Earlier Scientific Papers on ALH 84001 Explained,
with Insightful and Totally Objective Commentaries

Allan Treiman
Lunar and Planetary Institute

Many scientific papers have now been published on the possibility that the martian meteorite ALH 84001 contains traces of ancient martian life (McKay et al. 1996a). Many (probably most) of these papers are difficult to understand (even for specialists), and many do not really say why they are important. Here, I've tried to present the main arguments of these papers for the educated nonspecialist, and some sense of why they are important (or why not).


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.

The authors grew bacteria in a basalt rock to see if their shapes and structures were like those found in ALH 84001. The bacteria that grew were tubular or ovoid, and 0.5-3.5 micrometers long. Some bacterial shapes contained organic matter; others were hollow mineral shells of iron oxide-hydroxides. Some of the bacteria carried filaments, 0.02 to 0.2 micrometers in diameter and up to 6 micrometers long; isolated filaments like these, also preserved as iron minerals, were found throughout the rock. The purported bacterial features in ALH 84001 are very similar in shape and size to the mineralized bacteria and filaments.

The bacteria-shaped objects (BSOs) are among the most visually appealing hints of ancient life in ALH 84001. BSOs had not been observed in terrestrial basalt rocks that had bacterial colonies, so the authors grew bacteria in basalt and examined them. The basalt and the bacteria were from the Columbia River basalts of eastern Washington (Stevens and McKinley, 1995). Sterile basalt samples were "inoculated" with bacteria-rich sediment from deep within the Columbia River basalts, and incubated for up to 8 weeks at 30°C. All the inoculated samples showed abundant bacterial features on their grain surfaces, while uninoculated samples showed none.

In the inoculated samples, four types of whole micro-organism features were found. One variety (type 4) had been live cells: ellipsoidal to tubular objects 0.3-2.5 micrometers long; and composed of carbon and oxygen (no analysis for hydrogen) with lesser sodium, phosphorous, manganese sulfur, and chlorine. Two other varieties of "micro-organisms" had the same general shape but were hollow shells of iron-oxide-hydroxide minerals with different textures. These two varieties are interpreted as remains of bacteria, mineralized or fossilized by iron compounds. It is noteworthy that 12% of the observed bacterial shapes had become mineralized during the course of the experiment.

Many of the live cells had filaments attached (probably prosthecae*). Filaments without cells attached are abundant in the basalt -- they were 0.02 to 0.2 micrometers in diameter and as long as 6 micrometers. The filaments were composed of iron oxide-hydroxide minerals, just as were the mineralized bacteria.

The filaments are interpreted as biogenic features, mineralized appendages of micro-organisms. Other possible explanations include: abiotic mineral precipitates; inorganic products of biological activity; or nanobacteria (this being quite controversial). These filaments are like many of the bacteria-shaped objects figured earlier and here in ALH 84001 (McKay et al., 1996, 1997), and so it appears reasonable to interpret many of the bacteria-shaped objects in ALH 84001 as "...mineralized, unattached cellular filaments."

This paper continues the plausibility argument that the bacteria-shaped objects (BSOs) in ALH 84001 are, in fact, related to bacteria. Here, and in McKay et al. (1997), the group acknowledges that some of the BSOs they described in 1996 are too small to be micro-organisms as we know them with Earth-style biochemical processes and structures. But is interesting and intriguing that many of the features they found in their experimental sample bear a passing (or striking) similarity to the features in ALH 84001.

But the timescale of the experiments raises a further concern for ALH 84001. Apparently, it only took 8 weeks to grow bacteria, move them around, have them become mineralized, and dissolve away all their organic constituents. So, if the BSOs in ALH 84001 are bacteria or filaments, could they have grown and been mineralized in Antarctica, rather than on Mars??? Bacteria do grow in meteorites in Antarctica (Steele et al., 1998), and ALH 84001 had ~15,000 years there to let the bugs live, die, and become preserved. Granted, much of that time was probably spent frozen in ice, and nearly all of it was at temperatures below the 30°C of the experiments (everything happens slower at low temperatures). But still, couldn't there have been time to form the BSOs on Earth? And then, how one could distinguish mineralized martian bacteria from mineralized Earth bacteria, when all that remains is a hollow mineral shell?

A puzzling aspect of this work is the absence of new images from ALH 84001. This paper would have been a great place to showcase biodiversity in the meteorite. But two of the three images from ALH 84001 were distributed in late 1996 (after their paper was published); the third image was shown (I think) in March 1997.

* As opposed to flagellae or fimbrae, for the bio-geek.

Citations:

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. Nature 390, 455-456.

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

Stevens T. O. and McKinley J. P. (1995) Geochemically produced hydrogen supports microbial ecosystems in deep basalt aquifers. Science 270, 450-454.


Riciputi L. R. and Greenwood J. P. (1998) Analysis of sulfur and carbon isotope ratios in mixed matrices by secondary ion mass spectrometry: Implications for mass bias corrections. Intl. J. Mass Spectrom. 179, 65-71.

The authors analyzed sulfur isotope ratios in fine-grained mixtures of sulfide, oxide, and carbonate minerals. Using their standard method of extreme energy filtering, the authors' analyses of sulfur isotope ratios in sulfides were not changed if the sulfides were mixed with iron oxide or calcium-magnesium carbonate. So, it is clear that (1) analyses of mixed targets are minimally affected by 16O16O2- ions masquerading for 32S-2, and (2) correction factors for mixed targets are those for the host phase for the element in question.

Earlier this year, I reviewed a paper by these authors on sulfur isotopes in ALH 84001 (Greenwood et al., 1997), where they concluded that sulfide minerals in the carbonate globules formed without life. At the time, I complained that the paper was weak because it did not document the validity of some new twists on their analytical techniques. In this paper, Riciputi and Greenwood have documented their analytical procedures in full, and it is worth going back to the original paper.

Sulfur occurs as two stable (not radioactive) isotopes with masses of 32 and 34, 32S and 34S. Most sources of sulfur have abundance ratios of 34S/32S that are very similar to the average in the solar system. However, sulfur that has been processed by bacteria (or other life forms) can have distinctly different abundances of these isotopes. The greatest changes in S isotopes come from sulfate-reducing bacteria, which take sulfate ions (SO42-) from water and convert them to sulfide ions (S2-) in water or as solid sulfide minerals. Sulfate-reducing bacteria, when they have lots of sulfate in water around them, can form sulfide minerals with ~5% less 34S than the sulfate in the water. This difference is easily detected, and has been used (on Earth) as a guide to the action of these bacteria.

Greenwood et al. (1997) analyzed sulfides in the carbonate globules of ALH 84001 as a test of whether sulfate-reducing bacteria had been involved. The problem (at the time) with their work was that the material they analyzed was not just sulfide minerals, but also magnesite (a carbonate mineral) and magnetite (an oxide) mineral. Their correction procedures were calibrated using pure sulfides, and so they might have been inappropriate for the mixed target in ALH 84001. Also, analyzing sulfides and oxygen-bearing and oxide samples together might yield inaccurate results because the two-oxygen ion 16O16O2- can masquerade for 32S-2. In this current paper, the authors have shown that neither problem affects their analyses of sulfur isotope ratios in the carbonate globules. Greenwood et al. (1997) found that sulfur in the globules had the identical isotope ratio (within uncertainty) with sulfur in nonbiological minerals in ALH 84001 (pyrite, FeS2). So, Greenwood et al. (1997) have truly shown that sulfide minerals in the carbonate globules show no isotopic signature of a biological origin. Specifically, these sulfides were not produced by living organisms that use the sulfate-reduction mechanism used by common Earth bacteria. This conclusion seems fairly certain, albeit on a fairly restricted hypothesis.

Citations:

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.

Hutcheon I. D., Kent A. J. R., Ryerson F. J., and Phinney D. L. (1998) The temperature of formation of carbonate minerals in martian meteorite ALH84001: Constraints from cation diffusion (abstract). Eos 79, F967.

The authors are conducting element diffusion experiments in carbonate minerals as a way to understan d how the very sharp chemical composition boundaries in ALH 84001 carbonates can be preserved. From their preliminary results, Mg diffusion in calcite is fairly slow (diffusivity D = 3.0 x 10-18 cm2s-1) at 450°C. If this rate is relevant to the magnesium-iron carbonates of ALH 84001, the observed chemical gradients would have been obliterated in a few thousand years at 450°C or higher.

This preliminary diffusivity value for Mg in calcite is consistent with the earlier results of Fisler and Cygan (1998), although their experimental uncertainties are very large. Extrapolating Fisler and Cygan's results (900°C to 550°C) on Mg diffusion in calcite down to Hutcheon's temperature of 450°C gives a nominal D = 1.7 x 10-20 cm2s-1, and a permitted range of 7.5 x 10-26 to 3x10-15 cm2s-1. Obviously, the permitted range in Fisler and Cygan's diffusion parameters (principally the activation energy), can accommodate an enormous range of sins -- 10 orders of magnitude. (This is good enough only in astrophysics.)

In retrospect, it would have been extremely useful if Fisler and Cygan had reported their raw data, or even a table of diffusivities (D) and their uncertainties versus temperature. As is, the equation the equation they give doesn't give me any sense of how accurate it is, how accurate their raw measurements were, nor how reasonable an extrapolation to lower temperatures might be. I hope that Hutcheon et al. will report full data when their paper is written up.

Citations:

Fisler D. K. and Cygan R. T. (1998) Cation diffusion in calcite: Determining closure temperatures and the thermal history for the ALH 84001 meteorite. Meteor. Planet. Sci. 33, 785-789.


Steele A., Goddard D., Beech I. B., Tapper R. C., Stapleton D., and Smith J. R. (1998) Atomic force microscopy imaging of fragments from the Martian meteorite ALH84001. Jour. Microscopy 189, 2-7.

Some bacteria-shaped objects in ALH 84001 have been challenged as artificial, produced during the metal coating required for scanning electron microscopy. The authors show that metal lumps made during coating are 5-10 nm (billionths of a meter) across, so larger bacteria-shaped objects must have formed in another way.

McKay et al. (1996) described bacteria-shaped objects from ALH 84001 as remnants of martian micro-organisms. However, some of them have been criticized as being artificial, being droplets of the gold-palladium (Au/Pd) alloy that was deposited on the samples for scanning electron microscope (SEM) examination. To find out if the bacteria-shaped objects are blobs of Au/Pd metal, the authors looked at surfaces of ALH 84001 and reference samples with atomic force microscopy (AFM), which does not require that a sample be coated.

To see the effects of coating, the authors coated flat mica surfaces with Au/Pd for measured times. Before/after images with AFM showed that the Au/Pd coating was a layer of lumps, each ~5-10 nm (billionths of a meter) in diameter. The same size lumps cover coated surfaces of ALH 84001, so surface roughness at this scale is probably Au/Pd coating. Larger bacteria-shaped objects, to 750 nm long (McKay et al., 1996, 1977), are not from Au/Pd coating.

Bacteria-shaped objects in ALH 84001 do exist, as they have now been reported by four groups using different methods (McKay et al., 1996, 1997; Bradley et al., 1997; Sears and Kral, 1998; this work). The groups differ, of course, in how the objects are interpreted. McKay et al. (1996) called them microbial fossils; Bradley et al. (1997) found some that were clearly mineral surface effects, but McKay et al. (1997) responded that those were never considered microbial; and Sears and Kral (1998) gave evidence that they are Antarctic, not extraterrestrial. With this paper, it is clear that most of the bacteria-shaped objects of McKay et al. (1996, 1997) are not entirely artifacts from the Au/Pd coating.

Personally, I am still concerned about the small bacteria-shaped objects, especially the famous worm-shaped thingy, ~200 nm long but only ~20 nm in diameter. Its length would be affected little by Au/Pd coating, but what about its diameter? If the Au/Pd coating in this area is ~ 7 nm thick and it covers the sides and top of the object, then the "real worm" inside can only be about 6 nm in diameter!!

This 6 nm is much too small for a living micro-organism -- cell-bounding membranes themselves are ~ 5 nm thick (Nealson, 1997), leaving no volume at all for the chemical machinery of the cell! Even microtubules, the building blocks of bacterial appendages like cilia and flagellae, are ~25 nm in diameter! Under its Au/Pd coating, the worm-thing may be nothing more than a thin mineral ridge or wall (Bradley et al., 1997)

Citations:

Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1997) No 'nanofossils' in martian meteorite. Nature 390, 454-455.

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. Nature 390, 455-456.

Nealson K. H. (1997) The limits of life on Earth and searching for life on Mars. J. Geophys. Res. 102, 23675-23686.


Krähenbühl U., Noll K., Döbeli M., Grambole D., Herrmann F., and Tobler L. (1998) Exposure of ALH84001 and other achondrites on the Antarctic ice. Meteorit. Planet. Sci. 33, 665-670.

ALH 84001 landed on Earth about 13,000 years ago. Less than 500 years of that time was at the ice surface, based on the lack of fluorine enrichment at the surface of ALH 84001. This is very little time for it to become contaminated with Earth materials, e.g., organic compounds like PAHs.

After a meteorite lands in Antarctica, it may spend most of its time buried deeply in the ice, protected from atmospheric (and most biologic) forms of contamination (Cassidy et al., 1992). When a meteorite is exposed at the ice surface, it accumulates fluorine (F) from the atmosphere and its dust. The longer a meteorite has been exposed on the ice surface, the more F accumulates on it, and the deeper the F penetrates (Langenauer and Krähenbühl, 1993).

Fluorine abundances are measured by bombarding the meteorite with protons in a particle accelerator, and counting the number of gamma rays produced as F atoms are hit by the protons. This method can detect ~ 5 parts per million (ppm) F. ALH 84001 is not enriched in F at its surface, while other meteorites' surfaces are enriched up to 470 ppm F. The low abundance of F at ALH 84001's surface suggests that it was at the ice surface for a short time.

Calibrating the time at the ice's surface -- how much time leads to how much fluorine -- is based on other meteorites. The diogenite meteorite ALHA77256 is a good calibration for ALH84001 -- both have similar mineralogy and chemical compositions, find sites (both Allan Hills), and times since they fell to Earth (11,000 years vs. 13,000 years for ALH 84001). The surface of ALHA77256 is enriched in F by 114 ppm, while the surface of ALH 84001 has <5 ppm enrichment. If that 114 ppm represents 11,000 years exposed on the ice, then ALH 84001 was exposed less than 500 years. This short time span is a severe limit on the mechanisms and rates of terrestrial contamination of ALH 84001.

The short time ALH 84001 spent near the ice surface, if confirmed, would seem to suggest that many of its possible biogenic features (e.g., PAHs, bacteria-shaped objects) are not from terrestrial contamination and alteration. Although 500 years is a lot by our reckoning, it presents very little time for flow of melted ice through ALH 84001 to deposit PAH organics (e.g., Becker et al., 1997), growth of Earth bacteria (Steele et al., 1998), chemical alteration to produce bacteria-shaped objects (Sears and Kral, 1998), and possibly even introduction of soluble organics like amino acids (Bada et al., 1998; Jull et al., 1998). Particularly for the PAHs, this supports the suggestion that they are preterrestrial (martian).

This is a new technique, and needs much more work to be completely calibrated (as the authors freely admit). Different parts of a meteorite can show different F enrichments, and different kinds of meteorites take up F differently (from chemical composition, grain size, and porosity). Does the flux of F to Antarctica vary with time? Could F-rich water, as from a lake beneath the glaciers, come into contact with a meteorite? These complications aside, the F data here confirm the optical microscopy observations that ALH 84001 shows very limited effects of weathering on Earth.

