Allan Treiman
Lunar and Planetary Institute
Sadly, I am now ending this Internet web page, where I have tried to present technical papers about the martian meteorite ALH84001 for the non-specialist. With time, I have formed fairly strong opinions about the hypothesis of McKay et al. (1996), and have found it harder and harder to be insightful and remain objective. I have enjoyed writing for this web page, and I hope that my efforts have been helpful. Thank you for your support.
Allan Treiman. December 12, 2000.
The first paragraph after the title is an "executive summary" or sound bite of the article. It is followed, in indented normal typeface, by a longer summary. Last, in double-indented italic typeface, are my insightful and totally objective commentaries, worth exactly what you paid for them.
As before, the papers on ALH 84001 are given in reverse chronological order of publication date. Double line rules are used to distinguish the more recent additions from previous listings.
Relevance � Direct
Topic � Origin of PAHs.
McKay et al. (1996) reported organic material, PAHs, in the carbonate globules of ALH84001, and claimed it as evidence of ancient martian life. Based on thermochemical calculations, the authors find that the PAHs (and the proportions of the various PAH species) could reasonable have been produced by inorganically, without biological influences.
McKay et al. (1996) claimed that carbonaceous matter, the PAHs, in ALH84001 was derived from Martian microbes. Anders (1996) countered that these PAHs were likely formed without any biological activity, similar to an industrial processes called the Fischer-Tropsch reaction. Here, the authors use equilibrium thermochemical data to calculate whether PAHs in ALH84001 could possibly have formed in place, inorganically, under likely physical and chemical conditions. Their goal was to see if equilibrium thermochemical data could predict the presence and proportions of PAHs found by McKay et al. (1996) and Clemett et al. (1998): abundant PAHs compared to other carbon compounds (aliphatics), the apparent deficiency in small PAH compounds, and the low abundance of PAHs with attached chains of carbon atoms.
Zolotov and Shock assumed a very simple "scenario" for PAH formation. At some time, ALH84001 was suffused with a gas of hydrogen, carbon dioxide, and carbon monoxide at a high temperature, 1100°C. This could have been when the rock formed, or after an impact shock event (Treiman, 1998). The gas mixture then cooled down, and PAHs formed from it. Because the original gas composition is not known, Zolotov and Shock studied a wide range of reasonable guesses. For each gas mixture, they calculated the temperature at which each type of PAH molecule would form and condense from the gas to a solid or liquid. A higher condensation temperature suggests that a PAH would be more abundant because it would form first and because reactions are usually faster at higher temperature.
Zolotov and Shock found that PAHs are favored to form from these gases at temperatures below 230�C. Gas mixtures rich in carbon monoxide and hydrogen were more likely to produce PAHs than other gas mixtures (i.e., had higher condensation temperatures). They also found that, in general, heavier PAHs would be more abundant than lighter PAHs, and that PAHs without side-chains were favored where the gas was rich in carbon compared to hydrogen or oxygen (i.e., with little water).
These reactions are very slow without a catalyst. Fortunately, magnetite is a superb catalyst for Fischer-Tropsch reactions (see Anders, 1966), and is fairly abundant in the carbonate globules in ALH84001. Much of the PAHs seem to be associated with the magnetite-rich layers (Flynn et al., 1988), which is consistent with PAH catalysis by the magnetite. [It seems likely that chromite, which is abundant in the rest of ALH84001, is also a good catalyst.]
Zolotov and Shock think it somewhat unlikely that the PAHs formed directly from magmatic gases. ALH84001 cooled down slowly from magmatic temperatures. During slow cooling, the magmatic gases would have equilibrated with each other to yield less and less carbon monoxide. Less carbon monoxide means less PAH produced (i.e., lower condensation temperatures).
On the other hand, an impact event after the carbonates formed would provide everything needed to produce PAHs. The impact would have heated the carbonate globules enough to decompose their iron carbonate into magnetite and gas rich in carbon monoxide. With a little water (in clays or along grain boundaries), the gas would also contain hydrogen and would produce abundant PAHs without side chains. The magnetite that formed by decomposition of iron carbonate would catalyze production of PAHs. And finally, the patchy distribution of carbonate globules, water-bearing minerals, and shock effects could lead reasonably to uneven distributions of PAH compounds within the rock.
Somehow, I failed to review this paper when it first came out in February.
The carefully performed calculations of chemical equilibria and condensation temperatures (and thus perhaps abundances) of the various PAHs slide around the issue of kinetics � how fast these reactions will actually happen. I think the basic issue in applying this paper to ALH84001 is thermodynamics versus kinetics, or what can happen versus what will happen. Zolotov and Shock have done the "what can happen," but our (at least my) understanding of "what will happen" is so shaky that experiments will be needed. These should be done ASAP with and without catalysts like magnetite and chromite.
The geological scenario here is quite reasonable, especially in light of papers since this was published. There is much evidence for shock events after the carbonate globules formed (Treiman, 1998), and new experiments have shown that sub-micron magnetite grains similar to those in the ALH84001 carbonates can form as siderite decomposes (Golden et al., 2000). There are hydrous minerals in the carbonate globules, some of which predate the shock event (Brearley, 2000). Together, this paper and Golden et al. (2000) provide a single, simple, non-biological hypothesis for formation of two of McKay's� signs of martian life: the organic matter and the submicron magnetites.
Relevance � Supporting.
Topic � Recognizing physical signs or clues to ancient life.
In hot springs, microbes leave several distinctive signs of their presence even if the microbes themselves cannot be recognized. Microbial biofilm (or slime) is good biomarker � nearly all microbes make it, it resists weathering, and it is readily preserved by silica. Spheres of the minerals silica and fluorite (< 500 nm diameter) are common, but are not fossilized cells. Fluorite spheres, however, may only form from water containing organic compounds
As a first exploration of possible biomarkers for detection of ancient life on Mars, the authors examined the travertine deposits (and their microbes) from four similar hot springs. Le Zitelle springs (Italy) flow at 61°C, pH = 6.3, and deposit travertine of needly crystals of calcite and aragonite (both crystal forms of CaCO3). Narrow Gauge springs at Yellowstone National Park flow at 64-70�C, pH = 5.7-6.6, and deposit calcite and aragonite. Jemez Springs in northern New Mexico flow at 71°C, pH of 7.7, and deposit travertine made of calcite (without aragonite). Hot Springs National Park in Arkansas includes many springs that flow at 60-70�C and at pH of 7.1 � 7.5. They all produce travertine of aragonite, with or without calcite (these minerals are both calcium carbonate).
Physical biomarkers were found in samples from all sites. Preserved microbes were rare, especially in samples that had not been collected and preserved wet. Commonly, outlines or shapes of microbes were preserved in EPS (Extracellular Polysaccharide Slime, or biofilm) though the cells were not. Microbes are apparently poorly preserved in calcite and aragonite, though silica minerals (where deposited) can preserve their cell outlines with great fidelity. Similarly, organic matter was rare in the travertines, except for EPS.
Biofilm or EPS was ubiquitous in all the studied samples, and is a distinctive marker for microbial activity. EPS is recognized as stranded, blobby, or knobby sheets, films, or mats that coat mineral surfaces. It commonly preserves elongate shapes or voids in the sizes and shapes of microbes. EPS appears to be relatively stable to weathering and oxidation — it is found seemingly almost as deposited, in new and old samples where the organic matter of bacteria is gone. Crystals of carbonate minerals readily grow around EPS, thus enclosing and entombing it. EPS also appears to be replaced readily by spherules of silica and by iron silicate minerals.
Small mineral spheres, 50 � 500 nm diameter, were found in all travertine samples. Similar spheres have been identified as the fossil remains of very small microbes (Folk, 1993, 1994). Spheres of silica (probably opal, SiO2) were present in all samples, and ranged from 50 � 300 nm. Spheres composed of radiating fibers of fluorite (calcium fluoride, CaF2), 200 � 500 nm diameter, were present in most travertine samples. The silica spheres are not bacteria or fossil bacteria, being solid and containing no evidence of internal structure. Silica spheres like these can form without direct assistance of microbes or their products, and so are not (in the strict sense) biomarkers. However, silica spheres seem to form preferentially on and in microbial cell walls and EPS, and so act to preserve these biogenic structures. The fluorite spheres are not bacteria or fossil bacteria either, being solid and containing no hint of internal structure. However, the fluorite spherules can be considered a physical biomarker, as formation as spherules rather than cubes seems to require organic matter in the water.
Minerals can be physically attacked by microbes, leaving distinctive pitted surfaces. These mineral surfaces can be preserved indefinitely and thus be markers of biologic activity.
This massive paper is a first step towards understanding what signs a martian microbiota may have left for us to see. It was, of course, inspired by the ALH84001 controversy and done by members of D. McKay�s research group, but does not mention the meteorite by name. Most notable for ALH84001 research is that the authors reported no cells or biogenic structures comparable to the bacteria-shaped objects featured by McKay et al. (1996) and Thomas-Keprta et al. (1998). Objects like those in the famous "bugs on parade" or "worm" images were not found. Filaments like that reported by Thomas-Keprta et al. (1998) were found in the travertine, as were biofilm structures like that reported in McKay et al. (1997). However, there has been persistent doubt that either the filament or biofilm in ALH84001 are Martian. Given that ALH84001 laid on the Earth�s surface for thousands of years and is infested with terrestrial microbes (Steele et al., 1999), the cautious mind will infer that the filament and biofilm are terrestrial until proven otherwise.
References:
Anders E. (1996) Evaluating the evidence for past life on Mars (letter). Science 274, 2119-2121.
Brearley A.J. (2000) Hydrous phases in ALH84001: Further evidence for preterrestrial alteration. Lunar Planet. Sci. XXXI, A bstract #1203, Lunar and Planetary Institute, Houston (CD-ROM).
Clemett S.J., Dulay M.T., Gilette J.S., Chillier X.D.F., Mahajan T.B., and Zare R.N. (1998) Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. Faraday Discussions (Royal Soc. Chem.) 109, 417-436.
Folk R. L. (1993) SEM imaging of bacteria and nannobacteria in carbonate sediments and rocks. J. Sed. Pet. 63, 990-999.
Folk R. L. (1994) Interaction between bacteria, nannobacteria, and mineral precipitation in hot springs of central Italy. Geograph. Phys. Quart. 48, 233-246.
Flynn G.J., Keller L.P., Jacobsen C., and Wirick S. (1998) Organic carbon in carbonate and rim from Allan Hills 84001 (abstract). p. 13-14 in Workshop on the Issue Martian Meteorites: Where do we stand, and where are we going? Lunar and Planetary Institute, Contrib. # 956.
Golden D.C., Ming D.W., Schwandt C.S., Lauer H.V., Socki R.A., Morris R.V., Lofgren G.E., and McKay G.A. (2000) Inorganic formation of zoned Fe-Mg-Ca carbonate globules with magnetite and sulfide rims similar to those in martian meteorite ALH84001. Lunar Planet. Sci. XXXI, Abstract #1799, Lunar and Planetary Institute, Houston (CD-ROM).
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.Jr., 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., Stapleton D., Toporski J.K.W., Peters V., Bassinger V., Sharples G., Wynn-Williams D.D., and McKay D.S. (2000) Investigations into an unknown organism on the Martian meteorite Allan Hills 84001. Meteorit. Planet. Sci. 35, 273-241.
