not precipitate until the waters became extremely concentrated, as at the martian surface in a playa lake.

Wentworth S. J., Thomas-Keprta K. L., and McKay D. S. Alteration products and secondary minerals in martian meteorite Allan Hills 84001.

The authors examined ALH 84001 to characterize its weathering products, both terrestrial and preterrestrial, aside from the carbonate globules. ALH 84001 was weathered very little inside or outside. The fusion crust shows traces of typical Antarctic minerals: Ca- and Mg-sulfates, NaCl, and silica. However, these minerals are not present inside the meteorite (farther than 200 µm). The most abundant weathering effects inside ALH 84001 involve dissolution, pitting of carbonates and etching of pyroxene. Traces of smectite clay are associated with pyroxene. Blade-shaped crystals of an Mg-carbonate [hydromagnesite or nesquehonite?] and patches rich in Fe and S (weathered pyrite?) are also present. It is not known if these interior effects represent terrestrial or pre-terrestrial weathering.

Signs of Life: Biomarkers and Biogenicity Posters

Friedmann E. I., Wierzchos J., and Ascaso C. Chains of magnetite crystals in Allan Hills 84001: Evidence of biological origin.

Previously, submicrometer-sized magnetite crystals in ALH 84001 have been likened to crystals produced by magnetotactic bacteria on Earth. These bacteria produce chains of magnetite crystals to help them align themselves with the Earth's magnetic field. The authors report having found numerous chains of magnetite crystals in ALH 84001, similar to those chains formed by Earth bacteria. The authors imaged polished sections in back-scattered-electron mode in a scanning electron microscope. Chains of magnetite crystals occur in the carbonate masses; chains comprise up to 15 magnetite crystals in a row. Individually, the crystals are parallelepipeds or irregular grains 30-90 nm long and 20-50 nm wide. These shapes and sizes are common among terrestrial magnetotactic bacteria. In one chain, the individual magnetite crystals are >200 nm each in length. The authors suggest that the dead remains of martian magnetotactic bacteria were suspended in carbonate-rich fluid, on Mars, flushed into the rock, and deposited with (and in) the carbonate masses. All terrestrial magnetotactic bacteria are anaerobic, which would be consistent with their survival on early Mars.

Garcia-Ruiz J. M. Biomimetic but abiotic carbonates: New geochemical markers for primitive environments.

Interpretation of morphological features in ALH 84001 as biogenic structures relies on the idea that inorganic mineral deposits do not form in shapes, sizes, and symmetries like those produced by life. However, complex inorganic shapes that look like biological structures can be made in the laboratory without any life present. Carbonate minerals grown in silica-rich alkaline brines form unusual, bizarre shapes that seem to mimic biology (thus 'biomimetic'). Similar biomimetic shapes of silica can grow under alkaline conditions, such as might have been possible in the early histories of the Earth and Mars.

Gibson E. K. Jr., McKay D. S., Thomas-Keprta K., Westall F., and Romanek C. A. How do the properties of Allan Hills 84001 compare with accepted criteria for evidence of ancient life?

The authors give eight criteria which should be met for general acceptance of evidence for past life in a geological sample. (1) The geological context of ALH8 4001 is compatible with life, as it comes from a place (Mars) where liquid water was present. (2) The age and geologic setting of ALH 84001 are known well enough; its carbonates formed 4.0 billion years ago in the martian highlands, which are known to have hosted liquid water at that time. (3) ALH 84001 contains evidence of cellular mophologies. Although nonbiological explanations have been offered for some of bacteria-shaped objects, many are consistent with a biological origin. (4) ALH 84001 may contain evidence of biotic colonial or community structures in its possible biofilm structures (McKay et al., 1997). (5) ALH 84001 contains possible biominerals and evidence for biogenic chemical disequilibria) in the form of the magnetite crystals in its carbonate globules. These magnetites are indistinguishable from biogenic (magnetotactic) magnetites. (6) ALH 84001 contains isotopic signatures suggestive of biologic activity, in that its carbon-isotopic compositions are "in the direction of known biogenic carbon signatures." (7) ALH 84001 contains organic biomarkers in the form of its PAHs. [8] The above signatures are truly indigenous to the meteorite.

Some of the criteria are close to being satisfied ([5], [7], [8]) and others are not completely satisfied ([1], [4]). No one of these criteria has been completely violated by published data.

Thomas-Keprta K. L., Bazylinski D. A., Wentworth S. J., McKay D. S., Golden D. C., Gibson E. K. Jr. and Romanek C. S. Mineral biomarkers in martian meteorite ALH84001?