P.S. It would be very interesting to learn the ice surface exposure ages of meteorites that contain live Earth bacteria and fungi (Steele et al., 1998).

Citations:

Bada J. L., Glavin D. P., McDonald G. D., and Becker L. (1998) A search for endogenous amino acids in 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.

Cassidy W., Harvey R. P., Schutt J., Delisle G., and Yanai K. (1992) The meteorite collection sites of Antarctica. Meteoritics 27, 490-525.

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, 366-369.

Langenauer M. and Krähenbühl U. (1993) Depth profiles and surface enrichment of the halogens in four Antarctic H5 chondrites and in two non-Antarctic chondrites. Meteoritics 28, 490-525.

Sears D. W. G. and Kral T. A. (1998) Martian "microfossils" in lunar meteorites? Meteorit. Planet. Sci. 33, 791-794.

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


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.

Carbonate minerals in ALH 84001 have structures and textures like those of shock-melted sulfides and metals in other meteorites. This similarity suggests that the ALH 84001 carbonates also formed from shock-induced melts. In particular, the authors report that carbonate pancakes are actually lenticular (lentil- or lens-shaped), which is difficult to explain if they formed from aqueous solutions. If the carbonates formed from shock melts, they could not be associated with martian life.

Mineral interrelationships (textures) and compositions in ALH 84001 suggest that its carbonate globules crystallized from carbonate melts, created during an impact shock event. The carbonate melt crystallized rapidly to its present forms: disks along fractures, nodules, veins, and irregular disseminations (the same textures described earlier by many others, including Mittlefehldt, 1994; Treiman, 1995; D. McKay et al., 1996; G. McKay et al., 1997). Carbonate disks are actually lenticular (like lenses or lentil beans, thicker at the center than the edges) along fractures in pyroxene -- on either side of a disk, its fracture continues with minimal open space (their Figure 1). This texture formed by simultaneous crystallization of the melt and deformation of the host pyroxene. The melts were injected into open fractures, which immediately started closing. As the carbonate solidified (from the center outward), the surrounding pyroxene deformed around it, sealing the fracture away from the center of the solidifying carbonate. If the deformation had been after the disks formed, their edges would have been crushed; if the deformation had been before the disks formed, they would not be lenticular. The deformation produced many microfractures in the pyroxene around the carbonates. Magnesite-rich "microdisks," ~20 µm diameter, are commonly associated with the larger disks in fractures.

Formation of most of the carbonates' features is consistent with deposition from fluids in the fractures, but the fractures in ALH 84001 formed after deposition of the carbonates. ALH 84001 does not contain open fractures that are partially filled by carbonate disks, which should be present if the carbonates were deposited from water-rich fluid. Similarly, there are no open fractures adjacent to areas with disseminated carbonate.

Similar textures are present in highly shocked chondrite meteorites (not martian). In them, shock-melted sulfide minerals and iron metal form elongate and irregular drops and patches in surrounding mineral, or completely fill fractures in surrounding minerals. Shock-melted plagioclase also forms similar fracture-fillings, and are sometimes mixed with the sulfide/metal. So, the textures of carbonate disks and disseminations in ALH84001 are not consistent with deposition from a fluid, or with deformation of the host pyroxene before carbonate formation. These textures are, however, consistent with formation of the carbonates from shock melts.

The conceptual basis for considering the carbonates in ALH 84001 as impact melts (Scott et al., 1997) is clarified here: Similar structures formed from shock melts in other meteorites, especially in chondrites. Blebs of metal and sulfide minerals aligned on curving surfaces (healed fractures) in the chondrites are similar to the lenticular carbonates in ALH 84001. Sheets and masses of metal and sulfide filling fracture patterns in the chondrites are similar to the disseminated carbonates in ALH 84001(their Figure 5). However, different processes can yield similar textures, and the matter remains unsettled.

The most important observation here is that the carbonate disks are not simple pancakes in open fractures -- they they are lenticular (i.e., thicker in the center than the edges). As the authors explain, it is difficult to explain the lenticular disks as deposits from water-rich solutions.

To me, lenticular carbonate disks are not a problem -- Earth weathering of the Tatahouine meteorite made some in only a half-century (Barrat et al., 1998). But the lenticular shape and the chemical zoning pattern (Fig. 1) spell trouble. First, imagine that the carbonates formed by filling open lens-shaped spaces. Why would the centers of the carbonates always be at the center of the lenses?

 

Wouldn't growth tend to start, and zoning tend to be centered, at the edge of the lens (Fig. 2)? Second, imagine that the carbonates formed by replacement of feldspathic glass or feldspar, which just happened to be present in lens shapes. Again, wouldn't zoning tend to be centered at the edge of the lens (Fig. 2)? Third, imagine that the carbonates formed by dissolution of the pyroxene coupled to precipitation of carbonate (Barrat et al., 1998)? Wouldn't one expect chemical zoning to follow the outline of the lens, as in Figure 3?

But I think that formation from carbonate melt does not explain the zoning pattern of the lens-shaped carbonates, nor the details of their chemical zoning. Imagine, as the authors suggest, that the carbonate lenses originate as droplets of carbonate melt along fractures. Melts crystallize first where they are coolest (in general), and melts cool from the outside in (because heat of crystallization is so much larger than the specific heat of the surrounding pyroxene). So, one could expect carbonates to crystallizing with zoning patterns like that in Figure 2. Second, the chemical zoning in the carbonate disks is not as expected from crystallization of a carbonate melt. Starting with a melt of the average composition of the disks, the crystallization sequence should be magnesite, ferroan magnesite, and ferroan dolomite (=ankerite). This is nearly what is observed in the ALH 84001 disks, except in reverse order!!! Ferroan dolomite was first, followed by ferroan magnesite, followed by magnesite (crystals in the disks clearly grew from the inside outward, e.g., Treiman and Romanek, 1998). Other problems with Scott's model are outlined in Treiman (1998).

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.

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., Yamaguchi A., and Krot A. N. (1997) Petrological evidence for shock melting of carbonates in the martian meteorite ALH 84001. Nature 387, 377-379.

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 ALH 84001 revised: Multiple shock events. Meteorit. Planet. Sci. 33, 753-764.

Treiman A.H. and Romanek C.S. (1998) Chemical and stable isotopic disequilibrium in carbonate minerals of martian meteorite ALH 84001: Inconsistent with high formation temperature. Meteorit. Planet. Sci. 33, 737-742.


Wadhwa M. and Crozaz G. (1998) The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions. Meteorit. Planet. Sci. 33, 685-692.

ALH 84001 was originally an igneous rock, formed 4.5 billion years ago from a basalt magma. The source rock for this basalt had been melted previously, in accord with inferences from radioactive isotope measurements. This confirms again that Mars differentiated very early in its history, more than 4.5 billion years ago, into distinct crust and mantle. Minerals that crystallized late from the ALH 84001 magma, however, were modified by late igneous processes.

To help determine the chemical composition of the magma from which ALH 84001 formed, the authors analyzed trace-element abundances in AL H 84001's minerals with an ion microprobe. The magma composition is calculated from the mineral analysis using experimental data on how the elements partition between basalt magmas and minerals. Original igneous zoning of orthopyroxene in ALH 84001 is preserved in the abundances of many trace elements, like titanium, yttrium, zirconium, and the rare earths. The composition of the earliest parent magma can be calculated from the orthopyroxene with the lowest abundances of incompatible trace elements (those that prefer to be in magma compared to orthopyroxene). This parent magma is depleted in very incompatible elements (the light rare earths) compared to less-incompatible elements (the heavy rare earths). This depletion was inherited from the basalt's source in the martian mantle, which must have been melted to produce basalt before the melting that produced ALH 84001's magma.

In some places, however, this depletion in very incompatible elements is masked by a late magmatic process, infiltration metasomatism, which produced local enrichments in these elements. In infiltration metasomatism, the last dregs of a crystallizing magma percolate through pores and cracks in the nearly-solid rock. These last dregs are rich in very incompatible elements -- places where the dregs stop moving can become very enriched in the incompatible elements.

This story of multiple melting in the martian mantle confirms results from radioactive isotope studies -- very soon after it formed, Mars separated itself into core, mantle, and crust. The authors also find a hint from the distribution of europium (a rare earth element) that the early martian mantle was much more reducing (less oxygen) than it appears to be now (Warren and Kallemeyn, 1996).

This work is part of the larger effort to understand the martian meteorites and their implications for the origin and evolution of Mars. It has little bearing on the "life in ALH 84001" controversy.

For geochemists (everyone else can stop reading now), it is interesting that the rare earth elements in ALH 84001 are different from those in other martian meteorites. This makes it unlikely that ALH 84001 is closely related to the lherzolites (ALHA77005, Y793605, LEW88516), as some have suggested. The notion of infiltration metasomatism has drawn fire in the past (some from me), but it is a well-recognized and documented effect in plutonic rocks on Earth -- no reason why it should not also happen on Mars.

Citations:

Warren P. H. and Kallemeyn G. W. (1996) Siderophile trace elements in ALH 84001, other SNC meteorites and eucrites: Evidence of heterogeneity, possibly time-linked, in the mantle of Mars. Meteoritics Planet. Sci. 31, 97-105.


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.

Many magnetite grains in the ALH 84001 carbonates grew with their crystal structure in registry with the structure of their host carbonate crystals; this growth pattern is called epitaxy. Epitaxial growth of magnetite crystals, both elongate and compact, on carbonate is not consistent with the bacterial origin proposed by McKay et al. (1996) in which the magnetite crystals formed inside bacteria and were released after the bacteria died.

Carbonate globules in ALH 84001 contain small crystals of magnetite similar to those that are formed by some Earth bacteria (McKay et al., 1996). The authors have argued that these magnetites did (or could have) form hot, >500°C, and so are not related to life (Bradley et al., 1996, 1997). Here, they continue documenting features of the magnetites that may be inconsistent with a biological origin: morphology, especially those with elongate shapes; internal structures, especially screw dislocations; and alignments of crystal directions between them and their host carbonate minerals.

Most of the magnetite crystals in ALH 84001 have compact shapes (octahedra, teardrops, or parallelepipeds) that are similar to those of magnetites produced by some bacteria (McKay et al., 1996; Thomas-Keprta et al., 1998). However, identical magnetite crystals can grow inorganically at high temperature; an example here is from oxidation of iron (metal) vapor.

Bogard D. D. and Johnson D. H. (1998) Relative abundances of argon, krypton, and xenon in the Martian atmosphere as measured in Martian meteorites. Geochim. Cosmochim. Acta 62, 1829-1835.

Noble gas isotope abundances in martian basalt meteorites are consistent with mixing of gas from three sources: the martian atmosphere, the martian mantle, and the Earth's atmosphere (contamination). The noble gas composition of the martian atmosphere inferred from the meteorites is somewhat different from the in situ analyses performed by the Viking lander spacecraft. Noble gas abundances in ALH 84001 are consistent with mixes of martian atmosphere and martian mantle gas dissolved into cold water.

The authors analyzed fragments of martian basalts (EETA 79001, Shergotty, Y 793607, ALHA 77005) and of ALH 84001 for abundances of isotopes of the noble gas elements argon (Ar), krypton (Kr), and xenon (Xe). Details of the analyses will be published later -- here the authors give the isotope ratios 36Ar/ 132Xe, 84Kr/132Xe, and 129Xe/132Xe, which have been used to define the composition of the martian atmosphere. All but one analysis of noble gases in the martian basalts (new and literature) are consistent with mixtures of three components: the martian atmosphere, the martian mantle, and the Earth's atmosphere (contamination).

 

36Ar/132Xe

84Kr/132Xe

129Xe/132Xe

Earth Atmosphere

1350

27.8

0.98

Mars Mantle

15.1

1.2

1.03

Mars Atmosphere

900 ± 100

20.5 ± 2.5

2.60 ± 0.05

Viking Lander Analysis of Mars Atmosphere

~ 355

~ 11.5

2.5 ± 1

As in previous analyses, the martian atmosphere component is most abundant in shock glasses in the meteorites, and was probably trapped there when the meteorites were ejected from Mars, less than 3 million years ago.

The martian atmosphere component is similar to, but not identical to, the analyses of the martian atmosphere made on Mars by the Viking lander spacecraft in 1976. The martian atmosphere component defined here is a better approximation to the real noble gas composition of the martian atmosphere than are the Viking lander analyses.

Noble gas abundances in ALH 84001, as in previous analyses, are like those of the nakhlite martian meteorites and not simple mixtures of these three components. As noted earlier (Drake et al., 1994), noble gas abundances in these meteorites are like mixtures of martian mantle and martian atmosphere components that have then lost ~90% of their Ar and ~40% of their Kr (relative to Xe). This loss of light noble gases is consistent with their solubility in cold water, suggesting that the noble gases in ALH 84001 were deposited there by cold water.

The real point of this paper is to revise the noble gas ratios and abundances in the martian atmosphere. The authors say, in effect, that the meteorites in laboratories on Earth provide a better analysis of the martian atmosphere than the Viking lander instruments did in on Mars in 1976. Indirectly, this is a strong endorsement of Mars Sample Return!

It is odd (in a way) that the meteorites now define the martian atmosphere, because the Viking analyses (now superseded) are what Dr. Bogard and colleagues used in 1983 to "prove" that the meteorites were from Mars! However, this paper cannot be construed as proof that the martian meteorites are not from Mars! The revisions to the isotopic composition of the martian atmosphere, while impressive in the table above, still show the same traits that distinguish martian atmosphere from all other known gases in the solar system: high 36Ar/132Xe, 84Kr/132Xe, and 129Xe/132Xe! It seems most likely that the Viking analyses were a little less accurate than advertised, and not that the martian atmosphere has changed much over the last 3 million years.

As for ALH 84001, this paper changes little in the ongoing debate. It has been known that that the noble gas isotope composition of ALH 84001 is comparable to those in the nakhlites and of shergottite noble gases dissolved in water (Swindle et al., 1995). Unfortunately for those who would take this fact as evidence that the ALH 84001 carbonates were deposited by liquid water, most of the xenon in ALH 84001 is in its pyroxene (Gilmour et al., 1997).

Citations:

Drake M.J., Owen T., Swindle T.D., and Musselwhite D. (1994) Fractionated martian atmosphere in the nakhlites? Meteoritics 29, 854-859.

Gilmour J.D., Lyon I.C., Saxton J.M., Turner G., and Whitby J.A. (1997) Oxygen and noble gas isotope constraints on the origin of ALH 84001 carbonate (abstract). Lunar Planet. Sci. XXVIII, 421-422.

Swindle T.D., Grier J.A., and Burkland M.D. (1995) Noble gases in orthopyroxenite ALH84001: A different kind of martian meteorite with an atmospheric signature. Geochim. Cosmochim. Acta 59, 793-801.


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.

Theories that the carbonate minerals in ALH 84001 formed at high temperature all have serious flaws. In their stead, Warren proposes that the carbonates form ed during evaporation of surface water or groundwater on Mars. Carbonate mineral s (as in the meteorite) would have formed early -- sulfate would remain in solut ion and be carried elsewhere. These hypotheses permit, but do not require, biolo gical action.

Warren's paper explains some serious problems in current theories about the carbonate minerals in ALH 84001, and presents new theories involving carbonate formation at low temperature from briny water. First, the problems.

Theory 1. Carbonates formed by chemical reaction between the rock and carbon-dioxide-rich vapors during an impact event, as at the Haughton Crater, Canada (Harvey and McSween, 1996). Carbonates in the Haughton rocks are not similar at all to those in ALH 84001. In the Haughton rocks, carbonate grains are small and abundant, and the host rocks have abundant void spaces (where vapor was); neither is true for ALH 84001. In addition, formation of ALH 84001 carbonates from olivine or pyroxene in the host rock should produce nearly as much silica (SiO2) as carbonate, but ALH 84001 has much less silica than carbonate.