Thomas-Keprta K.L., McKay D.S., Wentworth S.J., Stevens T.O., Taunton A.E., Allen C.A., Coleman A., Gibson E.K.Jr., and Romanek C.S. (1998) Bacterial mineralization patterns in basaltic aquifers: Implications for possible life in martian meteorite ALH84001. Geology 26, 1031-1035.
Treiman A.H. (1998) The history of ALH 84001 revised: Multiple shock events. Meteorit. Planet. Sci. 33, 753-764.
Relevance - Direct!
Topic - Origin of carbonate globules.
McKay et al. (1996a) suggested that the carbonate globules in ALH84001 formed under the influence of martian microbiota. The authors here have grown comparable carbonate globules in the laboratory under abiotic conditions, at 150�C, from geologically reasonable aqueous solutions. Thus, the carbonate globules themselves are not direct evidence for martian life.
The carbonate globules in ALH84001 host putative signs of martian life (McKay et al. 1996a), but the origin of the globules has been controversial (e.g., Harvey and McSween, 1996; Scott et al., 1997; Valley et al., 1997; McSween and Harvey, 1998; Warren, 1998). Could the globules form at temperatures conducive to life-as-we-know-it? Moreover, could they form ONLY with the assistance of life? To help answer these questions, Golden and colleagues tried to grow carbonate globules like those in ALH84001 in the lab.
To grow carbonate globules, Golden started with water solutions rich in calcium, magnesium, and iron. The water was boiled and passed through a 0.2-micrometer filter. Carbon dioxide was bubbled through the solutions until carbonate had just begun to form. For experiments at 25�C, the solutions were placed in closed containers. For experiments at 150�C, the solutions were put in a �hydrothermal� cell, which allowed the high pressure required to keep water liquid at that temperature, ~ 4.7 bars (or atmospheres). After the experiments were over (24 to 96 hours), the products were freeze-dried and analyzed using X-ray diffraction (for mineral content), SEM (for textures and elemental content), and TEM for sub-micron textures.
The 150�C experiments yielded ellipsoidal globules, 10-50 micrometers diameter, of radiating crystals of carbonate minerals and rare grains of magnetite. The globule cores were iron-rich (siderite or ankerite) and the rims were magnesium-rich (magnesite), similar to the chemical zoning in ALH84001 globules. Some globules showed concentric alternations of Fe-rich and Fe-poor carbonate.
The 25�C experiments yielded spheres, 10 - 100 micrometers diameter, of amorphous Fe-rich carbonate. After 96 hours, these had been converted to crystalline Ca-carbonate (calcite). Mg-rich solutions produced similar results, but with Mg-rich calcite as the final product.
The products of the 150�C experiments show most of the characteristics of the globules in ALH84001: ellipsoidal shapes, crystals radiating from their centers, concentric chemical zoning from Fe-rich to Mg-rich, and with scattered magnetites. Quoting the paper, �The most important aspect of this synthesis is that carbonates with chemical zoning, composition, size, and appearance similar to those in ALH84001 can be achieved by purely inorganic means and at a relatively low temperature.�
These experiments (and those of Golden et al., 2000b) are critical to evaluating the claims of McKay et al. (1996a). In 1996, McKay claimed that the chemical zoning of the carbonate globules in ALH84001 indicated the action of martian life. That claim was softened (McKay et al., 1996b) after Anders (1996) noted that the chemical zoning pattern of the globules was consistent with the inorganic stability relationships of the carbonate minerals. Now, Golden et al. have shown that, in fact, carbonate globules like those in ALH84001 can form inorganically.Golden�s globules do not match those in ALH84001 exactly, but that is not a problem. It would be miraculous if their first experiments hit the exact chemical, physical, and biological conditions responsible for the ALH84001 globules. Of course, this work does not refute McKay�s assertion that the globules were made (or mediated) by life. However, it does provide an reasonable abiotic hypothesis for the globules. If you don�t like martian life in ALH84001, cite this paper!
I think that that Golden�s experiments are a bit ambiguous about the influence of microbiota. The experiments did not follow biological sterile procedures, but biological contamination is deemed unlikely because of the filtration and the high temperature. However, viable bacterial organisms or their spores may be smaller than 0.2 micrometers permitted by their filtering - the so-called nannobacteria and nanobes (Folk et al., 1993; Kajander and Ciftciouglu, 1998; Uwins et al., 1998). These organisms, if they really exist, would pass through the filter unhindered. Similarly, in this new world of extremophile bacteria (those that love environments we would consider unhealthy), it may be that some could survive and thrive at 150�C.
Relevance - Supporting.
Topic - Origin of bacteria-shaped objects.
Spheres, ovoids, and feathery shapes composed of carbonate minerals can be produced in the laboratory from aqueous precipitates of metastable carbonate and carbonate-hydroxide phases. These shapes are comparable to some in ALH84001 interpreted as biogenic.
One controversy about ALH84001 is whether its bacteria-shaped objects could have formed inorganically. While synthesizing fine particulates of oxides and carbonates for materials science applications, the authors have made �bacteria-shaped objects� of the stable carbonate mineral calcite from metastable carbonate minerals vaterite and aragonite.
The objects were grown from a solution of calcium chloride that was saturated in carbonate by bubbling carbon dioxide gas through it. Precipitation of calcium carbonate was induced by bubbling ammonia gas through the solution, until it achieved a pH of 8.5. The reaction temperature controlled which calcium carbonate formed. Above 80�C, they produced clusters of radiating crystals of calcite. At ~70�C, �floral� structures of aragonite precipitated. In addition, at 25�C, spherules of vaterite formed, each one being between 2 and 5 micrometers in diameter. These spherules are composed of smaller ovoids, each ovoid being 0.05 to 0.1 micrometers (50 to 100 nanometers) in diameter
These vaterite spherules are comparable in shape and size to putative biological features noted in the ALH84001 and Tatahouine meteorites (McKay et al., 1996; Barrat et al., 1999). Vaterite, being unstable, can transform to the stable mineral calcite, and can do so without changing its globular shape. Thus, bacteria-shaped objects like those in ALH84001 can form from abiotic processes.
I was somewhat disappointed by this paper, because its title seemed to promise more than it delivered. Although the paper does describe the inorganic production of bacteria-shaped objects, its relevance to ALH84001 seems limited, its experimental procedures were not suited to the question of life on Mars, and its presentation is flawed.This paper is welcome as an addition to several others written or cited on the formation of boigenic-shaped objects in inorganic systems (e.g., Peiming and Odler, 1990; Meeker and Hinkley, 1993; Bradley et al., 1997; Sears and Kral, 1997; Ksanfomality, 1988; Kirkland et al., 1999). However, this paper really does not address the chemical system of ALH84001. First, most of its figures are of rare-earth carbonate and oxide particles, which are irrelevant though appealing. Second, the calcium carbonate minerals (calcite, vaterite, aragonite) are of limited relevance. ALH84001 contains very little calcite, and its bacteria-shaped objects are not associated with calcite but with other carbonate minerals: magnesite, ankerite, and siderite. The proposed formation mechanism will not work with the later minerals, as they do not form in the vaterite or aragonite structures
Unfortunately, experimental procedures here were not designed with biological chemistry in mind. The experiments were not done in sterile conditions, and no effort was (apparently) made to exclude bacteria or other biological entities. Granted, the formation of vaterite spherules was probably inorganic, but the authors could not prove the absence of biological controls or catalysis.
Finally, this paper does not meet the usual editorial standards of Geochimica et Cosmochimica Acta. The text contains a critical error on the sizes of the vaterite globules - it says they are up to 50 micrometers diameter while a figure caption says 50 nanometers. Several articles cited in the text are not in the reference list, and one reference does not give the title of the article (a GCA standard). It is puzzling that the help of a Professor J.D. Gleason is acknowledged, but he is listed as an author on the page headers!
Relevance - Ancilliary.
Topic - Evolution of martian atmosphere.
ALH84001 contains nitrogen that is enriched in the heavy isotope of nitrogen, 15 N. This nitrogen, and other atmosphere gases, was trapped in the meteorite 4 billion years ago. The elemental composition of this atmosphere gas (specifically the ratio Ar/N) is not identical to the present martian atmosphere, although its N isotopic composition is similar.
This paper is not directly relevant to possible traces of Martian life in ALH84001, but is important to understanding the evolution of Mars' atmosphere and climate.
Relevance - Ancilliary
Topic - Evolution of martian atmosphere.
There are small amounts of hydrogen in the carbonate globules and the feldspar-rich glasses in ALH84001. This hydrogen is variably enriched in the heavy isotope of hydrogen, deuterium, and is comparable to the hydrogen in young martian rocks, like EETA79001. Not all of the hydrogen is so enriched in deuterium (are �lighter�), which means that the heavy martian hydrogen mixed (on Earth) with typical (lighter) Earth hydrogen. As the carbonate globules and the feldspar-rich glasses formed at about 4.0 billion years ago, Mars� hydrogen must have been heavy at that time.
By current theories, the enrichment in deuterium is actually a depletion in normal (light) hydrogen, which was lost as most of Mars� atmosphere was stripped off to space. The lighter elements and isotopes could float higher in the martian atmosphere, and so could escape to space more readily. If so, the hydrogen (and thus water) along with the bulk of Mars atmosphere must have escaped mostly before 4.0 billion years ago
This paper is not directly relevant to possible traces of Martian life in ALH84001, but is important to understanding the evolution of Mars' atmosphere and climate.
Relevance -- Direct!
Topic -- Terrestrial Contamination
Portions of ALH84001 were infested with terrestrial bacteria, which can be recognized as masses of branching fibers with characteristic bumps and club-shaped ends. These bacteria are identical to others that grow in Antarctica, and so probably grew there. Terrestrial microbiota growing in martian meteorites are a severe complication to recognition of martian microbiota and martian bio-organic compounds.
Steele and colleagues examined many chips of ALH84001 to see if it showed any evidence of being infested with terrestrial microorganisms. This work was prompted, in part, by concerns that the putative bacteria found by McKay et al. (1996) might be Earthlings and not Martians. Steele's continuing work has shown that bacteria and fungi take up residence in meteorites very rapidly after they fall to Earth - this is a huge concern for ALH84001, as it has been on Earth for many thousands of years.
Steele examined the surfaces of forty chips of ALH84001 by scanning electron microscopy (SEM). For comparison, the authors also examined Antarctic rocks that contained a variety of bacterial and fungal microbes ("cryptendolithic communities"), and four chondritic (asteroidal) meteorite samples from Antarctica. Of the chip samples of ALH84001, two from the same original are on the meteorite contained biological surface features, and they were examined further with a different style SEM, and an atomic force microscope (AFM). With these techniques, and chemical analyses via X-rays produced in the SEM, Steele and co-workers were able to identify the biological organisms.
The organisms are present in dense colonies of Y-branched fibers, each fiber ~ 200-300 nanometers in diameter, with bumps scattered on their surfaces and club-like endings. The fibers generally form a dense mat, with some rising upward. These features are nearly identical to those of Actinomycetes, a type of bacteria that are common inhabitants of normal rocks in Antarctica. The bumps on the fiber surfaces are similar to the fruiting (spore-forming) bodies of the bacteria. Fracture surfaces adjacent to the colonies are covered with a featureless material, which is probably "extracellular polymeric substance" or EPS, i.e. bacterial slime.