The authors extracted thousands of submicrometer magnetite grains from ALH 84001 by dissolving carbonate globules in acid. The magnetites come in three shapes: whiskers, irregular grains, and elongate prisms. Only 6% of the magnetites were whiskers, with length/width >3.67% of the magnetites are irregular (including cuboidal and teardrop grains). 27% of the magnetites are elongate hexagonal prisms with length/width <2.6, and nearly all have length/width between 1 and 2. The elongate prism magnetites are identical to those produced by some specific terrestrial bacteria: size, high chemical purity, length/width ratio, elongation along crystallographic direction (111), hexagonal cross section, and rectangular outline when viewed perpendicular to the elongation. No known inorganic magnetites share these features, so the elongate prism magnetites are viewed " likely evidence for primitive life on early Mars."

Zolotov M. Yu. and Shock E. L. Abiotic synthesis of hydrocarbons on Mars: Theoretical modeling of metastable equilibria.

Polycyclic aromatic hydrocarbons (PAHs) were found in ALH 84001 by McKay et al. (1996) and inferred to be traces of martian biota. Anders (1996), however, suggested that the same PAHs might have formed inorganically. Here the authors evaluate Anders' idea by modeling chemical equilibria among likely gases and organic molecules at temperatures and pressures relevant for Mars. Under highly oxidizing conditions in fluids (HM buffer) or with martian soil/regolith, organic molecules are not stable. Under reducing conditions like those in the martian meteorites (QFM buffer), hydrocarbons can form at temperatures below about 180°C. Lower temperatures and lower abundances of oxygen gas (higher pressures of hydrogen gas) increases the yield of hydrocarbons, both PAHs and alkanes (non-aromatic hydrocarbons). In addition, higher temperatures and slightly higher abundances of oxygen favor production of PAHs from other hydrocarbons. On Mars, the abundance of basaltic rocks would favor low oxygen abundances and high hydrogen abundances in groundwater and in volcanic systems. Under these conditions, PAHs could readily be formed without the action of biology. However, the oxidizing conditions observed in the martian soil do not favor formation of PAHs.

Key Research: Life on Earth Posters

Thomas-Keprta K. L., McKay D. S., Wentworth S. J., Stevens T. O., Taunton A. E., Allen C. C., Gibson E. K. Jr. and Romanek C. S. Mineralization of bacteria in terrestrial basaltic rocks: Comparison with possible biogenic features in martian meteorite Allan Hills 84001.

Little is known about the preservation of micro-organisms in igneous rocks. Rocks from the Columbia River Basalts (CRB) do contain indigenous bacteria, so the authors studied their preservation and mineralization (fossilization) by growing some under controlled conditions. Chips of fresh basalt cultured in sterile solutions showed no bacteria-shaped objects or possible cellular appendages. However, chips of the same basalt inoculated with CRB bacteria had bacteria-shaped objects of three types: (1) oval forms with smooth surfaces, 1-2.5 µm long, made mostly of iron, manganese, and oxygen with a little phosphorus; (2) oval forms of the same size with rough surfaces of thousands of thread-like filaments of the iron hydroxide hydrate mineral ferrihydrite, with some detectable phosphorus; and (3) tubular forms 0.3-2.4 µm long with a single filament at one end, composed mostly of carbon, oxygen (and hydrogen), and sodium, with traces of phosphorus, chlorine, and sulfur. In addition, objects like type 3 filaments were found - they are made of ferrihydrite. Type 3 objects are cells. Types 1 and 2 objects are hollow, 'fossilized' bacteria with no sign of internal structures. These results suggest that bacteria in basaltic rocks can become mineralized rapidly and lose detectable internal structures on the timescales of laboratory experiments. Although these experiments do not show that bacteria-shaped objects in ALH 84001 are fossilized bacteria, they do show a plausible way of forming the observed features from bacteria.

References: See the ALH papers Web site for more on most of these references.

Harvey R. P. and McSween H. Y. Jr. (1996) A possible high-temperature origin for the carbonates in the martian meteorite ALH 84001. Nature 382, 49-51.

McKay D. S., Gibson E. K. Jr., Thomas-Keprta K. L., Vali H., Romanek C. S., Clemett S. J., Chillier X. D. F., Maechling C. R., and Zare R. N. (1996) Search for past life on Mars: Possible relic biogenic activity in martian meteorite ALH 84001. Science 273, 924-930.

McKay D. S., Gibson E. K., Thomas-Keprta K., Romanek C. S., and Allen C. C. (1997) Possible biofilms in ALH 84001. Lunar Planet. Sci. XXVIII, 919-920.

Steele A., Goddard D. T., Toporski J. K. W., Stapleton D., Wynn-Williams D. D., and McKay D. (1998) Terrestrial contamination of an Antarctic chondrite (abstract). Meteor. Planet. Sci. 33, A149.

Thomas-Keprta K. L., Romanek C. S., Wentworth S. J., McKay D. S., Fisler D., Golden D. C., and Gibson E. K. (1997) TEM analysis of fine-grained minerals in the carbonate globules of martian meteorite ALH 84001 (abstract). Lunar. Planet Sci. XXVIII, 1433-1434.

Warren P. H. (1998) Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH 84001 meteorite. Jour. Geophys. Res. 103, 16759-16773.