Theory 2. The carbonates crystallized from carbonate-rich magma, which was formed and injected into cracks during a single impact event (Scott et al., 1997). The heat of melting carbonates should have produced lots of vapor, and ALH 84001 has little void space where the vapors would have been. Carbonate magmas forced into cracks ought to form continuous veinlets (dikelets) of carbonate minerals, not the isolated carbonate pancakes of ALH 84001. The round shapes of the carbonate pancakes are ascribed to the surface tension of the carbonate magma; however, this same surface tension would prevent formation of the abundant "patchy" or "lacy" carbonates in ALH 84001.

Theory 3. The carbonates were deposited from hot water (hydrothermal) solutions. Warren notes (as have others) that hot water coursing through ALH 84001, for the duration of a typical hydrothermal system, ought to produce abundant water-bearing silicate minerals (like clays, talc, and serpentine) from its pyroxene. ALH 84001 contains essentially no water-bearing silicate minerals.

As alternative mechanisms, Warren suggests that the carbonates in ALH 84001 were deposited at "room temperature" or colder from saline (briny) waters, either by the evaporation of waters deposited by floods, or by evaporation of groundwater. In the first idea, the flood water would have ponded to form a salt lake, playa, or sabkha. The flood waters would have been rich in carbonate, magnesia, and alkalis, and would have saturated the soil (regolith, for purists) beneath the ponds. As the water evaporated, it would percolate into the soil and deposit its dissolved carbonate in globules. Early growth of Ca-bearing carbonate minerals (like ankerite) could have consumed so much Ca that common Ca-sulfates (gypsum, andhyrite) could not form. On the other hand, crusts of carbonate or other minerals could have formed on the lakes and kept them from evaporating to the point where calcium sulfate minerals (which are not seen in ALH 84001) would grow.

The second setting is a carbonate-rich soil (or regolith) initiated by a sudden influx of groundwater, an underground "flood." As the groundwater flowed downhill and evaporated upward through the overlying soil, its dissolved minerals would be deposited into the soil and its rocks. Near the water source, uphill, carbonate minerals such as in ALH 84001 would be deposited. Sulfate minerals would only be deposited farther downhill as the water became more concentrated.

Both models explain the abundance of sulfate in the martian soil as the last minerals formed by evaporation, and explain why carbonate minerals are not detectable (so far) at the martian surface (they are underground). Evaporation increases the d18O value of oxygen in the remaining water, which might explain the oxygen isotope zoning in the ALH 84001 carbonates (cores with low d18O, rims with high d18O; vis. Leshin et al., 1998). The compositional zoning of the carbonates might be explained by changing water composition evaporation, or by influxes of new waters. Neither model requires, nor forbids, biological activity.

Warren's critiques of earlier models of carbonate formation in ALH 84001 deserve serious attention. His critique of "hydrothermal" scenarios is especially strong -- all have known that hydrothermal deposition ought also to produce abundant water-bearing silicates -- and Warren has begun quantifying the rates of alteration and the maximum duration of such a hydrothermal system.

This is the first of many current and upcoming contributions suggesting that the carbonate minerals in ALH 84001 formed during evaporation of briny water (McSween and Harvey, 1998; Scot and Krot, 1998). My feelings are mixed. I think I first suggested that the ALH 84001 carbonates formed from briny water at low temperature (Treiman, 1995). But, the closest known analogs to the ALH 84001 carbonates formed in volcanos on Spitsbergen Island within the last 25,000 years (Treiman et al., 1998), an unlikely place to find evaporitic environments!

Warrens' models place carbonate formation in ALH 84001 nicely within the current concepts of Mars geology and chemistry. However, he gives no examples where carbonates like those in ALH 84001 formed on Earth beneath evaporite lakes, playas, sabkhas, or calcite soils. Mg-Fe carbonates (magnesite-siderite) are abundant in ALH 84001 but are quite rare in these environments -- Ca carbonates (calcite and aragonite) are most abundant, and Ca-Mg-carbonate (dolomite) is much less common. Nor have I heard of evaporitic carbonate deposits having the shapes nor zoning of those in ALH 84001. The closest analogy I know of are the calcite disks, formed on Earth, in cracks in the Tatahouine meteorite (Barrat et al., 1998). However, these formed at the ground surface and not in the subsurface environments Warren hypothesizes.

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.

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.

McSween H.Y. Jr. and Harvey R.P. (1998) Brine evaporation: An alternative model for the formation of carbonates in ALH84001 (abstract). Meteor. Planet. Sci. 33, A103.

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. and Krot A. (1998) Formation of pre-impact interstitial carbonates in the ALH84001 martian meteorite (abstract). Meteor. Planet. Sci. 33, A139-A141.

Treiman A.H. (1995) A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age. Meteoritics 30, 294-302.

Treiman A.H., Ionov D.A., Amundsen H.E.F., Bunch T., and Blake D.F. (1998) A terrestrial analog for carbonates in ALH 84001: Ankerite-magnesite carbonates in mantle xenoliths and basalts from Spitsbergen (Svalbard), Norway (abstract). In Lunar Planet. Sci. XXIX, Abstract #1630, Lunar and Planetary Institute, Houston (CD-ROM).


A few magnetites are ~5 times as long as wide (Bradley et al., 1996), about 75 nm by 15 nm on average [1 nm = 1 billionth of a meter = 1 thousandth of a micrometer (or micron)]. Elongate magnetites like these are unusual, and are best known in deposits from hot vapors, T > ~ 500°C; the authors show examples from volcanic gas vents and laboratory experiments. However, elongate magnetites can be made by some bacteria (Thomas-Keprta et al., 1998).

Some elongate magnetites in the ALH 84001 carbonates contain structure defects called screw dislocations. These defects are common in magnetites grown from hot vapor (Bradley et al., 1996), but have not been seen in magnetite formed at lower temperature or by bacteria. Screw dislocations themselves are not proof of high temperature, as other kinds of crystals grown at low temperatures contain them. Now, however, there is no evidence of screw dislocations in biogenic magnetites.

Most important, the authors show that some magnetite crystals grew in orientations controlled by the crystal structures of their host carbonate crystals or by other magnetite crystals (viz. Bradley et al., 1996; Blake et al., 1998; Brearley, 1998). This relationship is called epitaxy. Epitaxial magnetite in ALH 84001 grew on the carbonate in two types of orientations. Some magnetite crystals grew epitaxially on others, in one case into a stack or chain of four crystals. Epitaxial growth of magnetite is only at reported at T > 300°C.

As for martian biology, all features of the ALH 84001 magnetites could have formed readily by vapor-phase growth at high temperatures: compact and elongate crystals, crystals with screw dislocations, epitaxial orientations on the host carbonates. Elongate magnetites with screw dislocations are not known, at this time, from bacteria. Further, it seems unlikely that loose magnetite crystals (broken from dead bacteria) would become oriented with the crystal structures of the host carbonates. Similarly, the aligned "nanofossils" on carbonate (Kerr, 1997) are likely inorganic magnetite crystals aligned epitaxially on the host carbonate grains.

This careful paper covers a lot of ground, and delves deeply into materials science and crystallography. The images are beautiful, and clearly demonstrate the similar orientations of the epitaxial magnetite grains in the ALH 84001 carbonates. The epitaxial relationships here are a strong challenge to the McKay et al. hypothesis.

This paper exemplifies two trends in the debate about ALH 84001. First is the start of a "rapprochement" between the biological sciences (low-temperature) and materials sciences (high-temperature). Before this, the problems raised by ALH 84001 had not been important in either field (e.g., "Would it matter if elongate magnetites in bacteria have screw dislocations?" or "Who would want to grow elongate magnetites at low temperature?"). Second is recognition that the McKay hypothesis will not be proved or disproved rapidly. Bradley et al. use language such as " strong possibility," "consistent with," and "problematic" rather than the language of conquest or absolute refutation. ALH 84001 is maddeningly difficult to work with -- it seems to provide few unambiguous clues (or are we blind to them?). On the other hand, the hypothesis of McKay et al. (1996) is so multifaceted and ambiguous that, Hydra-like, it is hard to slay.

Finally, a few technical comments. (1) Epitaxial growth of magnetite on carbonates may occur at temperatures below 300°C. Holser and Schneer (1961) show photos of magnetite grown on calcite in an apparent epitaxial relationship at 320°C, and may have grown comparable magnetites at 200°C. The lack of evidence for low-temperature epitaxy may merely represent a lack of data and interest (before now). (2) The mineral name "chalybite" is not accepted by the International Mineralogical Association (IMA); siderite is the proper name for natural FeCO3. (3) When giving Miller indexes for planes or directions in siderite (or calcite or dolomite) it is helpful to say whether the indexes are for rhombohedral or hexagonal coordinates. For instance, the cleavage plane directions in the ALH 84001 carbonates is {1 0 0} in rhombohedral, and {1 0 1} or {1 0 -1&nb sp;1} in hexagonal.

Citations:

Blake D. F., Treiman A. H., Cady S., Nelson C., and Krishnan K. (1998) Characterization of magnetite within carbonate in ALH84001 (abstract). In Lunar Planet. Sci. XXIX, Abstract #1347, Lunar and Planetary Institute, Houston (CD-ROM).

Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1996) Magnetite whiskers and platelets in ALH 84001 Martian meteorite: Evidence of vapor phase growth. Geochim. Cosmochim. Acta 60, 5149-5155.

Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1997) No 'nanofossils' in martian meteorite. Nature 390, 454-455.

Brearley A. J. (1998) Magnetite in ALH 84001: Product of the decomposition of ferroan carbonate (abstract). In Lunar Planet. Sci. XXIX, Abstract #1757, Lunar and Planetary Institute, Houston (CD-ROM).

Holser W. T. and Schneer C. J. (1961) Hydrothermal magnetite. Geol. Soc. Amer. Bull. 72, 369-386.

Kerr R. R. (1997) Martian "microbes" cover their tracks. Science 276, 30-31.

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.

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

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). In Lunar Planet. Sci. XXIX, Abstract #1494, Lunar and Planetary Institute, Houston (CD-ROM).


Sears D. W. G. and Kral T. A. (1998) Martian "microfossils" in lunar meteorites? Meteorit. Planet. Sci. 33, 791-794.

The authors found bacteria-shaped objects in ALH 84001, just as McKay et al. (1996) did. They found similar or identical objects in Antarctic meteorites that formed originally on Moon. Because the Moon is devoid of life, the bacteria-shaped objects in the Moon meteorites must have formed in Antarctica. It seems reasonable, then, that the bacteria-shaped objects in ALH 84001 also formed in Antarctica.

The authors examined lunar meteorites from Antarctica for bacteria-shaped objects (BSOs). The only history these meteorites share with ALH 84001 is their residence on Earth in Antarctica. So, if similar bacterial or organic signatures are found in both, the signatures probably represent their Antarctic experiences, and not common evolutions on such different bodies as the Moon and Mars.

The authors examined broken fragments of ALH 84001 and four lunar meteorites from Antarctica (ALH 81005, MAC 88104, MAC 88105, and QUE 93069). They used the same procedures as McKay et al. (1996), even using the same Au-Pd sputterer to coat their samples and using the same scanning electron microscopes.

Images of ALH 84001 fracture surfaces showed the same kinds of BSOs as described by McKay et al. (1996): elongate forms ~100 nm long and ~ 20 nm diameter; and spherical forms ~ 20 nm in diameter. BSOs in ALH 84001 were essentially identical to those in the lunar meteorites. The ALH 84001 sample contained many more elongate BSOs than spherical, while the lunar meteorites had fewer elongate than spherical. Some of the elongate BSOs appeared to be growing out of crystals, or breaking of edges of crystals, similar to the features found by Bradley et al. (1997).

BSOs in lunar meteorites are not related to extraterrestrial biology, as the Moon is assumed to be sterile. The presence of these BSOs in both martian and lunar meteorites suggests that BSOs are acquired during residence in Antarctica, the only environment (besides deep space) common to both types of meteorites. The actual identity of the BSOs is not known. They could be biota from Earth, minerals grown in the Antarctic environment, or coating artifacts.

This modest paper is the first confirmation by another group that bacteria-shaped objects (BSOs), accepted as such by D. McKay, are present in ALH 84001. The BSOs described by Bradley et al. (1997) were not accepted by McKay et al. (1997) as the same type of BSOs that were identified as possible martian microfossils (McKay et al., 1996).

The author's finding of similar BSOs in lunar and martian meteorites seems to suggest that the BSOs in both postdate their fall to Earth -- they are not fossilized martian bacteria. However, their conclusions (and those of McKay et al., 1996) remain suspect until we know what the BSOs, accepted BSOs, really are. What is their chemical composition? What minerals are they made of? What is their internal structure? What are their stable isotope ratios, especially of oxygen and carbon? This kind of data should have been available from the beginning, and any discussion of BSOs and "life in ALH 84001" is dubious without them!

Citations:

Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1997) No 'nanofossils' in martian meteorite. Nature 390, 454-455.

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 Va li H. (1997) No 'nanofossils' in martian meteorite: reply. Nature 390, 455-456.


Treiman A. H. (1998) The history of ALH 84001 revised: Multiple shock events. Meteorit. Planet. Sci. 33, 753-764.

ALH 84001 has experienced a complex geologic history, including at least five asteroid impact events, at least two of which were after deposition of the carbonate globules (hosts to putative signs of life). One impact after carbonate deposition left the rock hot enough to melt the feldspar glass, move fragments of carbonate globules in that melt, and produce bubbles of gas. Any possible signs of martian life in the carbonates experienced this shock history, and have been affected/degraded by it. The postcarbonate shock events may have produced chemical or mineralogical effects, which must not be confused with those related to original formation of the carbonates.

Observations on ALH 84001 concerning possible ancient martian life must be understood in the context of the surrounding rock and the geological events it has experienced. The author has constructed a chronology, a history, of ALH 84001 that encompasses nearly all petrographic and chronological data about it. It involves four or five impact events, approximately twice as many deformations, and four chemical events.

ALH 84001 began as an accumulation of pyroxene crystals in a body of basaltic magma in Mars. The chemical composition of the magma is not yet known. The compositions of its minerals homogenized on cooling.

Next, the parent rock of ALH 84001 was near "ground zero" for an asteroid impact, which caused severe deformation and heating (the granular bands). From the style of deformation, it seems likely that ALH 84001 was in the basement rock beneath the impact crater, possibly even part of its central uplift. After it cooled, the rock was again shocked in an asteroid impact, which converted any feldspar present to feldspar glass and may have produced large gas bubbles.

Then came deposition of the carbonate globules. The author argues (as in Treiman, 1995) that the carbonate grains replace feldspar-composition glass in the meteorite, and explains the lacy and framework carbonate textures (McKay et al., 1997) as replacement of feldspar glass among fragments of crystalline feldspar. Treiman disagrees strongly with the "one impact causes all" theory of Scott et al. (1997).

Following deposition of the carbonates, the parent rock of ALH 84001 was struck by another asteroid, which caused the feldspathic glass to become molten, and then flowed into cracks in the rock, carrying fragments of carbonate with it. Gas bubbles were produced in this impact, which might have allowed vapor-deposition of magnetites and vapor mobility of alkali elements and chromium.

Finally, ALH 84001 was involved in one or two more impacts, which launched it off of Mars and produced the varying orientations of the meteorite's trapped magnetic field. ALH 84001 fell to Earth, in Antarctica, about 15,000 years ago.