The similarity of these organisms to Actinomycetes suggests that they are Actinomycetes that grew in the meteorite on Earth. This idea is consistent with the spatial distribution of the colonies - they were found on only a few chips that were within 1 mm of the meteorite's outer surface on Earth (its fusion crust). There are plenty of food and fuel for microorganisms inside the meteorite, so it seems reasonable that their presence near its outer surface means that the organisms came from outside the meteorite, while it was on Earth.
Enough features are preserved to guess something of the bacteria's life style. The colonies are all on centered in the rims of carbonate globules, suggesting they started there in the regions richest in iron sulfide and oxide (fuel) and carbon (food). The abundant EPS and the club-like ends of fibers are defenses against low temperature and dry conditions, consistent with growth (and death) in Antarctica.
Finding these terrestrial microorganisms in ALH84001 does not disprove the contention of early Martian life. But the search for signs of martian life in meteorites must take careful account of contamination and infestation by terrestrial microbiota.
Over the last few years, Steele and colleagues have found that terrestrial microbiota are nearly ubiquitous in meteorites (Steele et al., 1998, 1999, 2000; Toporski et al. 1999 and submitted). Not only in meteorites that have lain in the dirt or ice for years (Burckle and Delaney, 1998; Gillet et al. 2000, reviewed below), but also in "fresh" falls, collected shortly after arriving at Earth and kept clean and safe in museums (Toporski et al., 1999). This is not meant as a slur on museums or curatorship, but as a recognition that life is everywhere on Earth, and that biologist must take near-heroic measures to keep itinerant cells and spores out of their experiments.
Relevance - Supporting.
Topic - Very small bacteria.
A non-martian meteorite that fell in the Sahara desert contains very small bacteria-shaped objects (<200 nanometers diameter) associated with Earth-weathering carbonate. Similar bacteria are abundant in the Saharan soil, and have been grown in the laboratory. So, it is possible that similarly sized and shaped objects in ALH84001 actually are fossilized bacteria.
Gillet and co-workers examined recently collected fragments of the Tatahouine meteorite, which fell in the Sahara desert 1931. In earlier work, they showed that the Tatahouine had weathered significantly on Earth, most notably with formation of carbonate disks along fractures (Barrat et al., 1998, 1999). In these earlier works, they noted the presence of bacteria-shaped object along fractures and in the carbonate disks, and speculated that they might be fossilized bacteria.
These bacteria-shaped objects (BSOs) were examined by scanning and transmission electron microscopy. BSOs are abundant and of two shapes: ovoids ( 70-300 nanometers diameter) and rods (100-600 nanometers long, 70-80 nanometers diameter). The BSOs appear to rest on, or be embedded in, the carbonate minerals (calcite) or the silicate minerals of the meteorite. Chemically, the BSOs consist of carbon, oxygen, and nitrogen, with traces of phosphorus and sulfur (the SEM cannot detect hydrogen). Small crystals of salt (NaCl) and "lite salt" (KCl) were scattered in the BSOs.
Similar BSOs were found in the soil where the meteorites were collected. This soil was incubated in a soy broth (a standard microbiology method), and soil bacteria grew in the broth. The bacteria were rods of 100-200 nanometers length, and ovoids of ~ 500 nanometers diameter. DNA sequencing showed that these two shapes came from the same kind of bacteria, which had not been described before. When these bacteria were grown on agar ("seaweed jello") with added calcium carbonate, they formed disk-shaped colonies in porous aggregates of calcium carbonate crystals. These disks have the same sizes and shapes as the carbonate disks in the Tatahouine meteorite. So, it seems very likely that this type of bacteria infested the meteorite after it fell to Earth, and produced the meteorite's BSOs and the carbonate disks.
The BSOs in the meteorite are, however, factors of 2 - 3 smaller (in length and diameter) than the live bacteria. Gillet and co-workers suggest that the bacteria in the meteorite were dwarfs because they rock had little of the nutrients they needed to grow. Now, the small sizes of the BSOs in ALH84001 cannot be used to refute a biologic origin.
This work is part of the ongoing debate on how small free-living (not parasitic) organisms can be. The size limit generally accepted by renowned scientists is spherical with a diameter of about 200 nanometers, or billionths of a meter (Nealson, 1997; Knoll and Osborne, 1999). This is still much larger than the putative cell fossils that McKay et al. (1996) reported. McKay's group have recanted a biological interpretation for some of their images (McKay et al., 1997), and have suggested that others might be fossil bacterial appendages rather than whole bacteria. However, a small and vocal minority of biologists claim that bacteria down to ~ 50 nanometers diameter can be found alive and grown in the laboratory. This work is one case - other are Uwins et al. (1998) and Kajander et al. (1999).
To be fair, R. Folk has argued for many years that spherical and ellipsoidal forms like these in rocks were fossilized "nannobacteria" (e.g., Folk, 1993, 1997). However, Folk was never able to test his objects using biological or organic chemical methods, and several types have turned out to be inorganic, or to be artifacts of sample preparation (Folk and Lynch, 1997; Kirkland et al., 1999).
Relevance - Ancillary.
Topic - Sulfur isotopes as signs of life.
Sulfur isotopic ratios in the martian meteorites have been strongly affected by chemical reactions induced by light, which must have happened in the martian atmosphere. At first glance, these effects could be attributed mistakenly to sulfur-eating bacteria.
Viking and Pathfinder data showed that the surface of Mars is rich in sulfur, but its source is not known. Similarly, the martian meteorites contain relatively abundant sulfur minerals, crystallized from magma and deposited from liquid water. The waters once interacted with the martian atmosphere (shown by oxygen isotope ratios). To help understand the source and history of their sulfur, Farquhar and colleagues measured the isotopic composition of sulfur in several martian meteorites (but not ALH84001).
There are six varieties of sulfur atoms, isotopes, with masses of 32, 33, 34, and 36. These isotopes are all stable, not radioactive. Nearly all chemical and biological reactions separate the isotopes strictly according to the differences in their masses -- the ratio 33S/32S will be changed half as much as the ratio 34S/32S, and a quarter as much as the ratio the ratio 36S/32S. These ratios are given in d notation, where d33S is the difference in parts per thousand between the ratio 33S/32S in a sample and in a standard. Only in photochemical reactions, those initiated by light, do sulfur isotope ratios change without strict accordance to mass differences (i.e., non-mass-dependent fractionation).
Farquhar and colleagues found that sulfur isotopes in some of the martian meteorites were very different from the solar system average, and that nearly all had isotope ratios that were out of strict accordance with mass differences. This suggests that much of the sulfur in the sulfur in these martian meteorites had cycled through the martian atmosphere (along with at least some of the oxygen; Karlsson et al., 1983?).
To validate this conclusion, Farquhar and colleagues studied the photochemical reactions of two common sulfur gases, H2S and SO2, to see if they made non-mass-dependent fractionations. In fact, photochemical breakdowns of both gases produced large mass-independent fractionations of sulfur isotopes.
Finally, Farquhar notes that biological systems can also produce large fractionations of sulfur isotope ratios. Geochemists and biologists usually only measure d34S, and would tend to blame biology for large changes in this value. However, photochemical reactions can do the same, and one cannot tell biochemistry from photochemistry without analyzing other sulfur isotopes, particularly d36S.
Farquhar's paper was reported as the death knell for McKay's (1996) hypothesis of ancient martian life in ALH84001. This fit of journalistic aggrandizement was inaccurate and unhelpful. Farquhar did not analyze, nor even mention, ALH84001. McKay's did not mention sulfur isotopes (others did: Shearer and Papike, 1996; Shearer et al., 1996; Greenwood et al., 1997).
Beyond the obvious caution about using sulfur isotopes signs of life, the real importance of this paper is in the chemical connections between Mars' atmosphere and subsurface. Today, the atmosphere and subsurface are nearly independent. However, Farquhar found evidence for mass-independent fractionation of sulfur isotopes even in the high-temperature, magmatic sulfides in EETA79001, which means that atmospherically processed sulfur has penetrated deep into Mars and was caught up in the martian basalts.
One puzzle from this paper is abundances of the heaviest sulfur isotope, 36S. Excesses or deficits in d36S ought to track along with the absolute values of other S isotope ratios. For instance if the excess d33S (beyond mass-dependent fractionation) is large, then the excess d36S ought to be large positive or large negative. Excess d33S and d36S beyond mass dependent fractionation are named D33S and D36S. However, all the D36S values are positive, despite having a mix of positive and negative D33S.
Unfortunately, the text equations for D33S and D36S in the text have misplaced parentheses, but are correct in the table legend.
References:
Barrat J.A., Gillet Ph., Lecuyer C., Sheppard S.M.F., and Lesourd M. (1998) Formation of carbonates in the Tatahouine meteorite. Science 280, 412-414.
Barrat J.A., Gillet Ph., Lesourd M., Blichert-Toft J., and Popeau G.R. (1999) The Tatahouine diogenite: Mineralogical and chemical effects of sixty-three years of terrestrial residence. Meteorit. Planet. Sci. 34, 91-97.
Burckle L.H. and Delaney J.S. (1998) Microfossils in chondritic meteorites from Antarctica? Stay tuned (abstract). Meteor. Planet. Sci. 33, A26-A27.
Folk R.L. (1993) Nannobacteria in carbonate sediments and rocks. J. Sed. Petrol. 63, 990-999.
Folk R.L. (1997) In defense of nannobacteria (letter). Science 274, 1287.
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Greenwood J.P., Riciputi L.R., and McSween H.Y.Jr. (1997) Sulfide isotope compositions in shergottites and ALH 84001, and possible implications for life on Mars. Geochim. Cosmochim. Acta 61, 4449-4453.
Kajander E.O., Bj�rklund M., �ift�ioglu N. (1999) Nanobacteria and man. pp. 195-204 in Enigmatic Microorganisms and Life in Extreme Environmental Habitats. (Ed. J. Seckcbach). Kluwer, Dordrecht, Netherlands.
Kirkland B.L., Lynch F.L., Rahnis M.A., Folk R.L., Molineux I.J., and McLean R.J.C. (1999) Alternative origins for nannobacteria-like objects in calcite. Geology 27, 347-350.
Knoll A. and Osborne M.J. eds. (1999) Size Limits Of Very Small Microorganisms: Proceedings of a Workshop. National Academy Press, Washington. 148 p.
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.Jr., Thomas-Keprta K.L., and Vali H. (1997) No 'nanofossils in martian meteorite: reply (letter). Nature 390, 455-456.
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ALH84001 and other martian meteorites were altered, while on Mars, by aqueous solutions. Baker and colleagues modeled the alteration by reacting a terrestrial basalt with water solutions rich in carbon dioxide. Experiment temperatures were from 23 to 400°C, and durations from 4-7 days. Even at the lowest temperature, the basalt was noticeably altered in seven days. Alteration minerals included carbonates, silica, and water-bearing silicates. Carbonate minerals comparable to those in ALH84001 were produced at temperatures above 75°C. ALH84001 is less altered than the experimental products, suggesting that it was wet (on Mars) for a very short time.