Any signs of ancient martian life in the ALH 84001 carbonates experienced at least two impact events, one of which produced enough heat to melt feldspar and produce vapor. Vapor-deposited magnetite crystals could have formed in this event, after carbonate deposition (Bradley et al., 1998). Also, the paleomagnetic signatures used to infer a low temperature for carbonate deposition (Kirschvink et al., 1997) must also postdate this impact and thus postdate carbonate formation. Martian organic matter deposited with the carbonates would certainly have been modified in this impact heating event.

Finally, the author again emphasizes that ALH 84001 contains essentially no water and no martian water-bearing minerals. "If Mars were actually wet in its distant past, how can a bone-dry rock such as ALH 84001 be possible?"

This work builds on the earlier work of Treiman (1995), trying to extract a coherent geological history from the mess of ALH 84001. The reader will want to look at Table 1 (a summary of his hypothesis) and the photographs, and avoid the ponderous text entirely. The author found it necessary to devote a section to nomenclature and another (much longer) to an enumerated list of observations and "facts." Only in the narrative history is the text passably entertaining; perhaps the author should consider writing science fantasy rather than fact.

Although the theory purports to encompass all "verified and credible observations," the choice of observations obviously dictates the results. So, it is no surprise that this work is basically in agreement with Treiman (1995). Scott et al.'s (1997) theory is singled out for debunking, and its observations are given short shrift.

The complexity of Treiman's theory is its greatest weakness, even though a martian highland rock might reasonably have experienced so many impact and chemical events. Nearly any observation about ALH 84001 can be easily blamed on one event or another in Treiman's scenario. How can one evaluate or disprove a theory this floppy? If it cannot be disproved, is it science? If it is not science, can it really advance our understanding of ALH 84001?

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.

Kirschvink J. L., Maine A. T., and Vali H. (1997) Paleomagnetic evi dence of a low-temperature origin of carbonates in the martian meteorite ALH 84001. Science 275, 1629-1633.

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., Yamaguchi A., and Krot A.N. (1997) Petrological evidence for shock melting of carbonates in the martian meteorite ALH 84001. Nature 387, 377-379.

Treiman A.H. (1995) A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age. Meteoritics 30, 294-302.


Treiman A. H. and Romanek C. S. (1998) Bulk and stable isotopic compositions of carbonate minerals in Martian meteorite ALH 84001: No proof of high formation temperature. Meteorit. Planet. Sci. 33, 737-742.

Several arguments for a high-temperature origin of the ALH 84001 carbonates, >650°C, rely on chemical or isotopic equilibria (or quasi-equilibria) among the carbonates or between carbonates and other minerals (Mittlefehldt, 1994; Harvey and McSween, 1996). Here, the authors argue that many of these arguments for high temperatures are incorrect, and that the chemical and isotopic compositions of the carbonates are consistent with formation at low temperature. The authors conclude that, at this time, the chemical and isotopic compositions of the ALH 84001 carbonates are not strong support for a high-temperature origin.

Here, the authors question the validity or significance of several early arguments that the carbonate masses in ALH 84001 formed at high temperatures, >650°C. Mittlefehldt (1994) and Harvey and McSween (1996) saw that the chemical compositions of some carbonates were in a range that might be expected for formation T ~700°C. Harvey and McSween also cited these arguments in support of a high formation temperature: calcite-dolomite thermometry; presence of coexisting calcite, dolomite, and magnesite; and isotope ratios of carbon and oxygen in the carbonates. Treiman and Romanek call each of these arguments into doubt.

The compositions of the Ca-poor carbonates, magnesites, in ALH 84001 have abundances of Ca, Mg, and Fe that appear to follow the position of the magnesite-dolomite unmixing curve for 700°C (Mittlefehldt, 1994; Harvey and McSween, 1996). However, that curve is not applicable unless the magnesite has equilibrated with dolomite, i.e., both minerals are present and have the proper Fe/Mg ratios. However, most of the magnesite in ALH 84001 is not associated with dolomite, and so the unmixing curve is not relevant. The relevant curve here is the spinode, which shows that the ALH 84001 magnesites could have formed at any temperature above 0°C.

The calcite-dolomite unmixing thermometer is not useful for ALH 84001, because the compositions of the calcite (rare as it is) suggest temperatures ranging from 0°C to 700°C. The presence of calcite, dolomite, and siderite suggested a temperature above 550°C. However, the Fe/Mg ratios for these minerals in ALH 84001 are not consistent with chemical equilibria, implying that the proposed temperature is invalid. It seems likely that the carbonates in ALH 84001 were never in chemical equilibria among themselves, and so these thermometers (based on equilibria) are not applicable.

Carbon isotopic ratios in the carbonates are not useful for thermometry because no one knows the carbon isotopic composition of the martian atmosphere. The atmosphere ratio is so poorly known that implied equilibrium temperatures are limited only to being between 0°C and >700°C. Oxygen isotopic compositions are not useful, as the ALH 84001 carbonates were never in oxygen isotopic equilibrium with the surrounding silicates (Farquhar et al., 1998).

This paper is primarily a critical comment on Harvey and McSween (1996), but Nature did not find it fit to print -- hence we are grateful to Meteoritics and Planetary Science. It hardly needs saying that Harvey and McSween do not agree with our results.

Figure 1 is nice. It shows that the carbonates grew from the globule cores outward, and shows sector zoning of their iron and magnesium abundances.

Citations:

Farquhar J., Thiemans M. H., and Jackson T. (1998) Atmosphere-surface interactions on Mars: D17O measurements of carbonate in ALH 84001. Science 280, 1580-1582.

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.

Mittlefehldt D. W. (1994) ALH84001, a cumulate orthopyroxenite member of the SNC meteorite group. Meteoritics 29, 214-221.

Scott E. R. D. and Krot A. N. (1998) Carbonates in martian met eorite ALH84001: petrologic evidence for an impact origin (abstract). In Lunar Planet. Sci. XXIX, Abstract #1786, Lunar and Planetary Institute, Houston (CD-ROM).


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.

Theories that the carbonate minerals in ALH 84001 formed at high temperature all have serious flaws. In their stead, Warren proposes that the carbonates form ed during evaporation of surface water or groundwater on Mars. Carbonate mineral s (as in the meteorite) would have formed early -- sulfate would remain in solut ion and be carried elsewhere. These hypotheses permit, but do not require, biolo gical action.

Warren's paper explains some serious problems in current theories about the carbonate minerals in ALH 84001, and presents new theories involving carbonate formation at low temperature from briny water. First, the problems.

Theory 1. Carbonates formed by chemical reaction between the rock and carbon-dioxide-rich vapors during an impact event, as at the Haughton Crater, Canada (Harvey and McSween, 1996). Carbonates in the Haughton rocks are not similar at all to those in ALH 84001. In the Haughton rocks, carbonate grains are small and abundant, and the host rocks have abundant void spaces (where vapor was); neither is true for ALH 84001. In addition, formation of ALH 84001 carbonates from olivine or pyroxene in the host rock should produce nearly as much silica (SiO2) as carbonate, but ALH 84001 has much less silica than carbonate.

Theory 2. The carbonates crystallized from carbonate-rich magma, which was formed and injected into cracks during a single impact event (Scott et al., 1997). The heat of melting carbonates should have produced lots of vapor, and ALH 84001 has little void space where the vapors would have been. Carbonate magmas forced into cracks ought to form continuous veinlets (dikelets) of carbonate minerals, not the isolated carbonate pancakes of ALH 84001. The round shapes of the carbonate pancakes are ascribed to the surface tension of the carbonate magma; however, this same surface tension would prevent formation of the abundant "patchy" or "lacy" carbonates in ALH 84001.

Theory 3. The carbonates were deposited from hot water (hydrothermal) solutions. Warren notes (as have others) that hot water coursing through ALH 84001, for the duration of a typical hydrothermal system, ought to produce abundant water-bearing silicate minerals (like clays, talc, and serpentine) from its pyroxene. ALH 84001 contains essentially no water-bearing silicate minerals.

As alternative mechanisms, Warren suggests that the carbonates in ALH 84001 were deposited at "room temperature" or colder from saline (briny) waters, either by the evaporation of waters deposited by floods, or by evaporation of groundwater. In the first idea, the flood water would have ponded to form a salt lake, playa, or sabkha. The flood waters would have been rich in carbonate, magnesia, and alkalis, and would have saturated the soil (regolith, for purists) beneath the ponds. As the water evaporated, it would percolate into the soil and deposit its dissolved carbonate in globules. Early growth of Ca-bearing carbonate minerals (like ankerite) could have consumed so much Ca that common Ca-sulfates (gypsum, andhyrite) could not form. On the other hand, crusts of carbonate or other minerals could have formed on the lakes and kept them from evaporating to the point where calcium sulfate minerals (which are not seen in ALH 84001) would grow.

The second setting is a carbonate-rich soil (or regolith) initiated by a sudden influx of groundwater, an underground "flood." As the groundwater flowed downhill and evaporated upward through the overlying soil, its dissolved minerals would be deposited into the soil and its rocks. Near the water source, uphill, carbonate minerals such as in ALH 84001 would be deposited. Sulfate minerals would only be deposited farther downhill as the water became more concentrated.

Both models explain the abundance of sulfate in the martian soil as the last minerals formed by evaporation, and explain why carbonate minerals are not detectable (so far) at the martian surface (they are underground). Evaporation increases the d18O value of oxygen in the remaining water, which might explain the oxygen isotope zoning in the ALH 84001 carbonates (cores with low d18O, rims with high d18O; vis. Leshin et al., 1998). The compositional zoning of the carbonates might be explained by changing water composition evaporation, or by influxes of new waters. Neither model requires, nor forbids, biological activity.

Warren's critiques of earlier models of carbonate formation in ALH 84001 deserve serious attention. His critique of "hydrothermal" scenarios is especially strong -- all have known that hydrothermal deposition ought also to produce abundant water-bearing silicates -- and Warren has begun quantifying the rates of alteration and the maximum duration of such a hydrothermal system.

This is the first of many current and upcoming contributions suggesting that the carbonate minerals in ALH 84001 formed during evaporation of briny water (McSween and Harvey, 1998; Scot and Krot, 1998). My feelings are mixed. I think I first suggested that the ALH 84001 carbonates formed from briny water at low temperature (Treiman, 1995). But, the closest known analogs to the ALH 84001 carbonates formed in volcanos on Spitsbergen Island within the last 25,000 years (Treiman et al., 1998), an unlikely place to find evaporitic environments!

Warrens' models place carbonate formation in ALH 84001 nicely within the current concepts of Mars geology and chemistry. However, he gives no examples where carbonates like those in ALH 84001 formed on Earth beneath evaporite lakes, playas, sabkhas, or calcite soils. Mg-Fe carbonates (magnesite-siderite) are abundant in ALH 84001 but are quite rare in these environments -- Ca carbonates (calcite and aragonite) are most abundant, and Ca-Mg-carbonate (dolomite) is much less common. Nor have I heard of evaporitic carbonate deposits having the shapes nor zoning of those in ALH 84001. The closest analogy I know of are the calcite disks, formed on Earth, in cracks in the Tatahouine meteorite (Barrat et al., 1998). However, these formed at the ground surface and not in the subsurface environments Warren hypothesizes.

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.

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.

McSween H.Y. Jr. and Harvey R.P. (1998) Brine evaporation: An alternative model for the formation of carbonates in ALH84001 (abstract). Meteor. Planet. Sci. 33, A103.

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. and Krot A. (1998) Formation of pre-impact interstitial carbonates in the ALH84001 martian meteorite (abstract). Meteor. Planet. Sci. 33, A139-A141.

Treiman A.H. (1995) A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age. Meteoritics 30, 294-302.

Treiman A.H., Ionov D.A., Amundsen H.E.F., Bunch T., and Blake D.F. (1998) A terrestrial analog for carbonates in ALH 84001: Ankerite-magnesite carbonates in mantle xenoliths and basalts from Spitsbergen (Svalbard), Norway (abstract). In Lunar Planet. Sci. XXIX, Abstract #1630, Lunar and Planetary Institute, Houston (CD-ROM).


Kring D. A., Swindle T. D., Gleason J. D., and Grier J.  A. (1998) Formation and relative ages of maskelynite and carbonate in the martian meteorite ALH 84001. Geochim. Cosmochim. Acta 62, 2155-2166.

The carbonate mineral globules in ALH 84001 formed by chemical replacement (not by filling globular void spaces, nor from carbonate magmas). The material replaced by the carbonates was feldspar-composition glass, not crystalline feldspar as previously suggested. This feldspar glass formed from crystalline feldspar in the impact shock event that produced the fine-grained crush zones in ALH 84001. The fluid responsible for carbonate deposition was relatively cool (<300°C), rich in carbon dioxide, and poor in water. The absolute age of the carbonate globules remains uncertain, as does the influence of living organisms in their formation.

In order to understand formation of the carbonate minerals in ALH 84001 (hosts to the possible evidence of ancient martian life), Kring and co-workers examined the relationships between carbonate grains and their surrounding minerals (their textures), and examined potassium-argon age dating of the ALH 84001 carbonates in light of new analyses for potassium in the carbonates.

Most of the carbonate globules in ALH 84001 formed by replacing feldspathic material -- what is now the feldspar-composition glass called maskelynite (vis. Treiman, 1995). In some places, carbonate mineral aggregates cross-cut the glass, implying that the carbonates are younger than the glass (or its precursor feldspar). In the fine-grained areas of ALH 84001 (the granular bands or crush zones), Kring and co-workers see that carbonate minerals and feldspar glass have the same kinds of shapes, suggesting that the carbonate replaced the glass (or precursor feldspar). Replacement of feldspar and feldspar glass by carbonate minerals is fairly common in Earth rocks.

The shapes of the carbonate globules suggest that they did not replace crystalline feldspars (with rare exceptions) but feldspar glass. When carbonate replaces crystalline feldspar (on Earth), it grows along parallel cracks (cleavages and twin planes) in the feldspar. This texture is present but very rare in ALH 84001 (Treiman, 1998). Most of the carbonate is as hemispherical globules, suggesting that it replaced a non-crystalline material -- a feldspar glass.

Kring and co-workers infer that this feldspar glass (now partly replaced by carbonate) formed in the same event that produced the rock's granular bands. The silicate and minerals in the rock were already chemically homogenized during early, post-igneous cooling. After deposition of the carbonate globules, ALH 84001 was subjected to another shock event that turned any remaining feldspars to glass and moved the feldspar glasses around.

It is still not clear when the carbonates were deposited. Carbonates in ALH 84001 contain less than 80 ppm of potassium (K), and only a few K-Ar age dates (actually Ar-Ar dates) for carbonates contain so little potassium (the rest are contaminated with feldspar glass). Those few ages are too imprecise to be useful (e.g., 2+2 billion years).

Replacement of feldspar glass by carbonates in ALH 84001 (dissolution of feldspar glass and precipitation of carbonates) must have been caused by reaction with fluids moving through the rock. Based on published experiments, the fluid was cooler than 300°C and was active for a short time, perhaps only a few years. The absence of hydrous silicate minerals suggests that the altering fluids were "carbonic," rich in carbon dioxide and poor in water. This mechanism does not support or refute biogenic activity, though the authors favor formation without action of living organisms.

The thrust of this paper is twofold: how the carbonate globules formed, and the geological history of the meteorite. The hypothesis of carbonate formation here is similar to parts of many others, including: a fluid rich in carbon dioxide (Harvey and McSween, 1996; Leshin et al., 1998); geologically low temperature (Treiman, 1995; Valley et al., 1997; Leshin et al., 1998; Warren, 1998); a short duration (Harvey and McSween, 1996; Scott et al., 1997; Warren, 1998); and a chemical link to dissolution of silicates (Treiman, 1995). However, each of the other theories has parts that disagree with Kring's, and a reconciliation still seems far off. If anything, there appears to be a growing consensus that the carbonates did form at relatively low temperatures, less than 300°C. This is still awfully hot for Earth-type life, which is not known to survive above about 120°C.

In the geological history of ALH 84001, Kring and co-workers disagree with Treiman's (1995) claim that the carbonates replaced crystalline feldspar. Rather, the textures suggest that the carbonates replaced feldspar glass. I agree with this part of Kring's work, and I've incorporated it into the revised history in Treiman (1998).

It's unfortunate that Kring was unable to derive an absolute age for carbonate formation. But the fact that ALH 84001 was shocked by asteroid impact events after the carbonates formed suggests that they date to the time of Mars' heavy asteroid bombardment, the Noachian epoch. This means that the carbonates would have been deposited more than 3.5 billion years ago, according to current estimates of Mars' chronology.

Citations:

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.

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.

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 ALH 84001 revised: Multiple shock events. Meteor. Planet. Sci. 33, in press.

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.


Farquhar J., Thiemens M. H., and Jackson T. (1998) Atmosphere-surface interactions on Mars: D17 O measurements of carbonate from ALH 84001. Science 280, 1580-1582.

Farquhar and colleagues found that the relative abundances of the three oxygen isotopes (16O, 17O, and 18O) in the carbonates of ALH 84001 are significantly different from those in its silicate minerals. The difference in oxygen isotope abundances, especially 17O, means oxygen in the carbonates must have come from a different source (or "reservoir") than oxygen in the other minerals. The authors suggest that the carbonate oxygen, rich in 17O, was produced in the martian atmosphere by light-induced chemical reactions between ozone and carbon dioxide. This high-17O oxygen would enter the carbonate minerals through the fluids that deposited them. The left-over low-17O gas might be stored in oxidized minerals at the martian surface.

The authors use oxygen isotope abundances as tracers for the origins of the carbonate minerals in ALH 84001. O atoms come in three varieties, stable isotopes, with masses of 16, 17, and 18: 16O, 17O, and 18 O. The relative abundances of these isotopes can be altered by physical and chemical reactions, usually in direct proportion to differences in mass; the abundance ratio 18O/16O will change twice as much as the ratio 17O/16O. These mass-dependent processes cannot produce every possible set of ratios -- from an initial isotope composition, they will only produce new compositions where the change in 18O/16O is twice the change in 17O/16O. Oxygen isotopes in all Earth rocks, water, and gases are related by these mass-dependent changes, but oxygen in the martian meteorites cannot be made from Earth oxygen by mass-dependent changes because it has more 17O to begin with. This difference is quantified as D17O: Earth oxygen defines D17O = 0; water-free minerals in the martian meteorites have D17O = +0.3 per mil (parts per mil).

The authors made the first analyses for the three oxygen isotopes in the acid-soluble (phosphoric acid) material of ALH 84001; it is probably mostly carbonate globules. That oxygen is "heavy," rich in the higher mass isotopes (d18O = +18.3 per mil; d17O = +10.3 per mil), and has D17O = +0.8 per mil. Oxygen from the other minerals is normal for martian meteorites, D17O = +0.3 per mil. Water in other martian meteorites also has D 17O = +0.8 per mil (Karlsson et al., 1992; Romanek et al., 1998). This difference in D 17O means that oxygen in the rocky part of Mars could not have ever mixed much with oxygen in Mars' water or atmosphere.

How did Mars' atmosphere and water come to have a different D17O than its rock? Three hypotheses have been offered: the high D17O is original, from comets that hit Mars early in its history; the high D17O evolved in the martian atmosphere by loss of lighter isotopes to space (mass-dependent, but in different proportions); and the high D17O evolved in the martian atmosphere through photochemical mass-independent processes. The first hypothesis can't be tested now; the second can produce the observed D 17O, but only by increasing the 18O/16O ratio to unreasonably high values.

The third mechanism, the authors' choice, can yield D17O oxygen in the martian atmosphere. In their mechanism, carbon dioxide absorbs ultraviolet light from the sun to yield free oxygen atoms (O), some of which combines to form oxygen molecules (O2). O2 can react with free O to form ozone (O3), but symmetrical ozone, 16O16O16O, forms more slowly than asymmetrical ozone, 18O16O16O or 17O16O16O (Gellene, 1996). In this way, the ozone is enriched in 17O and 18O, and has high D17O. The ozone's extra 17O and 18O can be transferred to carbon dioxide and water molecules, and finally to the carbonate minerals in ALH 84001.

If the ozone and its descendants are enriched in 17O and 18O, the remaining oxygen in the atmosphere must be depleted in these isotopes. Molecular oxygen may inherit oxygen deficient in 17O and 18O, and could become locked into martian soil as iron oxides.

To test models for making the high D1 7O oxygen, the authors compared oxygen and hydrogen isotope ratios in the martian meteorites. The martian atmosphere is rich in heavy hydrogen, deuterium, or 2H, compared to normal hydrogen, 1H (Leshin et al., 1996); its 2H/1H is approximately five times the average on Earth. Mars' high 2H/1H represents preferential loss of the lighter isotope, 1H, from its atmosphere to space. So, if the atmosphere is rich in heavy hydrogen and also has high D17O, samples with the highest 2H/1H ought to have the highest D17O. Analyses of the martian meteorites show the opposite, which may mean that hydrogen (i.e., water) in the martian atmosphere was replenished episodically. (Thanks to C. Romanek for assistance with this summary!)

This paper is critical for understanding the formation of carbonates in ALH 84001. It proves that the carbonates could never have equilibrated chemically or reacted much with the surrounding silicates because they have different D17O value s. This fact is difficult for theories that invoke chemical reactions among the carbonates, silicates, and/or fluids (e.g., Treiman, 1995; Harvey and McSween, 1996; Scott et al., 1997). Further, the paper confirms that Mars' atmosphere and hydrosphere (water) has been separate from Mars' interior and lavas for a very long time (Karlsson et al., 1992; Romanek et al., 1998). The authors' photochemical mechanism for the D17O differences will be very difficult to test telescopically (vis. Krasnopolsky et al., 1996); analyses on Mars or on returned samples will be needed.

The inverse correlation between 2H/1H and D17O is a problem; simple mechanisms apparently can't separate heavy hydrogen from high D17O oxygen if both formed in the atmosphere: not terrestrial contamination, not mixing between atmospheric and surface "reservoirs" on Mars, and not high-temperature alteration of rock. Farquhar's theory of episodic water releases (consistent with the many large floods that Mars has experience) says in effect that the heavy hydrogen formed at a different time than the high D17O oxygen. This model may be difficult to test quantitatively.

This paper was accompanied by a misleading press release. The press release touts the paper's results as conclusive disproof of a biologic origin for the ALH 84001 carbonates (McKay et al., 1996); this supposed disproof is wrong.

1) The second author is quoted: "So if these things [the carbonates globules] were biogenic, they should have equilibrated with the water. They didn't. They equilibrated with the atmosphere." This claim is false -- martian water (in the martian meteorites) has the same D 17O » +0.8 per mil as the ALH 84001 carbonates (Karlsson et al., 1992; Romanek et al., 1998). The identity in D 17O suggests that the carbonate did form from martian water -- it certainly is not consistent with the quote.

2) The second author is then quoted: "This data suggests that the carbonates were made by the interaction with the atmosphere rather than with the water on the surface, as would be required for a biologic process." So how could the carbonates form, if not aided by liquid water? Dry reaction of atmosphere and rock is incredibly slow (Stephens et al., 1995) and seems unlikely, as does formation of the carbonates in the martian atmosphere.

3) The second author is again quoted: "So what we're seeing looks like a garden variety precipitate of carbonates, rather than life." Well, "garden variety" carbonate rocks on Earth (limestone and marble) formed originally in oceans or lakes, and their carbonate minerals were originally made by living organisms (corals, algae, clams, etc.).

Further reading and references:

Boctor N.Z., Wang J., Alexander C.O.A., Hauri E., Bertka C.M., Fei Y., Humayun M. (1998) Petrology and hydrogen and sulfur isotope studies of mineral phases in martian meteorite ALH84001 (abstract). Lunar Planet. Sci. XXIX, Abstract #1787, CD-ROM. Lunar and Planetary Institute, Houston

Gellene G.I. (1996) An explanation for symmetry-induced isotopic fractionation in ozone. Science 274, 1344-1346. [Not a simple explanation]

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.

Karlsson H.R., Clayton R.N., Gibson E.K.Jr., and Mayeda T.K. (1992) Water in SNC meteorites: Evidence for a martian hydrosphere. Science 255, 1409-1411.

Krasnopolsky V.A., Mumma M.J., Bjoraker G.L., and Jennings D.E. (1996) Oxygen and carbon isotope ratios in martian carbon dioxide: Measurements and implications for atmospheric evolution. Icarus 124, 553-568.

Leshin L.A., Epstein S., and Stolper E.M. (1996) Hydrogen isotope geochemistry of SNC meteorites. Geochim. Cosmochim. Acta 60, 2635-2650.

Romanek C.S., Perry E.C., Treiman A.H., Socki R.A., Jones J.H., and Gibson E.K. Jr. (1998) Oxygen isotopic record of mineral alteration in the SNC meteorite Lafayette. Meteor. Planet. Sci. 33, in press.

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.

Stephens S.K., Stevenson D.J., and Rossman G.R. (1995) Carbonates on Mars: Experimental results (abstract). Lunar Planet. Sci. XXVI, 1355-1356.


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.

This paper shows that a rock like ALH 84001 can acquire carbonate "pancakes" and possible nanofossils in just a few years of Saharan desert conditions. The Tatahouine meteorite is not martian, although otherwise it is quite similar to ALH 84001. Some samples of Tatahouine were collected just after it fell to Earth in 1931 in southern Tunisia, and others were collected 63 years later. In those years, calcium carbonate from the soil was transported into the meteorite fragments and was deposited in fractures as unzoned rosette-shaped "pancakes" of calcite. Also in the fractures are calcite rods 70-80 nm wide and 100-600 nm long, and calcite spheres ~70 nm in diameter. These small calcite grains have similar shapes and sizes to some putative nanofossils in ALH 84001 and to some nanobacteria reported on Earth.

Barrat and co-workers compared fresh and weathered samples of the meteorite T atahouine to see how rapidly meteorites can be altered in a desert climate on Ea rth. Tatahouine is a good analog for ALH 84001, as it is composed of the same mi nerals (orthopyroxene, chromite, feldspar) in nearly the same proportions. Tatah ouine is not from Mars; however; it is different from ALH 84001 in lacking the c haracteristic oxygen isotope ratios of all martian meteorites. Tatahouine is cla ssified as a diogenite, just as ALH 84001 was originally.

Tatahouine fell in 1931 in southern Tunisia (near the town of Tatahouine), and many fragments were collected shortly thereafter. A co-worker returned to the fall site in 1964 and sieved additional samples of the meteorite from the soil. The new, weathered samples contain yellow or light orange mineral aggregates in their open fractures; these formed on Earth because fractures in the 1931 samples are completely clean. The mineral aggregates were deposited at ground surface temperatures, less than 60\260C. The mineral aggregates are calcite (calcium carbonate) with trace amounts of clay. The aggregates are "pancakes" with a rosette-like (radiating?) structure, and sit in saucer-shaped depressions on the walls of the fractures. Oxygen and carbon in the aggregates have nearly the same isotopic compositions as carbonates in the soil, confirming that the aggregates formed in place on Earth.

In the 1964 (weathered) samples, the surfaces of orthopyroxene, chromite, and calcite grains are commonly decorated with very small grains of calcite: rods 70-80 nm wide and 100-600 nm long, and spheres ~70 nm in diameter. These calcite grains are not present in the 1931 (unweathered) samples, and so must have formed during the 63 years in the desert. The calcite rods never showed preferred orientations, nor are rooted in the substrate mineral grains. There is no way at present to tell if the calcite rods and spheres are mineralized nanobacteria or inorganic precipitates.

This short paper is important for understanding ALH 84001 and its putative nanofossils: [1] it provides a good terrestrial analog to the carbonate pancakes in ALH 84001; [2] it suggests a new mechanism and environment for deposition of those carbonates; and [3] it shows how readily rocks can become altered on Earth, and presumably also in comparable environments on Mars.

[1] It has been difficult to agree on how the carbonate pancakes and globules in ALH 84001 formed, in part because few comparable carbonate deposits are known on Earth. Suggested temperatures have ranged from 0\260C to >700\260C, and environments from polar ice to deep crust under impact craters. This paper and Treiman et al. (1998) have now discovered environments on Earth where comparable carbonate deposits have formed. A noteworthy similarity between the ALH 84001 and Tatahouine carbonates is their presence in saucer-shaped depressions along fractures in pyroxene, as shown by Scott and Krot (1998). Barrat et al. suggest that the saucers reflect bacterial action, while Scott and Krot suggest (for ALH 84001) that the pyroxene was squeezed around the carbonate pancakes.

[2] Until now, all proposed theories for formation of the carbonates in ALH 84001 have implied that the rock was saturated in some fluid (be it cold water, hot vapors, or carbonate magma), and that carbonat e deposition was a single event or process. With this paper, Barrat and co-workers show that carbonate rosettes can be deposited in an environment that was probably never saturated with fluid (water) and where transport of carbonate was probably episodic.

[3] I am surprised at how quickly Tatahouine acquired carbonate rosettes in a hyperarid environment. Could the carbonates in ALH 84001 have formed so quickly from water solutions? Again, I am struck by how pristine ALH 84001 is -- how little it has interacted with water despite its age of 4.5 billion years.

P.S. In the movies Star Wars and Return of the Jedi, the scenes of Luke Skywalker's home planet Tatooine were shot in Tunisia.

P.P.S. 60°C = 140°F!

Citations:

Scott E.R.D. and Krot A.N. (1998) Carbonates in martian meteorite ALH84001: P etrologic evidence for an impact origin (abstract). In Lunar Planet. Sci. XXIX, Abstract #1786, Lunar and Planetary Institute, Houston (CD-ROM).

Treiman A.H., Ionov D.A., Amundsen H.E.F., Bunch T., and Blake D.F. (1998) A terrestrial analog for carbonates in ALH 84001: Ankerite-magnesite carbonates in mantle xenoliths and basalts from Spitsbergen (Svalbard), Norway (abstract). In Lunar Planet. Sci. XXIX, Abstract #1630, Lunar and Planetary Institute, Houston (CD-ROM).


Clemett S.J., Dulay M.T., Gilette J.S., Chillier X.D.F., Mahajan T.B., and Z are 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, in press.

Becker et al. (1997) suggested that polycyclic aromatic hydrocarbons (PAHs) in ALH 84001 did not form on Mars (McKay et al., 1996), but came from melted Antarctic ice and were deposited on the carbonates globules. Here, Clemett and co-workers refute Becker's claims by showing that: Antarctic ice from the Allan Hills contains essentially no PAHs; Allan Hills meteorites that lack indigenous PAHs contain no contaminant PAHs; carbonate minerals do not preferentially adsorb PAHs; and the spatial distribution of PAHs in ALH84001 is not consistent with contamination.

McKay et al. (1996) found abundant PAHs in ALH 84001, found that the PAHs are associated with the carbonate globules (hosts of other putative signs of martian life), and claimed the PAHs to be like those in decomposed bacteria. Becker et al. (1997) challenged these claims, asserting instead that the PAHs came from the Antarctic ice and were transported into the meteorite by seasonal meltwater. To support their view, Becker and co-workers presented analyses of the abundances of PAHs in Antarctic melt water, and performed experiments to show that PAHs were strongly adsorbed onto carbonate minerals.