Most of the martian meteorites were altered by water-bearing fluids, on Mars. The authors have performed experiments, altering an Earth basalt in their lab, to help define the temperature, fluid compositions, and durations of the martian alteration events. Baker and colleagues used a glass-rich, iron-rich basalt from the Columbia River area (Washington), and reacted it with mixtures of water and carbon dioxide (CO2) with added sulfate and chloride at a range of temperatures. Low temperature experiments, 23 and 75°C, were done with CO2-saturated water percolating through crushed rock. High-temperature experiment, 200 and 400°C, were done at pressures of 500 and 1000 atmospheres of mixed H2O-CO2 fluids. Experiments lasted 4-7 days, and all produced noticeable alteration of the basalt. "Calcite and siderite predominated in the low-temperature experiments; whereas magnesite, ankerite, and siderite were important at high temperatures." SiO2 minerals (quartz and/or opal) were produced in all water-rich experiments. At low temperature, the only silicate mineral was a zeolite, sacrofanite. At higher temperature, silicate minerals included the zeolite, a clay-like mineral (sepiolite), vesuvianite, biotite, and the amphibole richterite (all characteristic of low-grade metamorphism).
It is clear that glassy basalt begins altering very rapidly, less than a week, at room temperatures or higher. Even hydrous silicate minerals (zeolites, clay-like minerals, micas) can form this fast. Alteration is faster at higher temperature, and seems to be faster also with lesser proportions of CO2 in the fluid. Silica (SiO2) was rapidly leached from the glass, and either deposited elsewhere or removed in solution.
From these experiments, it appears that none of the martian meteorites could have interacted extensively with water-rich fluids. From its absence [really rarity] of hydrous silicates, it appears that ALH84001 could not have reacted extensively with aqueous fluid. "These experiments emphatically rule out long-term interaction with water as well as alteration by large volumes of water." However, alteration of crystalline basalt could be slower than of glassy basalt. These experiments do not constrain the duration alteration of the nakhlite meteorites, which are more altered than any experiment products. "Results of this study and absence of alteration outside the carbonate-lined vugs suggest that the EETA79001 parent rock did not interact with an aqueous fluid."
These are interesting, important experiments, particularly for the rates of alteration. The martian meteorites are now being examined in such detail that alteration of the sort produced here in less than a week should be clearly visible. These experiments re-enforce observations that the martian meteorites are little altered compared to terrestrial rocks. It is a bit of a geological problem for rocks on a warm, wet Mars (or even a cool wet Mars) to remain so unaltered for billions of years.
However, the alteration minerals Baker produced are only fair matches to those in the martian meteorites, which probably means that their experimental conditions were not completely appropriate. The CO2 pressures in all their experiments are much higher than is likely now and in the recent past. The low-temperature experiments were at P(CO2) of 1-2 bars (atmospheres), and the high-pressure experiments were at P(CO2) of 125 to 500 bars. Today, the martian atmosphere has P(CO2) = 0.006 bars. Such high CO2 pressures favor carbonate minerals over silicates, disfavor hydrous silicates (of any sort), and favor anhydrous versus hydrous carbonates (e.g., magnesite vs. nesquehonite; see Golden et al., 1999, 2000a).
Even so, the secondary minerals in ALH84001 are a decent match to the experimental products at higher temperature: ankerite, siderite, magnesite, silica(?), 'biotite' (see Brearley, 2000). These higher temperatures are, in general, consistent with the carbonate precipitation experiments, at 150°C, of Golden (1999, 2000a,b). The rarity of hydrous minerals in ALH84001 (Thomas-Keprta et al.,2000, Brearley, 2000), in conjunction with this work, suggests that the meteorite was wet for a VERY short time, perhaps less than a week out of its 4 billion year long history!
Alteration in the nakhlites is, however, completely different. Their alteration minerals are principally clays, which Baker did not make at all. Baker did produce silica and zeolites, which have not been reported in the nakhlites. The carbonate minerals in the nakhlite alterations, calcite and siderite, are like the low-T alterations in the experiments, which is in turn consistent with independent estimates that the alteration occurred at < 100°C (Treiman et al., 1993).
Oddly, the authors assert that the carbonates, sulfates, and clay mineraloids in EETA79001 did not form by aqueous alteration. If not via water, then how did they form?
The carbonate globules in ALH84001 (hosts to putative signs of ancient Martian life) formed 3.90�0.04 billion years ago. The Rb-Sr and U-Pb radioactive isotope chronometers give this same age, a time when the surface of Mars was rich in water (or ice) and was frequently hit by large asteroids. The age of the globules and the age of the largest asteroid impact onto ALH84001 are the same (within 300 million years), so the globules could have been formed by (or during) that impact.
To obtain the age when the carbonate globules in ALH84001 formed, Borg and colleagues analyzed isotope abundances of rubidium (Rb), strontium (Sr), uranium (U) and lead (Pb) as they dissolved carbonate globules in successively stronger and stronger acids. The weakest acids dissolved Earth contaminants, which had the common Earth lead. The stronger acids dissolved first Ca-Mg-Fe-rich carbonate (the ankerite/dolomite) and then the more magnesium-rich carbonate of the globules. The isotope ratios of Rb, Sr, U, and Pb in these various dissolved carbonates were analyzed to get ages. By Rb/Sr, the carbonates formed 3.90�0.04 billion years ago; by U-Pb, the carbonates formed 4.04�0.10 billion years ago. These two ages are the same within analytical uncertainties, and so are probably real and believable.
This age, when the carbonate globules formed, is also within error of the potassium-argon (K-Ar) age of the whole meteorite ALH84001, ~3.8-4.3 billion years ago. The K-Ar ages (actually Ar-Ar) represents the time when the whole rock was last heated enough to lose its trapped argon, likely above about 500°C. The similarity of the times of heating and carbonate formation suggests that the two events are related - perhaps that the carbonates formed because of the heat from an asteroid impact (Harvey and McSween, 1996; Scott et al., 1997, 1998). However, the uncertainties in the ages, �40 million years for the carbonates and �150 million years for the heating, leave time for nearly any mode of carbonate formation.
This paper is a superb, excruciatingly careful, study, and seems to leave little room for other possible ages. Given the rarity of carbonate in ALH84001, it may be difficult to get more precise ages than these. A possible weakness in the paper is the necessary assumption that all the carbonates started out with the same initial ratios of Sr isotopes (87Sr/86Sr) and of Pb (206Pb/204Pb). It is possible that the carbonates formed from mixtures of fluids with different chemical and isotopic compositions (Valley et al., 1997; Golden et al. 2000a), and that the "ages" here represent fluid mixing, not a date.
The similarity between the age of the carbonates and the heating event is intriguing, but perhaps not very significant given the uncertainties in the ages . Identical ages would fit the carbonate formation theories of Harvey and McSween (1996), and Scott et al. (1997, 1998), and also the possibility that the carbonates formed from hot water generated by the impact. However, the uncertainty in the time of the heating event, a few hundred million years, leaves a lot of time for the carbonates to have formed by nearly any other mechanism imaginable.
Minerals in ALH84001, including the carbonate globules that host putative traces of ancient martian life, have been subjected to the high pressures and temperatures of impact (shock) metamorphism. Any explanation of features in the globules must recognize the effects of this metamorphism.
Cooney and colleagues studied the shock history of ALH84001 by examining its minerals and glasses with optical (light) spectroscopy (infrared reflection and Raman spectroscopy).
The carbonates in ALH84001 are complex on a molecular scale. Multiple peaks in the Raman spectra suggest small-scale intergrowth of carbonates of different compositions. The peaks are significantly wider than those from well-crystalline minerals, suggesting structural disorder caused by shock. Silica in ALH84001 is amorphous, with small proportions of crystalline quartz and tridymite. The amorphous structure of the silica suggests peak shock pressures above 31 GPa. The feldspar-composition glasses in ALH84001 have been called maskelynite, which is supposed to be glass, never melted, formed by high shock pressure. However, these feldspar glasses were molten, as shown by spectra and textures (like bubbles). To make this glass, shock pressures must have been above 50 GPa.
Phosphate minerals include apatite and merrillite, which replaces the apatite. The apatite shows no sign of OH vibrations, meaning that it is completely free of water.
The mineralogical and spectroscopic data from this study are welcome, but don't break much new ground. The results on silica and feldspar glasses have (for the most part) been anticipated. Evidence that the carbonates have complex microstructures and evidence for shock-induced disorder confirms petrographic evidence (much developed by Scott) that the carbonates experienced significant shock. Most interesting in the broader picture is the absence of water in the phosphate minerals, which would normally have been expected to carry much of the rock's water, if ALH84001 actually ever had water. Considering how easily apatite exchanges its water and chlorine and fluorine (which is why fluoride affects teeth), how much water could have been involved in carbonate formation?
and
Scott et al. claim to find flaws in Warren's (1998) model for the low-temperature formation of the carbonate globules in ALH84001, hosts to putative traces of ancient martian life. Warren disagrees, and restates the problems he finds in Scott's et al. (1997, 1998) model, that the carbonate globules formed at high-temperature in an impact (shock) metamorphic event.
In proposing that the carbonate globules in ALH84001 formed at low temperature by evaporation of saline flood-waters, Warren (1998) noted several problems in the theory of Scott et al. (1997, 1998) that the globules formed as impact melts. Here, are my impressions of Scott and Warren discussing the credibility of their respective theories.
- DEPOSITION OF CARBONATE GLOBULES
SCOTT: Warren (1988) argued that the carbonate globules were deposited by water into cracks and holes. Scott says that water-deposited carbonates would be distributed more evenly than in ALH84001. Further, the rock has no void spaces now, and it seems unlikely that carbonates could have completely filled all of the original void space. Void spaces could not have been squeezed together after the carbonates formed, as the carbonates are not deformed significantly.
WARREN: In Earth rocks, carbonate minerals are commonly deposited unevenly, perhaps the norm and not the exception. ALH84001 has few void spaces now, because the rock was compacted in an impact event after the carbonates were formed. Scott is incorrect in saying that the carbonate globules are experienced no major shock events after they formed.
- FORMATION OF CARBONATE DISKS IN FRACTURES
SCOTT: It is difficult to understand how carbonate disks could form in the fractures (Warren, 1998). If the disks were deposited in disk-shaped holes, it is difficult to see how the holes could have been enlarged along thin fractures, and how the carbonate globules could have started growing in the exact center of each hole. If the disks were deposited along open fractures, it is difficult to understand how the fractures were closed by shock and have the pattern of fractures and defects that Scott et al. (1998) saw in pyroxenes.
WARREN: Disc-shaped holes can be formed along cracks in pyroxene; carbonate minerals were deposited, on Earth, in cavities like these, in a meteorite that fell in the Sahara (Barrat et al., 1998). Not all of the carbonate globules 'began growing at the centers of holes.' The fracture and defect patterns noted by Scott et al. (1998) are not particularly relevant, and have probably been affected by the salty waters.
- POSSIBLE INCONSISTENCIES IN SHOCK MODEL OF SCOTT
SCOTT: Warren (1998) pointed out "several apparent shortcomings" of the shock-melt model of Scott et al. (1997, 1998), who beg to differ. The carbonate globules in ALH84001 occur as isolated patches along fractures, which Warren claimed was inconsistent with origin as a shock melt. Scott responds that monomineralic shock melts (as he describes the carbonates) do commonly appear as isolated patches on healed fractures.
WARREN: The scattered distribution of globules still does not resemble how shock melts are distributed in other meteorites. In any case, Scott's "... appeal to vaguely defined complexities ..." is inadequate.
- OTHER CRITICISMS WARREN: Scott did not respond to several of Warren's (1998) criticisms of the shock-melt model for the carbonates, and Warren here elaborates on those problems. In particular, the shock melt model seems incapable of producing the large variations in oxygen isotope compositions of various layers of the globules and pancakes.
Comments and replies are the academic version of "'Did not!' 'Did so!' 'Did not!'" In my experience, they rarely help the non-specialist much and nearly never resolve a conflict of views (for instance: Treiman and Wallendahl, 1998; Lovely et al., 1998). This comment and reply are a microcosm of the larger issue: did the carbonate globules (and their putative traces of ancient martian life) form at high temperature or low. Although much recent work has favored low temperatures (Golden et al., 1999, 2000a,b; McSween and Harvey, 1998), the issue is not yet settled (Shearer et al., 1999).
This paper is identical in text and layout to McSween and Harvey (1998), which was reviewed here last year.
References:
Barrat J.A., Gillet Ph., Lesourd M., Blichert-Toft J., and Popeau G.R. (1999) The Tatahouine diogenite: Mineralogical and chemical effects of sixty-three years of terrestrial residence. Meteorit. Planet. Sci. 34, 91-97.
Brearley A.J. (2000) Hydrous phases in ALH84001: Further evidence for preterrestrial alteration. Lunar Planet. Sci. XXXI, Abstract #1203, Lunar and Planetary Institute, Houston (CD-ROM).
Golden D.C., Ming D.W., Schwandt C.S., Morris R.V., Yang S.V., and Lofgren G.E. (1999) An experimental study of kinetically-driven precipitation of Ca-Mg-Fe carbonates from solution: Implications for the low-temperature formation of carbonates in martian meteorite ALH84001. Lunar Planet. Sci. XXX, Abstract #1973, Lunar and Planetary Institute, Houston (CD-ROM).
Golden D.C., Ming D.W., Schwandt C.S., Lauer H.V., Socki R.A., Morris R.V., Lofgren G.E., and McKay G.A. (2000a) Inorganic formation of zoned Fe-Mg-Ca carbonate globules with magnetite and sulfide rims similar to those in martian meteorite ALH84001. Lunar Planet. Sci. XXXI, Abstract #1799, Lunar and Planetary Institute, Houston (CD-ROM).
Golden D.C., Ming D.W, Schwandt C.S, Morris R.V, Yang S.V, and Lofgren G.E. (2000b) An experimental study on kinetically-driven precipitation of Ca-Mg-Fe carbonates from solution: Implications for the low temperature formation of carbonates in martian meteorite Allan Hills 84001. Meteorit. Planet. Sci. 35, in press.
Harvey R.P. and McSween H.Y. Jr. (1996) A possible high-temperature origin for the carbonates in the martian meteorite ALH84001. Nature 382, 49-51.
Leshin L.A., McKeegan K.D., Carpenter P.K., and Harvey R.P. (1998) Oxygen isotopic constraints on the genesis of carbonates from Martian meteorite ALH 84001. Geochim. Cosmochim. Acta 62, 3-13.
Lovely D.R., Anderson R.T., Chapelle F.H. (1998) Reply to "The hydrogen chemistry of aquifers basalt. Science 282, 2196.
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.
McSween H.Y.Jr. and Harvey R.P. (1998) An evaporation model for formation of carbonates in the ALH84001 martian meteorite. Internat. Geol. Rev. 40, 774-783.
Shearer C.K., Leshin L.A., and Adcock C.T. (1999) Olivine in Martian meteorite Allan Hills 84001: Evidence for a high-temperature origin and implications for signs of life. Meteor. Planet. Sci. 34, 331-340.
Scott E.R.D., Yamaguchi A., and Krot A.N. (1997) Petrological evidence for shock melting of carbonates in the martian meteorite ALH 84001. Nature 387, 377-379.
Scott E.R.D., Krot A.N., and Yamaguchi A. (1998) Carbonates in fractures of Martian meteorite ALH 84001: Petrologic evidence for impact origin. Meteorit. Planet. Sci. 33, 709-719.
Treiman A.H., Barrett R.A. and Gooding J.L. (1993) Preterrestrial aqueous alteration of the Lafayette (SNC) meteorite. Meteoritics 28, 86-97.
Treiman A.H. (1998) The history of ALH 84001 revised: Multiple shock events. Meteorit. Planet. Sci. 33, 753-764.
Treiman A.H. and Wallendahl A. (1998) Hydrogen chemistry of basalt aquifers. Science 282, 2196.
Valley J.W., Eiler J.M., Graham C.M., Gibson E.K.Jr., Romanek C.S., and Stolper E.M. (1997) Low-temperature carbonate concretions in the martian meteorites ALH 84001: Evidence from stable isotopes and mineralogy. Science 275, 1633-1638.
Warren P.H. (1998) Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite. Jour. Geophys. Res. 103, 16759-16773.
ALH84001 contains at least two types of organic carbon. One is terrestrial contamination, and is associated with the carbonate globules. The second is associated with the pyroxene minerals in the meteorite, and includes polycyclic aromatic hydrocarbons (PAHs) and high-mass polymerized organic molecules (kerogen). This organic material has a carbon isotope composition that is not consistent with terrestrial contamination, and is inferred to be Martian. However, it is similar to organic material in carbonaceous meteorites, and so it represents meteorite material that fell onto Mars, not Martian life.
The ALH84001 meteorite contains organic carbon, compounds with carbon atoms bonded to other carbons, and these organic compounds were used as evidence for a fossil martian life in the meteorite (McKay et al., 1996). In this paper, Becker et al. have investigated the stable isotopic composition of the organic carbon as a test of whether it is terrestrial or biologic. Carbon isotopic compositions are in the standard d 13C notation, which gives the difference between the 13C/12C ratios in the sample and a standard, usually in parts per mil (the same as tenths of a percent).
To separate organic material from various minerals in ALH84001, Becker used one molar HCl to dissolve the carbonate minerals and release their organic matter. One sample was also treated in six molar HCl. Solutions and residues from acid treatment were analyzed for carbon isotope ratios using standard methods. Some were analyzed for polycyclic aromatic hydrocarbons (PAHs) by laser desorption mass spectrometry, LDMS. For comparison, samples of a carbonaceous chondrite meteorite were prepared the same way, heated at various temperatures, and analyzed by LDMS.
Organic compounds from the carbonate globules had d 13C = -26 per mil and are almost certainly Earth contamination (Jull et al., 1998). The residue after acid treatment gave d 13C = -8 per mil, which is a mixture of this Earth contamination and carbon from the carbonate globules, at d 13C = ~ +36 per mil (mass balance on carbon gives 87% from Earth contaminant and 13% from the carbonates). A sample treated first in 1 molar HCl and then in 6 molar HCl gave d 13C = -15 per mil.
PAHs from the carbonates are comparable to those found with carbonates by McKay et al. (1996) and Clemett et al. (1998). PAHs from the insoluble material (mostly the mineral pyroxene), were distinctly different from those in the carbonate globules (Stephan et al., 1998). Becker found no evidence that PAHs were strongly associated with carbonate globules (as did McKay et al. 1996). The insoluble materials contained a moderate abundance of high-mass organic molecules, generically called `kerogen.'
Organic matter associated with the carbonate globules is terrestrial contamination, based on these (and other) stable isotope results, presence of radioactive carbon-14, and presence of L-type amino acids. The organic matter in the residues has the unusual value d 13C = -15 per mil, is interpreted as pre-terrestrial (Martian) kerogen, "... the first definitive evidence [of] organic material present on Mars ..." These organics probably did not form at the martian surface because it is (and was) too oxidizing. The authors suggest that these organics were asteroidal in origin, and were delivered to Mars in carbonaceous meteorites.
From this work and that of Stephan et al. (1998), it seems clear that ALH84001 contains a variety of organic material -- in different places and with different chemical and isotopic compositions. This conclusion muddies the clear correlation of carbonates and PAHs that was crucial to McKay's (1996) theory. This paper is, to my mind, far from conclusive. Rather than complain about the author's experimental procedures or their logic, I want to discuss the carbon with d 13C = -15 per mil.
The authors interpret the strong-acid-residue organics, with d 13C = -15 per mil, as being polymerized organic material (kerogen) from carbonaceous chondrite meteorites (vis. Bell, 1996). To them, this means that these organics are actually Martian, the remnants of carbonaceous meteorites that fell onto Mars. However, there are other reasonable (?) explanations for the d 13C = -15 per mil organics: [1] a mixture of common organics and carbonate, [2] terrestrial contamination, and [3] martian biota.
[1] The carbon with d 13C = -15 per mil could be a mixture of ~ 91% organic material (d 13C = -26 per mil) and ~9% carbon from carbonate minerals (d 13C = +36 per mil). Although Becker et al. did treat their samples with strong acids to remove all the carbonates, some carbonate grains are embedded deep inside pyroxene grains (e.g., Scott et al., 1998) and could reasonably have been protected from acid attack. Becker et al. could only detect carbonate minerals more abundant than ~0.5% by weight, and this mechanism requires that only 10 ppm carbonate, 0.001%, survive acid treatment.
[2] The d 13C = -15 per mil organics could be terrestrial contaminants in at least two ways. First, the common organic carbon of d 13C ~ -22 per mil develops in the common photosynthetic mechanism, called C3. Some plants have slightly different mechanisms, called C4 and CAM, which produce organic material with d 13C ~ -15 per mil (Hoefs, 1980). So, it is possible that the PAHs in ALH84001 derive from mixed terrestrial sources (vis., Burckle and Delaney, 1999).
Second, plants with C3 photosynthesis (and the animals that feed on them) have different d 13C in their different types of organic compounds. For instance, cellulose in marine plankton tends to have d 13C = -22 per mil, while its protein tends to have d 13C near -15 per mil (Hoefs, 1980). So, it is possible that terrestrial organics entering ALH84001 have become separated according to their chemical nature, a kind of natural affinity extraction.
[3] The d 13C = -15 per mil organics could still be from martian biota, because the overall d 13C of carbon on Mars is different from that on Earth. If martian biota formed organics from carbon dioxide in a mechanism like C3 photosynthesis, their organics would have a d 13C value about 25 per mil lower than the carbon dioxide. The carbon isotope ratio in Mars' atmosphere is not well known, but is probably heavier (larger d 13C) than the Earth's atmosphere (e.g., Jakosky and Jones, 1997). If the martian atmosphere has d 13C ~ +10 per mil, then C3 photosynthesis would produce organic compounds with d 13C ~ -15 per mil, just what Becker et al. found.
Shearer C.K., Leshin L.A., and Adcock C.T. (1999) Olivine in Martian meteorite ALH84001: Evidence for high-temperature origin and implications for signs of life. Meteorit. Planet. Sci. 34, 331-340.