Here, Clemett and co-workers respond to Becker's assertions, and find them to be entirely without merit. Clemett and co-workers address four issues: [1] Are PAHs actually concentrated onto carbonate minerals? [2] Do ice and meltwater from the Allan Hills of Antarctica contain enough PAHs to act as a source of contamination? [3] Are other meteorites from the Allan Hills contaminated with terrestrial PAHs? [4] Is the spatial distribution of PAHs in ALH 84001 consistent with contaminati on?

[1] To explain McKay's finding of PAHs strongly associated with carbonate globules in ALH 84001, Becker and co-workers claimed that PAH molecules in solution attach themselves strongly to carbonate minerals and that the dilute solutions of PAHs from melted ice could provide all the needed PAHs. To assess these claims, Clemett and co-workers did two sets of experiments. First, they coated mixtures of calcium carbonate and quartz sand with PAHs (by drying a PAH solution), flushed the mineral mixtures with water for two weeks, and analyzed the mineral mixtures for PAHs. Neither mineral retained much of the PAHs, but the quartz retained significantly more than the calcium carbonate. Second, they made a saturated solution of naphthalene (a PAH) in water, mixed one batch of solution with calcium carbonate and another batch with titanium oxide (as a standard), and sampled the solutions periodically for 130 hours. In that time, naphthalene abundances in both solutions remained constant, meaning that it was not being concentrated on the calcium carbonate grains (or on the titanium oxide). These results directly contradict those of Becker et al., and Clemett points out some significant shortcomings in the design of the Becker's experiments.

[2] To measure the abundances of PAHs in Allan Hills ice and meltwater, Clemett and co-workers melted 150 gm of Allan Hills ice (c/o Johnson Space Center) and analyzed a concentrate using the same \265L2MS instrument that was used in the McKay et al. (1996) work. No PAHs were detected, so the ice contains less than 1 part per million of water-soluble PAHs. From the literature, Clemett et al. estimate that Antarctic ice actually contains less than 100 parts per trillion of PAHs, and that these are not the same PAHs found in the ALH 84001 meteorite. As PAHs are not strongly concentrated on carbonate minerals (see [1] above), the Allan Hills ice is not likely to be a source of PAH contamination for ALH 84001.

[3] If ALH 84001 were contaminated with Earth PAHs, other meteorites should be similarly contaminated. Clemett analyzed some Allan Hills meteorites that contain no indigenous PAHs and found that only one sample had detectable PAHs, and that at less than one-tenth the abundances in ALH 84001. Clemett and co-workers also analyzed carbonaceous micrometeorites from Antarctica, which ought to be easily contaminated because of their fine grain sizes and high surface areas. Each micrometeorite has a distinct pattern of PAH abundances, but none are similar to that in ALH 84001. Thus, there is no evidence that meteorites are contaminated by terrestrial PAHs in Antarctica.

[4] Finally, Clemett reconfirmed that ALH 84001 is depleted in PAHs near its fusion crust, the edge of the meteorite that was melted during its passage through the Earth's atmosphere. At four separate localities, PAHs are 10 times more abundant 1.5 mm inside the meteorite than just inside its fusion crust. One might expect that contaminant PAHs would be more abundant near the meteorite's exterior. Clemett and co-workers interpret this result to mean that ALH 84001 contained PAHs before it hit the Earth's atmosphere, and that the PAHs closest to the fusion crust were vaporized or burnt out as the fusion crust formed.

This paper will not be published until July 1998, but its material is timely and a preprint is freely available at http://zaresimon.Stanford.EDU/MicroL2MS/Faraday98/Faraday98.html.
These carefully documented results make a good case that the PAHs in ALH 84001 are not terrestrial contamination, and by implication got into the meteorite on Mars.

The issue of organics in ALH 84001 is evolving quickly and is far from settled. Stephan et al. (1998) found that PAHs were broadly distributed in the meteorite, and not concentrated in the carbonate globules. Compare this result with: "Not all spheroid cores in the analyzed chip contain PAH excesses; in fact, some cores do not have PAHs concentrations above background" (Thomas et al., 1995). Also, the carbon-14 results of Jull et al. (1998) imply that all or almost all the organic carbon in ALH 84001 is terrestrial contamination. It is not really clear if there are enough PAHs in the meteorite so that Jull could have detected them. Amino acids in ALH 84001 are clearly terrestrial in origin (Bada et al., 1998), so there is little doubt that the meteorite was contaminated by water-soluble organic materials here on Earth. But it remains completely unclear whether the PAHs (which are generally insoluble in water) are terrestria l or martian, and whether they are biogenic or abiogenic.

As a final grumpy note, I cannot understand why investigators of PAHs persist in using calcium carbonate as a potential substrate material. Above all, there is very little calcium carbonate in ALH 84001: Nearly all the carbonate present is dolomite-ankerite solid solution or magnesite-sider ite solid solution. While I don't think that PAH adsorption behavior will be radically different among the various rhombohedral carbonates, it would still be best to use experimental analogs as close as possible to the real material. Second, the calcium carbonate in these experiments is not adequately characterized. Calcium carbonate can form in three crystal structures: calcite, aragonite, and vaterite. Calcite itself has the same structure as siderite and magnesite, and so might be a good analog for adsorption experiments. But we don't know the crystal structure of the calcium carbonates in the experiments!

Citations:

Bada J.L., Glavin D.P., McDonald G.D., and Becker L. (1998) A search for endo genous amino acids in 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.

Jull A.J.T., Courtney C., Jeffrey D.A., and Beck J.W. (1998) Isotopic evidenc e for a terrestrial source of organic compounds found in Martian meteorites Alla n Hills 84001 and Elephant Moraine 79001. Science 279, 366-369.

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

Stephan T., Rost D., Jessberger E.K., and Greshake A. (1998) Polycyclic aroma tic hydrocarbons in ALH84001 analyzed with time-of-flight secondary ion mass spe ctrometry (abstract). In Lunar Planet. Sci. XXIX, Abstract #1263, Lunar a nd Planetary Institute, Houston (CD-ROM).

Thomas K.L., Romanek C.S., Clemett S.J., Gibson E.K., McKay D.S., Maechling D .R., and Zare R.N. (1995) Preliminary analysis of polycyclic aromatic hydrocarbo ns in the martian (SNC) meteorite ALH 84001 (abstract). In Lunar Planet. Sci. XXVI, 1409-1411.


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.

Oxygen isotope compositions (ratios of 18O to 16O) can be sensitive indicators of the conditions of formation of carbonate minerals, like those in ALH 84001 that host putative signs of ancient Martian life. Leshin et al. measured the oxygen isotope compositions of those carbonate minerals, using an ion probe so they could analyze spots only 10 micrometers across. They found a wide range of oxygen isotope ratios, which varied in concert with the elemental compositions of the carbonates, and studied two models to explain their data. First, the carbonates could have grown from an "infinite" reservoir of fluid. In this case, the variation in oxygen isotope composition comes from variations in temperature, and the maximum temperature of carbonate formation must have been above 250°C. Second, the carbonates could have grown at a constant temperature from a limited reservoir of fluid. In this case, the oxygen isotope compositions vary because of fractionation between the fluid and the growing carbonate minerals. Oxygen isotope ratios here do not limit the temperature of carbonate formation, but carbon isotope ratios imply that temperatures must have been 200°C. Both models imply temperatures too hot for life in the terrestrial mold.

Leshin et al. analyzed spots in the carbonate globules in ALH 84001 for their oxygen isotope ratios by ion microprobe. The ion microprobe allows for fairly precise analyses of the isotope ratio 18O/16O on spots as small as 10 micrometers across. Valley et al. (1997) did a similar study, but Leshin et al.'s sample had a much wider range of carbonate mineral compositions. Leshin et al. selected 18 carbonate spots (in three separate thin sections), analyzed their elemental compositions by electron microprobe, and then analyzed their oxygen isotope compositions. No more than four spots were in a single carbonate globule. Oxygen isotope ratios, given as d 18O with respect to standard mean ocean water, were precise to ±1 per mil (or part per thousand) at the 1s level. Accuracy was affected by significant instrumental fractionation effects, which varied according to the elemental composition of the carbonates. The instrumental fractionations were controlled using natural standards, and the reported d 18O values are inferred to be accurate to ±2 per mil.

Oxygen isotopic compositions of the carbonates ranged from +5.4 per mil to 25.3 per mil, a significantly greater range than had been reported before. d 18O of the carbonates varies with their calcium content -- dolomite (with high calcium) has the lowest d 18O value, while magnesite (very low calcium) has the highest d 18O. This strong zoning in oxygen isotopes, like the strong zoning in elemental composition, implies that the ALH 84001 carbonates did not experience high temperatures for any significant length of time, e.g. they could not have been at ~500°C for longer than a few hundred years. The zoning in oxygen isotopes also gives some important limits on possible origins for the carbonates in ALH 84001, and Leshin et al. explored those limits in two scenarios.

First, Leshin et al. considered an open system model, in which the carbonates grew from an "infinite" reservoir of fluid with carbonate dissolved in it. In this case, the variation in oxygen isotope composition comes from variations in temperature. Because there is no way of knowing the oxygen composition of the fluids that deposited the carbonate minerals (without knowing the temperature), Leshin et al. extrapolate from an end-member case. If the carbonate minerals with the highest d 18O formed at the lowest likely temperature, 0\260C, the maximum temperature of carbonate formation must have been above 250°C. So, if high d 18O carbonates formed at temperatures above 0\260C, the maximum temperature must have been even higher than 250°C.

Second, Leshin et al. considered a closed system model, in which the carbonates grew at constant temperature from a fluid reservoir of limited volume. Variations in oxygen isotope compositions would come from the fractionation of oxygen isotopes between the growing carbonate minerals and the fluid. For this scheme to work at all, to model increasing d 18O with increasing growth of the carbonates, the fluid must have been rich in CO2, not H2O. With limited data on mixed CO2-H2O fluids, Leshin et al. assumed for the model that fluid was pure CO2 (plus dissolved carbonates). They found that d 18O of the carbonates could be readily reproduced by this model at temperatures from 0°C to above 500°C. However, this model can fit carbon isotope ratios in the carbonates (literature data) only for temperatures above 200°C.

The implications of these models lend no support to the hypothesis that the carbonate globules in ALH 84001 formed with the assistance of a martian biota (McKay et al., 1996). The high temperatures implied by the closed system model seem inconsistent with living organisms in the terrestrial mold. The high temperatures and large temperature changes implied by the open system model also seem inconsistent with terrestrial-style biota.

Leshin et al. appear to have done a first-rate job with their analyses and standardization -- they were fortunate to have access to a sample that exposed a wide range of carbonate compositions. I appreciated the extent to which they documented their analyses and samples, although one could always want more. It would have been nice to see maps of all the areas where they took multiple oxygen analyses to understand exactly how the oxygen isotope zoning correlated with the chemical zoning. For instance, the two most Ca-rich carbonates they analyzed are from the same area (PTS168 area D) and the most Ca-rich has higher d 18O that the other. This trend is opposite to that of the analyses as a whole (poorer in Ca means higher d 18O). Similarly, analyses of low-Ca carbonates in one area (TS1,68 area 1) also show d 18O decreasing a bit (or being constant) with decreasing Ca.

Leshin's treatment of the models of variation in elemental and oxygen isotope variations seem reasonable, and is in accord with the new 17O measurements of Farquhar et al. (1998). But neither model explored by Leshin et al. seems to explain some of the zoning and structures in the carbonate globules. The chemical compositions of many globules do not change monotonically from core to rim, but show oscillations in Ca and Fe contents (e.g., Fig. 2 of Harvey & McSween, 1996; Fig 1. of Shearer and Adcock, 1997). The closed system model (at least in its simplest form) cannot replicate these chemical oscillations, nor the decrease in d 18O with decreasing Ca content in PTS168 area D. The open system model (at least in the simple form here) explains chemical and isotopic variations by changes in temperature only. However, the globules zones rich in magnetite and iron sulfides suggest that the availability of oxygen and sulfur (i.e., their fugacities) were also varying. Clearly, as Leshin et al. conclude, more data is needed.

Citations:

Farquhar J., Thiemens M.H., and Jackson T. (1998) D17O measurements of carbonate from ALH 84001: Implications for oxygen cycling between the atmosphere-hydrosphere and pedosphere of Mars (abstract). In Lunar Planet. Sci. XXIX, Abstract #1872, Lunar and Planetary Institute, Houston (CD-ROM).

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.

Shearer C.K. and Adcock C.T. (1998) The relationship between the carbonate and shock-produced glass in ALH 84001 (abstract). In Lunar Planet. Sci. XXIX, Abstract #1280, Lunar and Planetary Institute, Houston (CD-ROM).

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.


Collinson D. W. (1997) Magnetic properties of martian meteorites: Implications for an ancient martian magnetic field. Meteor. Planet. Sci. 32, 803-811.

The magnetic properties of ALH 84001 are complex. The natural remnant magnetism of ALH 84001 is carried by small amounts of magnetite (possibly that in the carbonate globules) and titanomagnetite (location unknown); no evidence for pyrrhotite was noted. ALH 84001 trapped impermanent ("viscous") magnetic fields in two separate directions; these impermanent magnetizations probably formed on Earth. ALH 84001 also carries a hard (relatively "permanent") remnant magnetization that probably originated on Mars. Adjacent fragments of ALH 84001 carry this hard magnetization in directions that differ by 40 in angle, which is anomalous if the meteorite originally contained a uniform remnant magnetization.

Collinson examined the magnetic properties of most of the martian meteorites, emphasizing their natural remnant magnetizations (NRM). The characteristics of the NRMs were studied by alternating field demagnetization. "... the rock is subjected to a decreasing alternating [magnetic] field of successively higher initial peak values, resulting in the removal of NRM components of different magnetic stabilities." The minerals that carry the NRM and their abundances were investigated by successively heating the samples and measuring their magnetizations (thermomagnetic signatures) and by measuring the bulk magnetic susceptibility of the samples.

The natural remnant magnetization (NRM) of ALH 84001 is probably carried by magnetite (Curie T ~ 550-600C) and titanomagnetite (Curie T ~ 350-400C). Abundances of both are very low -- magnetite is probably present at @ 0.001% by volume of the rock.

The NRM of three samples of ALH 84001 were examined by alternating field demagnetization -- two of the samples were originally adjacent in the whole rock (their edges fitted together). All three samples had similar NRM: a weak NRM seen in the lowest applied fields, a stronger NRM that gave stable magnetization directions with applied fields of ~ 20-35 milliTesla; and a weak "hard" NRM that persisted stably to applied fields of at least 100 milliTesla. In the two samples that were originally adjacent, the lowest-field NRM directions are different by about 40 suggesting that they were trapped after ALH 84001 was broken apart. In these same samples, the intermediate-field NRM directions are the same. Collinson interprets both of these NRMs as "viscous," produced by magnetic fields at room temperatures, and readily modified at room temperatures. The "hard" magnetization is likely either a thermal remnant magnetization (TRM) trapped as the meteorite cooled to below 350C or a chemical remnant magnetization trapped in magnetic crystals as they grew. It is puzzling that the two samples that were originally adjacent, carry this hard NRM in different directions, separated again by about 40° of arc.