Small grains of the mineral olivine are concentrated near broken and dispersed carbonate globules in ALH84001. The olivine formed at high temperatures, above 900°C, as shown by its elemental and oxygen isotope compositions. These facts suggest that the olivine formed when the carbonate globules were disrupted, and supports suggestions that the magnetite in the carbonate globules formed in a high-temperature event (i.e., without biology).
ALH84001 contains small quantities of the mineral olivine [(Mg,Fe)2SiO4], which Harvey & McSween (1996) claimed to be formed at the same time, in the same chemical reaction, as the carbonate globules. On the other hand, the olivine could be a relict igneous mineral (originally grown from magma) or have formed by metamorphism before or after the carbonates formed. To evaluate the relation between the olivine and carbonates, and thereby the hypothesis of ancient Martian life in ALH84001 (McKay et al., 1996), Shearer and co-workers have re-investigated the olivine: its occurrence, associations, chemical compositions, and oxygen isotopic composition.
Harvey & McSween (1996) reported that the olivine was all near carbonate gloubles, and Shearer and co-workers confirm this observation. They located over 110 olivine grains, all irregular in outline and < 40 µm across, in 3 thin sections of ALH84001. All of these olivine grains were in the abundant orthopyroxene, near (but not in) granulated zones. All but a few olivine grains were adjacent to carbonate globules that were disrupted by later shock events (McKay et al., 1997). This spatial distribution is not consistent with an igneous origin, in which the olivines would be concentrated near the cores of the pyroxenes, and not associated at all with the post-magmatic granulated zones. The spatial distribution also suggests that the olivines are somehow related to the deformation event that disrupted the carbonate globules (see my comments below).
The chemical compositions of the olivines and other minerals are fairly homogeneous, and suggest chemical reactions at a very high temperature. The elemental compositions of the olivine, pyroxene, and iron oxide minerals are as analyzed by others and are consistent with chemical equilibrium among them at >900°C (vis. Treiman 1995, 1998). The oxygen isotope compositions of two olivine grains (by ion microprobe) average d 18O = 5.1 per mil compared to a value for pyroxene of d 18O = 4.6 per mil. The difference in d 18O, if from chemical equilibrium, suggests at temperature above 700°C. These high temperature s, assuming they represent equilibrium, appear inconsistent with formation of olivine and carbonate at the same time (T~500°C; Harvey & McSween, 1996).
If the olivines did not form with the carbonates, it is critical to know whether they formed before or after the carbonates. Shearer infers that the olivines formed after the carbonates. From the spatial association of olivine with disrupted carbonate globules, Shearer infers that the olivines formed when the carbonates were disrupted, deformation D3 of Treiman (1998). Assuming that the glassy feldspar in ALH84001 flowed as a liquid in that event, Shearer gives it a minimum temperature of 1100°C. In this event, the olivine could have formed by dehydration of sheet silicate minerals (vis. Brearley, 1998), by reaction of some part of the carbonate globules with pyroxene, or reaction of pyroxene and iron oxide with loss of oxygen.
Shearer et al suggest that this event was responsible for formation of the submicron magnetite grains in the carbonate globules (vis. McKay et al., 1996; Thomas-Keprta et al., 1998) without any intervention of biological organisms.
The new data here are welcome, particularly oxygen isotope ratios, and evidence that the olivine grains in ALH84001 are spatially associated with carbonates and granulated zones. However, I think that one of Shearer's critical observations is incorrect. Also, their conclusion (that the olivines formed after the carbonates) seems inconsistent the rates of element diffusion in minerals.
First, much of Shearer's hypothesis rests on the observation that the olivine grains in ALH84001 are "...adjacent to fractures containing disrupted carbonate globules and feldspathic glass..." (page 333). They use this correlation to indicate that the olivine grains during the event that disrupted the carbonates (D3), which must have been after the carbonate globules formed. However, their own Figure 3c shows many olivine grains near carbonate globules that are (as well as I can see) completely undeformed and undisrupted. Similarly, Harvey & McSween (1996) show olivine grains in pyroxene near a similarly undisrupted globule. On the other hand, Shearer's own Figure 2 shows a disrupted carbonate globule in feldspathic glass, but without any olivine grains in the nearby orthopyroxene!
If the olivine is not associated only with disrupted carbonates, it need not have formed after the carbonates did. To my mind, the evidence seems to show that olivine, the pyroxenes, and the oxide minerals in ALH84001 all formed (and chemically equilibrated) before the carbonate globules formed (Treiman, 1995, 1998).
Second, Shearer's inference that the olivine formed after the carbonate globules seems inconsistent with how fast elements can move within minerals. Shearer and co-workers would have the olivines form from oxides, carbonates or pyroxene in a short pulse of high temperatures. The temperature and duration of the pulse must be enough to form 40 µm olivine crystals and to homogenize their chemical compositions and the compositions of nearby pyroxene and oxide minerals. The high-temperature pulse cannot have been long or hot enough to soften the razor-edge sharp chemical zones of the carbonate globules (Treiman and Romanek, 1998, show zoning in carbonate globules that is sharp on sub-micron scales).
However, the important elements here (Ca, Mg, and Fe) move faster in carbonate minerals than in either pyroxenes or olivine (Brady, 1995; Cygan and Fisler, 1998; Kent et al., 1999). Diffusion coefficients (measures of the speed of motion) at 800°C are on the order of 10-17 m2/sec in carbonates, ~10-18 m2/sec in olivine, and 10-21 -- 10-24 m2/sec in pyroxene; the smaller the number, the slower elements move. These data are from lab experiments, and are consistent with observations from nature (e.g., Treiman and Essene, 1995).
What do these number mean? Simply, that a thermal pulse adequate to form homogeneous olivine and pyroxene areas 40 µm across should have also homogenized carbonate grains of the same size. Since the carbonate grains are still zoned at sub-micron scales, they could NOT have experienced the same thermal pulse that produced the olivines, and must have formed AFTER the olivines.
References:
Bada J.L., Glavin D.P., McDonald G.D., and Becker L. (1998) A search for endogenous amino acids in the Martian meteorite ALH84001. Science 279, 362-365.
Becker L., Glavin D.P., and Bada J.L. (1997) Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta 61, 475-481.
Bell J.F. (1996) Evaluating the evidence for past life on Mars (letter). Science 274, 2121-2122.
Brady J.B. (1995) Diffusion Data for Silicate Minerals, Glasses, and Liquids. pp 269-290 in Mineral Physics & Crystallography: A Handbook of Physical Constants ( T.J. Ahrens, ed). American Geophysical Union, Wash. D.C.
Brearley A.J. (1998) Microstructures of feldspathic glass in ALH 84001 and evidence for post carbonate formation shock melting (abstract). Lunar Planet. Sci. XXIX, Abstract #1451, Lunar and Planetary Institute, Houston (CD-ROM).
Burckle L.H. and Delaney J.S. (1999) Terrestrial microfossils in Antarctic ordinary chondrites. Meteorit. Planet. Sci. 34, 475-478.
Clemett S.J., Dulay M.T., Gilette J.S., Chillier X.D.F., Mahajan T.B., and Zare R.N. (1998) Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. Faraday Discussions (Royal Soc. Chem.) 109, 417-436.
Faure G. (1977) Principles of Isotope Geology. J. Wyllie & Sons, N.Y. 464p.
Fisler D.K. and Cygan R.T. (1998) Cation diffusion in calcite: Determining closure temperatures and the thermal history for the ALH 84001 meteorite. Meteorit. Planet. Sci. 33, 785-789.
Harvey R.P. and McSween H.Y. Jr. (1996) A possible high-temperature origin for the carbonates in the martian meteorite ALH84001. Nature 382, 49-51.
Hoefs J. (1980) Stable Isotope Chemistry, 2nd ed. Springer, Berlin / New York. 208p.
Jakosky B.M. and Jones J.H. (1997) The history of Martian volatiles. Rev. Geophys. 35, 1-16.Jull A.J.T., Courtney C., Jeffrey D.A., and Beck J.W. (1998) Isotopic evidence for a terrestrial source of organic compounds found in Martian meteorites, Allan Hills 84001 and Elephant Moraine 79001. Science 279, 366-368.
Kent A.J.R., Hutcheon I.D., Ryerson F.J., and Phinney D.L. (1999) The temperature of formation of carbonates in martian meteorite ALH84001: Constraints from cation diffusion. Lunar Planet. Sci. XXX, Abstract #1473, Lunar and Planetary Institute, Houston (CD-ROM).
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 G.A., Mikouchi T., and Lofgren G.E. (1997) Carbonates and feldspathic glass in ALH84001: Additional complications (abstract). Meteor. Planet. Sci. 32, A87-A88.
Scott E.R.D., Krot A.N., and Yamaguchi A. (1998) Carbonates in fractures of Martian meteorite ALH 84001: Petrologic evidence for impact origin. Meteorit. Planet. Sci. 33, 709-719.
Stephan T., Heiss C.H., Rost D., and Jessberger E.K. (1999) Polycyclic aromatic hydrocarbons in meteorites: Allan Hills 84001, Murchison, and Orgueil. Lunar Planet. Sci. XXX, Abstract #1569, Lunar and Planetary Institute, Houston (CD-ROM).
Thomas-Keprta K.L., Bazylinski D.A., Golden D.C., Wentworth S.J., Gibson E.K.Jr., and McKay D.S. (1998) Magnetite from ALH84001 carbonate globules: Evidence of biogenic signatures? (abstract). Lunar Planet. Sci. XXIX, Abstract #1494, Lunar and Planetary Institute, Houston (CD-ROM).
Thomas-Keprta K.L., Wentworth S.J., McKay D.S., Bazylinski D., Bell M.S., Romanek C.S., Golden D.C., and Gibson E. K. Jr. (1999) On the origins of magnetite in martian meteorite ALH84001 (abstract). Lunar Planet. Sci. XXX, Abstract #1856, Lunar and Planetary Institute, Houston (CD-ROM).
Treiman A.H. (1995) A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age. Meteoritics 30, 294-302.
Treiman A.H. (1998) The history of Allan Hills 84001 revisited: Multiple shock events. Meteorit. Planet. Sci. 33, 753-764.
Treiman A.H. and Essene E.J. (1985) The Oka carbonatite complex, Quebec: Geology and evidence for silicate-carbonate liquid immiscibility. Amer. Mineral. 70, 1101-1113.
Treiman A.H. and Romanek C.S. (1998) Bulk and stable isotopic compositions of carbonate minerals in Allan Hills 84001: No proof of high formation temperature. Meteorit. Planet. Sci. 33, 737-742.
"All that glitters is not gold." Many rocks contain rounded and ovoid shapes, 25-300 nm long, that look like bacteria or fossilized bacteria. However, the authors have made such bacteria-shaped objects of inorganic minerals under sterile laboratory conditions, as well as in the presence of live bacteria. So, ancient bacterial fossils (or modern bacteria) in rocks cannot be recognized by shape alone.
Mineral surfaces in many Earth rocks are decorated with spherical, rod-shaped, and ovoid shaped objects of 25-300 nm (nanometers) size. These objects were proposed to be dwarf forms of bacteria, nannobacteria, or mineralized fossil nannobacteria (Folk, 1993) and are similar to shapes in ALH84001 suggested to be mineralized Martian micro-organisms (McKay et al., 1996). Many geologists have argued that these bacteria-shaped objects were not related to living organisms, but were inorganic mineral structures. The authors here did experiments to see if they could make these bacteria-shaped objects in the laboratory under controlled conditions, with and without bacteria present.
The authors dissolved calcium oxide (CaO) in water (organic-free), and let this solution to react with carbon dioxide (CO2) from the air. As carbon dioxide dissolved into the water at room temperature, grains of the mineral calcite (CaCO3) grew. Strict sterility was maintained at all time and closely monitored. Experiments were done with and without added organic compounds, with live bacteria, and with fragments of dead bacteria.
After one day of reaction, all of the experiments yielded rhombic-shaped crystals of calcite, and rounded calcite grains (20-50 nm across) which looked like nannobacteria. After two and three days of reaction, the proportion and size of calcite crystals increased, at the expense of the rounded grains. Experiments with bacteria and fragments yielded abundant rounded or irregular nm-scale objects, which could have been rounded calcite grains and organic globules. These globules persisted through the three days of the experiments.
The authors then took the larger calcite grains and etched them in dilute HCl. The etched surfaces were covered with ovoids, spherules, and segmented objects, which again appeared similar or identical to nannobacteria.
"These experiments show that nanometer-scale objects that resemble purported nannobacteria can be bacterial fragments. The experiments also show that similar objects can be produced under controlled conditions both by precipitation and etching. ... Our experiments do not disprove the existence of free-living organisms £ 50 nm. They do, however, suggest that many rounded, £ 50 nm objects found in nature could equally well be organic matter, anhedral protocrystals, amorphous precipitates, or features on etched mineral surfaces. ... These experiments show that, in dealing with the extremely fine scale textures in the natural environment, it will be difficult, if not impossible, to distinguish biologic from nonbiologic features solely on the basis of external morphology."
This short paper is an important contribution to the ALH84001 debate by demonstrating that bacteria-shaped objects (in ALH84001 and Nakhla; McKay et al., 1996, 1999) can form without biology. Articles in Geology magazine are limited to four pages, and I sincerely hope that the authors will publish a more detailed version of this work. In particular, it would be nice to read more experimental details, see many more images, see chemical analyses of the calcium carbonate materials, and learn why they are inferred to be calcite rather than aragonite or vaterite or amorphous. But the authors have done a pretty nice job with only four pages.
McKay et al. (1996) argued that the minerals of the carbonate globules in ALH84001 were formed by martian biota. Scott sees no proof of this idea, and finds rather that the magnetites, iron sulfides, and carbonates in the globules are better explained by inorganic processes. Scott sees that the carbonate globules formed during an intense impact shock event, from evaporite minerals that had been in the rock before it was shocked.
Here, Dr. Scott reviews evidence on McKay's group's biologic hypothesis for the origin of carbonate globules and their minerals in ALH84001, and on his (Scott's) abiologic hypothesis. Dr. Scott emphasizes the mineralogic issues, so does not discuss organic compounds or bacteria-shaped objects (except as they relate to mineralogy). First, he discusses problems with McKay et al.'s (1996) hypothesis of biogenic formation. Then, Scott discusses data that favor his theory of high-temperature shock-induced formation, as outlined earlier (Scott et al., 1997, 1998).
Magnetite (Fe3O4) grains are abundant in the carbonate globules, and McKay et al. (1996) suggested that the grains were formed by bacteria on Mars. However, shapes of some ALH84001 magnetites are not known from terrestrial bacteria, but all the known shapes have been reported in deposits from high-temperature (>500°C) fluids. Magnetite crystals in terrestrial bacteria are all "single magnetic domain" sized -- lengths and widths so the grains spontaneously become strongly magnetic. These grains are ideal for bacteria that need to sense magnetic fields around them. However, a significant proportion of magnetites in the ALH84001 carbonates are too small to be single magnetic domains (Thomas-Keprta et al., 1999), will not magnetize stably, and so are unlikely to have been made in bacteria. Many magnetite grains in ALH84001 are aligned with respect to the carbonate grains they sit on (Bradley et al., 1998), and no bacteria are known to do this. Bacteria usually hold their single-domain magnetites grains in linear chains, and McKay's group has shown a similar chain in ALH84001. However, grains in this chain are too small to be single domain, so the chain is likely not biogenic. Nor is it clear why magnetic bacteria would live in a rock that was full of magnetic materials; if the magnetite grains in the ALH84001 carbonates were washed into the rock, it is not clear why other mineral grains were not brought in too.
As before (Scott et al., 1997, 1998), the author sees convincing evidence that the carbonates now in ALH84001 formed at high temperature during an impact shock event. Above all, the carbonate globules do not "look like" low temperature products as they do not simply fill fractures and pore spaces. Other investigators have argued that the carbonates replaced earlier minerals (feldspar, olivine, pyroxene). This hypothesis is inadequate, as ALH84001 does not contain enough of the other minerals (clays, serpentines, silica) that should form during replacement. Melted plagioclase and silica glasses, and many other features, prove that ALH84001 experienced an intense shock heating event. Textures of the carbonates suggest that it, and feldspathic materials, were injected as liquids into fractures, which were then slammed shut in the shock event. The simultaneous timing of carbonate formation and shock heating (within 40 million years) are consistent with this scheme. The rarity of low-temperature alteration minerals (clays) reflects the high temperature of the shock event and the short time needed to cool the rock after shock.
For this scenario to work, ALH84001 must have contained carbonate minerals before it was shocked. Scott agrees with other workers that the carbonates were likely deposited as evaporites, from water that had interacted chemically with Mars' atmosphere.
A very valuable part of this contribution is Scott's discussion of the small magnetite grains in the ALH84001 carbonates. Thomas-Keprta et al. (1998, 1999) have emphasized the similarity of these magnetites to biogenic magnetosome magnetites. Here, Scott emphasizes the considerable differences between the ALH84001 magnetites and magnetosome magnetites. Scott also questions how biogenic magnetites could have gotten into ALH84001, a question I feel is key to understanding them. Much of the remainder of the paper is a welcome review of mineralogical and petrologic problems associated with a low-temperature biogenic origin for the carbonates. As I've said before, I think that Scott's hypothesis is unlikely -- but it deserves to be considered seriously and has led Scott to raise a slew of embarrassing questions about other hypotheses.
Citations:
Bradley J.P., McSween H.Y.Jr., and Harvey R.P. (1998) Epitaxial growth of nanophase magnetite in Martian meteorite ALH 84001: Implications for biogenic mineralization. Meteorit. Planet. Sci. 33, 765-773.
Greenwood J.P., Riciputi L.R., and McSween H.Y.Jr. (1997) Sulfide isotope compositions in shergottites and ALH 84001, and possible implications for life on Mars. Geochim. Cosmochim. Acta 61, 4449-4453.
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., Wentworth S.W., Thomas-Keprta K., Westall F., and Gibson E.K.Jr. (1999) Possible bacteria in Nakhla. Lunar Planet. Sci. XXX, Abstract #1816, Lunar and Planetary Institute, Houston (CD-ROM).
Scott E.R.D., Yamaguchi A., and Krot A.N. (1997) Petrological evidence for shock melting of carbonates in the martian meteorite ALH 84001. Nature 387, 377-379.
Scott E.R.D., Krot A.N., and Yamaguchi A. (1998) Carbonates in fractures of Martian meteorite ALH 84001: Petrologic evidence for impact origin. Meteorit. Planet. Sci. 33, 709-719.
Thomas-Keprta K.L., Bazylinski D.A., Golden D.C., Wentworth S.J., Gibson E.K.Jr., and McKay D.S. (1998) Magnetite from ALH84001 carbonate globules: Evidence of biogenic signatures? (abstract). Lunar Planet. Sci. XXIX, Abstract #1494, Lunar and Planetary Institute, Houston (CD-ROM).
Thomas-Keprta K.L., Wentworth S.J., McKay D.S., Bazylinski D., Bell M.S., Romanek C.S., Golden D.C., and Gibson E. K. Jr. (1999) On the origins of magnetite in martian meteorite ALH84001 (abstract). Lunar Planet. Sci. XXX, Abstract #1856, Lunar and Planetary Institute, Houston (CD-ROM).
The author recalls his experiences leading up to the NASA press conference at which D.S. McKay and colleagues announced their claim of evidence for ancient martian life in ALH 84001. The validity of the claim and its evidence are discussed in the context of two failed extraordinary claims in paleontology.
The course of science is littered with claims of extraordinary breakthroughs and insights. Correct claims are well-chronicled, and often led to Nobel prizes. Failed claims reveal a rarely-seen side of science: error, faith, despair, and sometimes redemption. Dr. Schopf considers three extraordinary claims: the fossil of a purported man drowned in the Noachian deluge; hoax fossils of birds, stars, spiderwebs, the name of God in Hebrew, etc., perpetrated upon a German paleontologist; and evidence for fossil life in meteorite ALH 84001.
In 1725, Dr. J.J. Scheutzer discovered fossil bones in limestone from Baden (Germany), and interpreted them as a human skeleton, from a man drowned in the Noachian flood. His discovery was hailed as scientific proof of the Bible, although many scholars harbored doubts. After Scheutzer's death, and the conquest of his home Holland by France in 1810, Georges Cuvier was granted permission to "clean" the specimen. Cuvier confirmed his suspicions (based on published drawings) that the skeleton actually came from a large salamander.
Also in 1725, Professor J. Beringer of Würzburg (Germany) dug up a collection of rocks with incised and relief depictions of animals (bees, scorpions, lizards, aphids, spiderwebs, a honeycomb, a bird with eggs), flowers, stars, a smiling sun, and the name of God in Hebrew. Professor Beringer quickly published a treatise on these marvelous rocks, but soon realized that he had been hoaxed. He requested a judicial hearing, where two rival academics were implicated. Beringer was exonerated, and continued his distinguished career.
In 1996, Dr. D. McKay and colleagues at the Johnson Space Center announced at a press conference that they had uncovered evidence, in a meteorite, for life in Mars' distant past. Their first line of evidence was the presence of carbonate, sulfide and oxide minerals together in the rock. McKay et al. suggested that the association of these minerals could be related to life, but the association does not prove life. Second, McKay's group found minute crystals of magnetic iron oxide, which are strikingly similar to those found in some Earth bacteria. Yet these particles cannot be distinguished from similar magnetic grains formed without life. The third line of evidence was the presence of organic chemical compounds associated with the carbonate minerals, yet the specific organic compounds discovered can form readily without any intervention from life. Finally, McKay and colleagues presented images of "features resembling terrestrial microorganisms..." as possibly being fossils of martian microorganisms.
The "features resembling terrestrial microorganisms..." are actually similar in shape only, not in size. They are up to 30 x 130 nm (billionths of a meter) long. This is about 1/200 the volume of the smallest known terrestrial organism, which is a parasite: it is too small to contain all the biochemistry needed for life as we know it. If we assume that the martian organism had a cell wall ~ 6 nm thick, its volume is 1/2000 that of the smallest known Earth organism.
The question of life in ALH 84001 cannot be answered definitively with the available data. Dr. Schopf draws three lessons from the stories of extraordinary scientific claims: that "headlines win," that scientists are human, and that science is self-correcting -- faulty conclusions will eventually be found out.