Collinson's results on the remnant magnetic properties of ALH 84001 do not solve any of the questions related to ancient martian life, but do add to the general understanding of the meteorite and its many complexities. It is interesting, although not sinister, that Collins on does not reference the study of remnant magnetism of Kirschvink et al. (1997). Kirschvink's work was published after Collinson's was submitted, but before it was revised to its final form. Collinson could have modified this manuscript to recognize Kirschvink's results, but was not required to do so.

[1] Magnetic minerals. Collinson found that the magnetic properties of ALH 84001 could be explained titanomagnetite and 0.001% of single-domain magnetite (grains ~50 nm in diameter). ALH 84001 contains about 1% of carbonate globules, and it seems reasonable that about 0.1% of their volume might be single-domain magnetites (e.g., McKay et al., 1996). Collinson saw no evidence that pyrrhotite, Fe1-xS, had an important magnetic signature. On the other hand, Kirschvink et al. (1997) attributed all of the magnetic signatures they found to pyrrhotite, even though they reported carbonate globules (presumably with single-domain magnetites) in their sample. Since pyrrhotite is exceedingly rare and titanomagnetite is not reported in ALH 84001, both results are puzzling.

[2] Magnetization directions and components. Both Collinson and Kirschvink et al. (1997) reported finding three distinct NRM components, with incomplete overlap. Collinson's lowest-field NRM does not seem to be present in Kirschvink's data. Both groups found a NRM that was stable between 20 and 35 milliTesla; and both groups found a NRM that was stable to ~ 80 milliTesla. Kirschvink et al. gave evidence for a NRM stable at even higher field strengths, while Collinson reported no comparable NRM. However, Collinson found that the two lower-field-strength of his NRM components were viscous and probably acquired on Earth, while Kirschvink et al. found no evidence of viscous NRM. Sorting this out will require more knowledge than I possess.

[3] Adjacent fragments. Both Collinson and Kirschvink et al. were able to study fragments of ALH 84001 that had been physically adjacent in the original rock. And both groups found that the adjacent fragments had different NRM directions, most notably for the "harder" NRM (stable at alternating fields to 80 milliTesla). Collinson's fragments showed about 40\060 angular separation between their NRM directions, and Kirschvink's fragments showed 50-75\060 angular separation. These results are hard to reconcile with the thermal history of ALH 84001, which includes a high-temperature event after the main brecciation event and after formation of the carbonate globules (Treiman, 1998).

Citations:

Kirschvink J.L., Maine A.T., and Vali H. (1997) Paleomagnetic evidence of a low-temperature origin of carbonates in the martian meteorite ALH 84001. Science 275, 1629-1633.

Treiman A.H. (1998) The history of ALH 84001 revised: Multiple shock events (abstract).In Lunar Planet. Sci. XXIX, Abstract #1195, Lunar and Planetary Institute, Houston (CD-ROM).


Gladman B. (1997) Destination Earth. Martian meteorite delivery. Icarus 130, 228-246.

Gladman's computer models of the orbits of meteorites from Mars, although not specifically aimed at ALH 84001, are important for the related issue of whether live martian organisms could get to Earth. The martian meteorites are assumed to have been ejected from Mars by asteroid impacts onto Mars, and the meteorites themselves spent between 0.6 and 15 million years in interplanetary space. However, several (perhaps dozens) of rocks ejected in each of these asteroid impacts on Mars will arrive on Earth in less than a year after departing Mars (Gladman and Burns, 1996). Spores of earth bacteria are quite capable of surviving that long in space, so each meteorite-forming impact onto Mars has a significant chance of inoculating the Earth with viable martian organisms, if martian organisms do exist or live near Mars' surface.

Citations:

Gladman B.J. and Burns J.A. (1996) Mars meteorite transfer: Simulation. Science 274, 161-162.


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, 366-369.

Jull and co-workers measured the abundances of stable and radioactive isotopes of carbon in ALH 84001. Most of carbon in ALH 84001 is from its carbonate mineral globules (as reported previously). Most of the remaining carbon is from Earth organic material, i.e., terrestrial contamination. A small fraction of the carbon (~8%) is too old to be Earth contamination, and is not (in chemistry and carbon isotopes) like carbon from the carbonate minerals. This carbon may be from organic material formed on Mars, or possibly a rare inorganic mineral (also from Mars).

Part of McKay et al.'s (1996) argument for traces of martian life in ALH 84001 is that the meteorite contains organic material, rich in PAH compounds, associated with its carbonate mineral globules. However, Becker et al. (1996) argued that this organic material is actually terrestrial contamination. To help resolve this issue, Jull and co-workers analyzed the isotopic composition of the carbon in the organic matter and the carbonate minerals of ALH 84001 (following Jull et al., 1997).

The principal clue used by Jull is the abundance of the radioactive isotope of carbon, carbon-14, in the organic material. Carbon-14 is used as an age-dating tool for archaeological and cultural artifacts (like the Shroud of Turin). Carbon-14 forms continuously and abundantly in the Earth's atmosphere. As soon as a carbon-bearing compound is isolated from the atmosphere (e.g., a tree dies and stops absorbing CO2 from the air), its carbon-14 starts decaying away with a half-life of 5730 years. Most of the organic matter in ALH 84001 contains significant amounts of carbon-14 -- which means that it is terrestrial contamination (there is no reasonable extraterrestrial source of so much carbon-14). Also, the carbon-14 gives an average age near 6000 years, which is approximately 7000 years after ALH 84001 fell to Earth. So, there is little doubt that most of the organic carbon in ALH 84001 is terrestrial contamination. In addition, the relative abundances of carbon-12 and carbon-13 (the d 13C value) in the ALH 84001 organics are typical or carbon from living things on Earth.

The carbon in carbonate minerals in ALH 84001 is clearly not terrestrial -- it has little or no carbon-14, and a d 13C value much higher than typical of Earth carbonates. Earlier, Jull et al. (1997) got similar results for carbonate minerals in a different sample of ALH 84001, although that sample had enough carbon-14 to suggest some chemical exchanges with Earth water.

However, a small part of the carbon in ALH 84001 might be martian organic material. This carbon was not dissolved away during acid treatment designed to remove carbonate minerals, so it is either organic or some (unknown) resistant mineral. This batch of carbon has no carbon-14, meaning that it is very old. Jull and coworkers take this ancient age to mean that this batch of carbon did not form on Earth -- it is martian.

This work appears to be carefully done, adequately documented, and carefully presented. It does not directly refute McKay et al.'s (1996) hypothesis of martian biological activity in ALH 84001, but it is not much of a confirmation, either. I have two comments about this work and possible evidence of martian biological activity in ALH 84001.

ALH 84001 contains hundreds of parts per million organic carbon, much more than other martian meteorites except EETA79001 (which Jull also analyzed in this paper). This high abundance of organic matter has been used to support claims of fossil martian biology in ALH 84001. However, ALH 84001 contains the same amount of organic carbon as do typical basalt meteorites from asteroids, even those found in Antarctica (Grady et al., 1997). Just as Jull and co-workers showed that most of the organic carbon in ALH 84001 is terrestrial contamination, Grady et al. (1997) showed that most of the carbon in asteroidal basalt meteorites is terrestrial contamination. In this way, ALH 84001 is quite average and was not contaminated any more than normal for a meteorite.

The most intriguing part of Jull's work, at least to me, is the extraterrestrial organic (?) material they found in ALH 84001. They found this carbon in a sample of ALH 84001 that had been treated to remove all of its carbonate minerals. At lower temperatures (<450C) this treated sample released the same terrestrial carbon (both 14C and d 13C) as the untreated sample. But at higher temperatures, the treated sample released some carbon without any 14C, meaning it was pre-terrestrial. This high-temperature, non-carbonate carbon could be organic matter, or could possibly be a rare, acid-resistant, as-yet-unidentified inorganic mineral. Many different kinds of organics can be released at these higher temperatures, including material like kerogen, graphite, and PAHs. So, it is tempting to say that Jull and co-workers detected the same PAHs that McKay et al. found (and probably also other high-temperature organic compounds). But most meteorite basalts from asteroids also contain about similar amounts of high-temperature carbon (10-30 parts per million; Grady et al. 1997). Could it be that basalts in the solar system just have this much of high-temperature carbon compounds, whether or not life was present?

Citations:

Becker L., Glavin D.P., and Bada J.L. (1997) Polycycic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta 61, 475-481.

Grady M.M., Wright I.P., and Pillinger C.T. (1997) Carbon in howardite, eucrite, and diogenite basaltic achondrites. Meteoritics Planet. Sci. 32, 863-87

Jull A.J.T., Eastoe C.J., and Cloudt S. (1997) Isotopic composition of carbonates in SNC meteorites, Allan Hills 84001 and Zagami. Jour. Geophys. Res. 102, 1663-1669.


Bada, J.L., Glavin D.P., McDonald G.D., and Becker L. (1998) A search for endogenous amino acids in martian meteorite ALH84001. Science 279, 362- 365.

Bada and co-workers analyzed ALH 84001 for amino acids, chemicals that are essential in life as we know it on Earth. In the meteorite's carbonate globules are small amounts of amino acids, which are nearly identical (in proportions of acid species and in their chemical handedness) to amino acids in Antarctic ice. So, Bada and co-workers conclude that (essentially) all of the amino acids in ALH 84001 are terrestrial contamination, carried into the meteorite by melted Antarctic ice.

Part of McKay et al.'s (1996) argument for traces of martian life in ALH 84001 is that the meteorite contains organic material mixed with its carbonate mineral globules. Last year, Bada's research group claimed the organic material is terrestrial contamination (Becker et al., 1996). Continuing this work, Bada and co-workers analyzed ALH 84001 and its carbonate minerals for amino acids. Amino acids are small organic molecules, the building blocks of proteins and enzymes in all living things on Earth. Earth life only uses a few of the many possible amino acids in fairly characteristic relative abundances, and only uses the L form of those amino acids. With these distinctive characters, amino acids are a sensitive test for Earth organic contamination in meteorites.

To analyze for amino acids, Bada and co-workers used a very sensitive technique developed in their laboratory. McKay et al. suggested that the signs of ancient martian life were associated with carbonate minerals in ALH 84001, so Bada and co-workers used a chemical extraction to separate amino acids in the carbonate globules from those elsewhere. First, they rinsed the samples of ALH 84001 in distilled water, and that extracted no amino acids at all. Then, they reacted the samples with weak hydrochloric acid, which should dissolve the carbonate minerals in the rock and release any amino acids associated with them. This acid solution was dried, and part of it analyzed for free amino acids (those not chemically bound to anything else). Another part of the solution was dried and treated to liberate amino acids that were bound to other molecules (for example, this treatment would break proteins into their constituent amino acids). And finally, they analyzed the remainder of the meteorite that was not dissolved in acid (including the pyroxene and chromite mineral grains) for bound amino acids.

Bada and co-workers found that the amino acids in ALH 84001 were most abundant as bound acids associated with the carbonate minerals. There were almost no amino acids in the distilled water wash, the acid-insoluble residue, or as free amino acids in the acid solution. The part of ALH 84001 that dissolved in acid contained about 10 parts per million total amino acids (almost all chemically bound), while the rest of the rock contained only 75 - 100 parts per billion of amino acids.

The amino acids in ALH 84001 are almost exactly in the same proportion as in the Antarctic ice -- the proportions of DL-serine to glycine to L-alanine are approximately 3:3:1. In addition, there is a little D-alanine in Antarctic ice and in ALH 84001 [ed. note: possibly from micrometeorites in the ice?]. This similarity of terrestrial and ALH 84001 amino acids leaves little doubt that they are primarily terrestrial contamination, derived from amino acids in the ice that was around ALH 84001.

The amino acids that Bada and co-workers found in ALH 84001 are from the Antarctic ice. But this fact is not a death blow to the hypothesis of that ALH 84001 contains traces of ancient martian life (McKay et al. 1996). Despite an exuberant press release from Scripps Oceanographic Institution, Bada's work is not a conclusive test of McKay's hypothesis. McKay et al. (1996) did not talk about amino acids, so the absence of preterrestrial amino acids does not refute their hypothesis at all. Of course, if Bada and co-workers had found abundant preterrestrial amino acids, it would have been strong support for McKay et al.'s hypothesis.

Two aspects of Bada's experiments are puzzling (although probably not very important). First, their acid treatment was designed to dissolve carbonate minerals, but it dissolved 20% of their carbonate-free sample of lunar rock. What actually dissolved from the lunar rock? Possibly feldspar? Could feldspar (or whatever) also have dissolved from ALH 84001, and would this change the conclusions? Second, their acid treatment seems to have increased the masses of their samples. For instance, sample 2 of ALH 84001 started at 463 milligrams, and ended up as 472.5 milligrams (text and Table 1). What is this extra mass? Could it be lab contamination that might carry amino acids?

Citations:

Becker L., Glavin D.P., and Bada J.L. (1997) Polycycic aromatic hydrocarb ons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar i ce. Geochim. Cosmochim. Acta 61, 475-481.


Bradley J.P., Harvey R.P., and McSween H.Y.Jr. (1997) No 'nanofossils' in martian meteorite orthopyroxenite. Nature 390, 454.

McKay D.S., Gibson E.K. Jr., Thomas-Keprta K., and Vali H. (1997) Reply to "No 'nanofossils' in martian meteorite orthopyroxenite." Nature 390, 455-456.

Bradley et al. claim that the possible nanofossils found by McKay et al. (1996) in martian meteorite ALH 84001 are actually irregularities in the surfaces of mineral grains. These irregularities were accentuated by the metal coating that had to be put on the samples for electron microscope examination. So, Bradley and co-workers reject the hypothesis that ALH 84001 carries nanofossils of ancient martian life.

In response, McKay et al. say that they also found the same surface irregularities, and that they are not possible martian nanofossils. The metal coating on the samples did not interfere with their identification of objects as nanofossils, because they did control experiments on metal coatings and know what the coating does. (G.J. Taylor has posted a nice summary of these letters.)

Bradley and co-workers examined fracture surfaces of ALH 84001 using nearly the same methods that McKay et al. (1996) used. They found sausage-shaped surface features, approximately 100 to 400 nanometers (billionths of a meter) long, that looked (to them) similar to the possible nanofossils in the McKay et al. (1996) paper and in later magazine articles and press briefings. Bradley found these sausage-shaped features on the carbonate minerals (as McKay's `nanofossils' were) and also on the host silicate minerals. By observing the sample from many angles (in their electron microscope), Bradley found that the 'sausages' were not sitting on the host minerals, but were actually ridges poking out of the host minerals.

Bradley also did a few experiments on how the metal coating on the samples changes the shapes of surface features. They found (as have others) that metal coatings tend to make surface features look segmented (the thicker, the more segmented) -- an appearance that McKay's group had suggested once to reflect cell boundaries.

McKay et al. respond that they have also seen ridges on minerals' surfaces that Bradley et al. found -- same sizes, shapes, and textures. McKay and co-workers suggest that the ridges are grains of clay minerals formed during `incipient' alteration of the host minerals. But these surface ridges, say McKay et al., are not the possible nanofossils they described in 1996 and subsequently. Their possible nanofossils differ from the Bradley ridges by not being parallel with each other, by intersecting with each other at distinct angles, by being curved, and by being rather isolated from each other.

McKay and colleagues also dispute that their identifications of possible nanofossils (here and earlier) were compromised by metal coatings on the samples. They reiterate that they did control experiments on the effects of metal coatings, and that the nanofossil morphologies do not result from coating. McKay also notes that some of Bradley's samples were coated with gold alone, rather than a gold-palladium alloy, and that gold coatings are known to make larger artifacts (artificial structures) than are gold-palladium.