This chapter is not a peer-reviewed scientific document, but gives a fascinating historical account of the ALH 84001 controversy. Dr. Schopf was the designated skeptic at the first NASA press conference on ALH 84001, the curmudgeon who found the flaws in the theory McKay built. Particularly intriguing is Schopf's story of his first meeting with McKay's group -- by his account, they had tentatively identified carbonate pancakes in ALH 84001 as whole fossil foraminifera.
This chapter was adapted from a longer work by Dr. Schopf, Cradle of Life, to be published in 1999 by Princeton University Press.
The carbonate globules in ALH84001, hosts to possible evidence of ancient martian life, could have precipitated from pore water fed from a saline lake. Some deposits from saline lakes show sequences of minerals like those in the globules, and most other data on the globules are consistent with this evaporitic origin. Although micro-organisms are abundant in saline lakes on Earth, this model does not require martian micro-organisms to produce the carbonate globules.
Several models for formation of the carbonate globules in ALH84001 (hosts to possible evidence of ancient martian life) have invoked very high temperatures (Harvey and McSween, 1996; Scott et al., 1997). These scenarios have proved difficult to test, so the authors explore an alternative model: precipitation at low temperature from the water of a saline, evaporating lake.
Saline lakes on Earth (like the Dead Sea) generally precipitate carbonate minerals as they evaporate: a common sequence is calcite, dolomite or ankerite, and magnesite. These minerals normally appear in concentric zones around the lake center, but can also be deposited in rocks beneath the lake. This sequence of minerals is similar to that in the ALH84001 globules: calcite, ankerite, magnesian siderite, and magnesite.
The carbonates globules in ALH84001 differ from known terrestrial evaporite deposits in several respects: some of their carbonates are rich in iron; their magnesite is rich in calcium; sulfate minerals are nearly absent; and they have layers rich in magnetite. The iron-rich carbonates are not a problem, however, as similar compositions are common in ancient evaporite sediments. As with ALH84001, this may reflect lower oxygen abundances in the air and water. The high calcium abundance in magnesite could be explained reasonably by chemical disequilibrium. The absence of sulfate minerals in the ALH84001 could arise in many ways. Pore spaces in the rock may have been filled before the lake water reached sulfate saturation. The lake may have been stratified, with sulfate precipitation at the top only. Water flowing into the lake could have prevented it from reaching sulfate saturation. And bacteria can prevent reduce the sulfate in solution to sulfide (although this seems unlikely; Greenwood et al., 1998). The magnetite-rich layers could represent sudden influxes of sediments into the lake.
This evaporite lake model is consistent with other data on ALH84001 and Mars: oxygen isotope measurements on ALH84001, the presence of lake deposits on Mars, and evaporite minerals in other martian meteorites. Leshin et al. (1998) showed that oxygen isotope ratios in the carbonate globules changed along with element composition, and the range in isotope ratios is consistent (in general terms) with isotope fractionation during evaporation of water from a lake. Some Viking and Mars Global Surveyor images of Mars appear to show evaporite deposits, both from playas and from crater-filling lakes. And salt minerals in the other martian meteorites include carbonates, sulfates, and halide minerals that would be expected from an evaporating lake.
Bacteria and other microbes are abundant in evaporite lakes on Earth, but are not necessary for precipitation of the carbonate minerals. Thus, this model does not necessarily support the hypotheses of McKay et al. (1996). However, it does suggest that evaporite lakes on Mars might be good targets for spacecraft exploration in the search for Martian life.
This paper and Warren's (1998) paper represent a new convergence of views about ALH84001 carbonates: if high-temperature models cannot be proved, why not theorize about low-temperature environments. These two sets of models differ in where the carbonate was deposited: this paper suggests deposition beneath a saline lake, while Warren (1998) suggests deposition from floodwaters as they percolated through the martian regolith. McSween and Harvey are careful to say that they have not abandoned a high-temperature origin for the carbonate globules (Harvey and McSween, 1996); they are merely considering another possible scenario.
Low-temperature evaporite scenarios like this one and Warren's (1998) explain two serious impediments to most other models: the fine scale zoning of the carbonates and the extreme rarity of hydrous silicate minerals in ALH84001. The fine-scale zoning must have been preserved through the formation and subsequent history of the carbonates. If the carbonates were deposited hot (>500°C), they must have cooled incredibly quickly or chemical diffusion would have erased the zoning. If the carbonates formed cold (as in an evaporite lake), time is no longer an issue. The absence of water-bearing silicate minerals in ALH84001 has been an embarrassment for scenarios of hydrothermal (hot-water) carbonates: the main mineral of ALH84001, orthopyroxene, reacts rapidly with hot (or warm) water to produce clays, serpentine and other water-bearing minerals. But in a "cold ," room-temperature environment, orthopyroxene may react too slowly to detect (see paper below by Barrat et al., 1999).
Both this paper and Warren's (1998) paper present plausible models for low-temperature formation of ALH84001's carbonate globules in a reasonable Martian setting. Both, however, have the carbonate globules filling open spaces in ALH84001, and this point is subject to dispute. My sense is that the carbonates did not fill open space, but somehow replaced other minerals (Treiman, 1995, 1998); textures of some other carbonate masses in ALH84001 also seem difficult to explain as open space fillings (e.g., McKay et al., 1998).
Also, neither paper shows a carbonate globule, like those in ALH84001, that actually formed in the environments they propose. The most similar carbonates from an evaporitic environment, described by Barrat et al. (1998, 1999), do not contain iron or magnesium carbonates; the terrestrial carbonates most similar to those of ALH84001 are not from an evaporite environment (Treiman et al., 1998; Blake et al., 1999; Mojzsis et al., 1999).
Citations:
Barrat J.A., Gillet Ph., Lécuyer C., Sheppard S.M.F., and Lesourd M. (1998) Formation of carbonates in the Tatahouine meteorite. Science 280, 412-414.
Barrat J.A., Gillet Ph., Lesourd M., Blichert-Toft J., and Popeau G.R. (1999) The Tatahouine diogenite: Mineralogical and chemical effects of sixty-three years of terrestrial residence. Meteor. Planet. Sci. 34, 91-97. [See below]
Blake D.M., Treiman A.H., Amundsen H.E.F., Mojzsis S.J., and Bunch T. (1999) Carbonate globules, analogous to those in ALH84001, from Spitzbergen, Norway: Formation in a hydrothermal environment. Lunar Planet. Sci. XXX, Abstract #1683, Lunar and Planetary Institute, Houston (CD-ROM).
Greenwood J.P., Riciputi L.R., and McSween H.Y.Jr. (1997) Sulfide isotope compositions in shergottites and ALH 84001, and possible implications for life on Mars. Geochim. Cosmochim. Acta 61, 4449-4453.
Harvey R.P. and McSween H.Y. Jr. (1996) A possible high-temperature origin for the carbonates in the martian meteorite ALH84001. Nature 382, 49-51.
Leshin L.A., McKeegan K.D., Carpenter P.K., and Harvey R.P. (1998) Oxygen isotopic constraints on the genesis of carbonates from Martian meteorite ALH 84001. Geochim. Cosmochim. Acta 62, 3-13.
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.
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Carbonate globules similar to those in ALH84001 were produced, in an asteroidal meteorite, during 63 years of weathering in north Africa. Small grains of calcite, the sizes and shapes of possible bacteria, are abundant in the globules and present on fracture surfaces in the meteorite and on solid objects in the soil. These small grains may be fossilized bacteria, and tests are in progress.
The Tatahouine meteorite fell in 1931 in Tunisia, and fragments were collected immediately. Like ALH84001, the Tatahouine meteorite is composed almost entirely of the minerals orthopyroxene and chromite; unlike ALH84001, Tatahouine is definitely not from Mars. Fragments of the meteorite were collected again in 1994, after 63 years in (and on) the soil. In that time, the meteorite acquired significant signs of alteration and chemical reaction with its environment. Its iron metal and sulfides were partially replaced by reddish iron oxides, i.e. it rusted. Some chemical elements in the soil were transported into the meteorite, notably strontium, rubidium, and the light rare earth elements (e.g., La, Ce, Nd). And the mineral calcite (CaCO3) was deposited in the meteorite's cracks as vein fillings and disc-shaped masses.
The calcite masses in Tatahouine are comparable in shape and size to the carbonate mineral pancakes in ALH84001. The calcite in Tatahouine definitely formed on Earth, because they are only found in the samples collected in 1994, and they have isotope ratios in carbon and oxygen that are identical to those in the soil around them (Barrat et al., 1998). The calcite masses are not associated with hydrous silicate minerals. In and around the calcite masses are abundant rod-shaped objects, 70-80 nm wide and 100-600 nm long. Also present are spherical objects, ~70 nm in diameter. Both kinds of shapes are too small for precise analyses, but are probably now made of calcite. These objects are also found on fractures through pyroxene and chromite crystals, on the outside surfaces of the meteorite fragments and on solid objects in the soil: "... gravels, gun bullets, and an old coin."
These small calcite shapes are clearly related to soil processes, and are very similar to the "bacteria" shapes shown by McKay and co-workers in the ALH84001 meteorite (McKay et al., 1996, 1997). The shapes could either be purely inorganic precipitates from the soil, or could be mineralized (fossilized) biological organisms. In either case, the weathered samples of the Tatahouine meteorite are good analogs for the formation of carbonate materials in ALH84001. Testing for biological activity is in progress.
This research was begun to explore how weathering on Earth affects the chemical compositions of meteorites. The analogy between Tatahouine and ALH84001 could not have been imagined when their work began.
The critical issue in this work is the degree of similarity between the carbonate masses in Tatahouine and those in ALH84001. One would want to know the chemical compositions of the Tatahouine carbonates, and whether they are zoned like those in ALH84001. It would be important to describe internal structures (layering) in Tatahouine carbonate, if they have any. And it would be important to document whether the carbonate masses fill open fractures or whether some pyroxene or chromite was dissolved to make room for them (as it appears in Fig. 1A of this paper and Fig. 1 of Barrat et al., 1998).
Since this paper was accepted for publication, the authors have continued to characterize the rod-shaped objects. Gillet et al. (1999) reported having cultured calcite-forming bacteria from the soil at the fall site. However, these bacteria are ten times as long as the calcite rods in Tatahouine, and it is not clear if they are related. DNA and other biochemical tests are in progress.
I have enjoyed reading Barrat's papers. He writes well, and has a dry sense of humor.
Citations:
Barrat J.A., Gillet Ph., Lécuyer C., Sheppard S.M.F., and Lesourd M. (1998) Formation of carbonates in the Tatahouine meteorite. Science 280, 412-414.
Gillet Ph., Barrat J.A., Blichert-Toft J., Fiquet G., Guyot F., Jambon A., Lecuer V.C., Sheppart D., Malavergne Martinez I. (1999) From chemical and mineralogical models to geophysical tests of the martian interior (abstract). Program and Proceedings, International Symposium Mars Exploration Program & Sample Return Missions.
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.Jr., Thomas-Keprta K.L., and Vali H. (1997) No 'nano fossils' in martian meteorite: reply. Nature 390, 455-456.