This exchange focuses on two important issues about the possible martian nanofossils in ALH 84001: 1) how can you recognize a nanofossil, and 2) how does laboratory preparation change the surfaces of the samples. Unfortunately, short "correspondence and reply" tidbits (Nature Nuggets®) cannot carry enough scientific "meat" to resolve these issues.

1) How can you recognize that a shape in ALH 84001 is a martian nanofossil? In 1996, McKay et al. cited "...regularly shaped ovoid and elongate forms ranging from 20 to 100 nanometers in longest dimension" as possible nanofossils (their Figure 6 and Kerr, 1996). At their big NASA press conference, McKay and colleagues also presented an image of aligned sausage-shaped objects in a grid-formation as being possible nanofossils. Bradley et al. found features that matched these characteristics, and showed that they were not biological.

Here, McKay et al. seem to have changed their definition of martian nanofossils. Nanofossils are still elongate and ovoid. Now, however: they do not appear in parallel, but display "intersecting alignments;" they are relatively isolated from each other; they are significantly curved (their Fig. 2c); and they are much larger, up to 750 nanometers long. With these new criteria, many of McKay's own objects may not qualify as nanofossils: the ovoids of Figure 6a in McKay et al. (1996); the famous segmented worm shape (Kerr, 1996); and the aligned sausage-shaped objects.

2) How does the metal coating (for electron microscopy) affect the surfaces of minerals in ALH 84001? This question has been argued, mostly in private, since McKay et al. (1996) was published. In other words, are some of the `nanofossils' in ALH 84001 completely artificial, made during metal coating, and completely irrelevant to life on Mars? Believable answers to these questions will only come from carefully controlled experiments, where fragments of ALH 84001 are coated with various thicknesses of different metals and alloys. Bradley et al. report that they did a few experiments in this program; McKay et al. report that they did a series of experiments on a different sample (a lunar glass). Unfortunately, neither set of experiments has been reported in any detail, and I am still not sure of what metal coatings (Au or Au/Pd) do to surface morphology at these very small sizes.

Citations:

Kerr R.A. (1996) Ancient life on Mars? Science 273, 864-866.


Murty S.V.S. and Mohapatra R.K. (1997) Nitrogen and heavy noble gases in ALH 84001: Signatures of ancient martian atmosphere. Geochim. Cosmochim. Acta 61, 5417-5428.

About 4.0 billion years ago, traces of noble gases and nitrogen from the martian atmosphere were trapped in ALH 84001. The isotopic compositions and relative abundances of the heavy noble gases xenon (Xe) and krypton (Kr) are similar to the present-day martian atmosphere. So, Mars' unusual Xe and Kr compositions and abundances were set earlier than 4.0 billion years ago. Argon trapped in ALH 84001 has less 40Ar from radioactive 40K (potassium) that Mars' present-day atmosphere, suggesting that it has continued to gain 40Ar over time [ed. note: e.g., by volcanic outgassing]. Nitrogen trapped in ALH 84001 has much less of the heavy isotope 15N, consistent with loss of the light isotope 14N (and other lightweight gases) from Mars' atmosphere over the last 4 billion years.

The elemental and isotopic composition of the martian atmosphere has been a real puzzle. It is greatly depleted in the light stable isotopes of all gas elements, from hydrogen to xenon. For instance, the abundance ratios of light to heavy xenon isotopes (e.g., 128Xe/136Xe) are approximately 0.7 times that in the Sun (Zahnle, 1993). It is a mystery how and when the lightweight isotopes were removed, but a separate process must have acted for each element (Pepin, 1994). Any process strong enough to remove a lot of, say, 128Xe compared to 136Xe, would certainly have removed all of the lighter gaseous elements completely (like krypton, argon, and nitrogen). Similarly, any process capable of separating 36Ar from 38Ar to the extent seen in the martian atmosphere would have removed essentially all of its nitrogen.

One way to help understand the martian atmosphere would be to learn how its composition has changed through time. Its present-day atmosphere (analyzed by Earth telescopes and the Viking landers) is the same as the atmosphere of 180 million years ago, as trapped in some martian meteorites (most notably EETA79001). Recognizing that ALH 84001 has retained noble gases (like argon) for 4.0 billion years, Murty and Mohapatra investigated whether it might contain trapped martian atmosphere from that time. They used standard techniques -- separating the meteorite into its minerals by their density, heating the samples up in steps of 200C (or more) to 1600C, and collecting the gases given off by each sample in each temperature step. The gases were separated, and the isotopic composition of each element was measured with a mass spectrometer.

Murty and Mohapatra found that ALH 84001 contains significant quantities of nitrogen, argon, krypton, and xenon gases. Most gases (xenon, krypton, nitrogen, and 36Argon) all were released by the samples at nearly the same temperatures, suggesting that they are from the same trapped atmosphere component. ALH 84001 contains a nitrogen component comparable to Mars `mantle' (the Chassigny meteorite) and a trapped component with d 15N ³ +85per mil; the current Mars atmosphere has d 15N » +620 per mil. From the isotopic composition of the argon (in mineral and temperature and temperature separates), the authors estimate that the trapped gas has 40Ar/36Ar £ 1400, while the current Mars atmosphere has a value of 2400. The trapped gas in ALH 84001 has 14N/36Ar about 60 times the value for the current Mars atmosphere. The Kr and Xe isotope compositions of most of the trapped gas are similar to the current martian atmosphere, or current atmosphere as modified by groundwater processes.

Murty and Mohapatra infer that this trapped gas component is a sample of the martian atmosphere from 4.0 billion years ago, the age when argon gas was last lost from ALH 84001. The ancient and modern atmospheres have similar isotopes and relative abundances of xenon and krypton (the heaviest noble gases), which means that the hydrodynamic escape processes that set these abundances (Pepin, 1994) were complete by 4.0 billion years ago. The higher 40Ar/36Ar in the current atmosphere reflects production of 40Ar from potassium over the history of Mars. And the decrease in 14N/36Ar may reflect loss of nitrogen (through sputtering) into space over the last 4.0 billion years.

This work is not directly related to the "life in ALH 84001" folderol. It is part of the long-term effort to learn about Mars' ancient environments through clues in the martian meteorites. The noble gases and nitrogen hold great promise in unraveling the evolution of Mars' atmosphere, particularly why it is so thin now (surface pressure of ~ 1/200 that of Earth) and where its water has gone. But this work, no matter how good, is not likely to be the final word from ALH 84001. The uncertainty here is not from Murty and Mohapatra's analyses, but in the inherent variability of samples of ALH 84001 and the many assumptions that must be made to unravel the noble gas story.

First, it appears that different samples of ALH 84001 contain different quantities, proportions, and isotope compositions of the noble gases and nitrogen. This is perhaps not too surprising, as the mineral proportions and chemical composition of ALH 84001 are rather variable, for instance potassium abundances (108 vs. 200 parts per million: Mittlefehldt, 1994; Dreibus et al., 1994). For the noble gases, this variability can appear as differences in the proportion of 40Ar that comes from radioactive potassium (this paper; Turner et al., 1997), and as differences in xenon isotope ratios (Fig. 9 of this paper vs. Fig. 2 of Swindle et al., 1995 and Fig. 3 of Miura et al., 1995). Variability like these in elemental and isotopic abundances suggests that the gases in ALH 84001 came from many different sources and were not mixed well. It will be possible, eventually, to sort out the different sources (or components) of gas; now, it seems to be a muddle.

Second, interpretation of noble gas and nitrogen abundances is not simple, and relies on some (fairly complex) correction schemes and underlying assumptions. Different research groups have not treated their data the same way; so when their results appear in conflict, it may be difficult for a non-specialist (like me) to understand why. For instance, all groups so far have agreed that some of the argon in ALH 84001 comes from atmosphere trapped in the mineral grains. Turner et al. (1997) present evidence that this trapped gas is like argon from the Earth's atmosphere: 40Ar/36Ar = 295. Murty and Mohapatra infer that the trapped argon is ancient martian, with 40Ar/36Ar £ 1410. Miura et al. (1995) and Goswami et al. (1997) use the current martian atmosphere value of 40Ar/36Ar » 2400. Swindle et al. (1995) do not infer a specific 40Ar/36Ar for the trapped component. Is each group justified, given their data and the intrinsic variability of ALH 84001, or have some (or all) of them made unjustified simplifications in their data treatment

Citations:

Dreibus G., Burghele A., Jochum K.P., Spettel B., Wlotzka F., and Wänke H. (1994) Chemical and mineral composition of ALH 84001: A martian orthopyroxenite (abstract). Meteoritics 29, 461.

Goswami J.N., Sinha N., Murty S.V.S., Mohapatra R.K., and Clement C.J. (1997) Nuclear tracks and light noble gases in Allan Hills 84001: Pre-atmospheric size, fall characteristics, cosmic ray exposure duration, and formation age. Meteoritics Planet. Sci. 32, 91-96.

Mittlefehldt D.W. (1994) Errata. Meteoritics 29, 900.

Miura Y.N., Nagao K., Sugiura N., Sagawa H., and Matsubara K. (1995) Orthopyroxenite ALH84001 and shergottite ALHA77005: Additional evidence for a martian origin from noble gases. Geochim. Cosmochim. Acta 59, 2105-2113.

Pepin R.O. (1994) Evolution of the martian atmosphere. Icarus 111, 289-304.

Swindle T.D., Grier J.A., and Burkland M.K. (1995) Noble gases in orthopyroxenite ALH84001: A different kind of martian meteorite with an atmospheric signature. Geochim. Cosmochim. Acta 59, 793-801.

Turner G., Knott S.F., Ash R.D., and Gilmour J.D. (1997) Ar-Ar chronology of the Martian meteorite ALH84001: Evidence for the timing of the early bombardment of Mars. Geochim. Cosmochim. Acta 61, 3835-3850.

Zahnle K.J. (1993) Xenonological constraints on the impact erosion of the early martian atmosphere. Jour. Geophys. Res. 98, 10899-10913.


Greenwood J.P., Riciputi L.R., and McSween H.Y.Jr. (1997) Sulfide isotopic compositions in shergottites and ALH 84001, and possible implications for life on Mars. Geochim. Cosmochim. Acta 61, 4449-4453.

The authors measured the abundance ratio of sulfur isotopes (34S/32S) in minerals of martian meteorites to see if the sulfur in ALH 84001 had been processed by sulfate-reducing bacteria, as implied by McKay et al. (1996). They found no evidence for the action of sulfate-reducing bacteria in ALH 84001, and so reject the McKay et al. (1996) hypothesis that ALH 84001 contains traces of ancient martian life.

The element sulfur occurs as two stable (not radioactive) isotopes with masses of 32 and 34, 32S and 34S. Most sources of sulfur have abundance ratios of 34S/32S that are very similar to the average in the solar system. However, sulfur that has been processed by bacteria (or other life forms) can have distinctly different abundances of these isotopes. The greatest changes in S isotopes come from sulfate-reducing bacteria, which take sulfate ions (SO42-) from water and convert them to sulfide ions (S2-) in water or as solid sulfide minerals. Sulfate-reducing bacteria, when they have lots of sulfate in water around them, can form sulfide minerals with ~5% less 34S than the sulfate in the water. This difference is easily detected, and has been used (on Earth) as a guide to the action of these bacteria.

To estimate the sulfur isotope ratio for bulk Mars, Greenwood et al. measured sulfur isotope ratios the martian basalt meteorites (Shergotty, Zagami, EETA79001, LEW88516, and QUE94201). The sulfur isotope ratios for these meteorites are within 0.3% of the solar system average. In ALH 84001, they first measured sulfur isotopes in millimeter-sized grains of pyrite (FeS2), which are not associated with the possible traces of ancient martian life (Gibson et al., 1996; but see Shearer et al., 1997). The pyrite had variable and slightly `heavier' sulfur than the other martian meteorites, with 34S/32S from approximately 0.2 to 0.75% larger than the solar system average; this agrees with earlier work of Shearer et al. (1996). Finally, they analyzed the sulfur-rich outer zone of a single carbonate globule from ALH 84001 iron sulfide minerals in the carbonate globules were claimed by McKay et al. (1996) to have formed through the action of martian biological organisms. The outer parts of the carbonate globules contain carbonate and oxide minerals in addition to the sulfides, so Greenwood et al. did not get so precise a result here as for the pure sulfide minerals. Also, they had to apply a small correction for pairs of oxygen atoms masquerading as sulfur. But the 34S/32S for the sulfide-rich region of the carbonate globule is identical to the nonbiological pyrite in ALH 84001: 0.6% larger than the solar system average.

The nonbiological and possibly biological sulfide minerals in ALH 84001 have nearly identical 34S/32S ratios. Greenwood et al. take this similarity to suggest that sulfur (in the possibly biological sulfides) in the carbonate globules was not processed by sulfate-reducing bacteria that the McKay et al. (1996) hypothesis is wrong. Rather, they suggest that all the sulfides in ALH 84001 formed from a high-temperature fluid (too hot for life as we know it), probably generated by an asteroid impact onto Mars. The variations in sulfur isotope ratios suggest mixing of `light' and `heavy' sulfur, the former perhaps from igneous rocks, the latter perhaps from Mars' surface.

This paper is much weaker than it could have been because the authors did not document their experiments adequately. The analyses of sulfur isotopes in the pure sulfide minerals (pyrite and pyrrhotite) seem superb; they follow carefully described procedures, are based on good standards, and are repeatable. But the analysis of sulfur isotopes in the carbonate globule, the critical analysis for evaluating the hypothesis of ancient martian life (McKay et al., 1996), will be suspect until Greenwood et al. document it fully.

The problem with Greenwood's analysis for sulfur isotopes in the carbonate globule is that they did not analyze only sulfide minerals. Their instrument, an ion microprobe, shoots cesium ions at the sample, and collects ions from the sample that are sputtered off by the cesium. Sulfur come off as S2- ions, both as the `light' 32S2- and the `heavy' 34S2-. Two problems are possible when the sulfur is present as sulfides among other minerals, like carbonates and oxides.

If the sulfide minerals are mixed with oxide and carbonate minerals, the ion 16O16O 2- might be formed in abundance (from the carbonates and oxides) and might pass as 32S2-, as both ions have the same masses and charges. If there were lots of 16O16O 2- passing for 32S2-, the sulfur would appear `lighter' than it really is.

It is also possible that having sulfur-bearing minerals among other minerals influences the way that the sulfur sputters off the sample and into the analyzer. For instance, sulfur in sulfides mixed with carbonates and oxides might sputter more like a sulfate than a sulfide, and require a different correction procedure.

Greenwood et al. were aware of these potential problems, and reported that they: 1) corrected for the presence of 16O16O 2- (less than 0.2% in their value of 34S/32S); and 2) did experiments to show that their sulfur isotope correction procedures gave consistent results for 34S/32S with or without admixed carbonates and oxides. But they gave no details on the 16O16O 2- correction, and no results for the experiments on mixtures. Since we cannot see the details of their corrections, and the results of their experiments, we are really asked to take on faith that Greenwood did both properly. Some scientists, trusting the authors implicitly, will take their work on faith. Others, who do not accept the conclusions of Greenwood et al., will point to these problems as cause for discounting the paper entirely. And those who wish to "trust, but verify" will merely be disappointed.

Citations:

Gibson E.K.Jr., McKay D.S., Thomas-Keprta K.L., and Romanek C.S. (1996) E valuating the evidence for past life on Mars (letter). Science 274, 2125.

Shearer C.K., Layne G.D., Papike J.J., and Spilde M.N. (1996) Sulfur isotope systematics in alteration assemblages in martian meteorite ALH 84001. Geochim. Cosmochim. Acta 60, 2921-2926.

Shearer C.K., Spilde M.N., Wiedenbeck M., and Papike J.J. (1997) The petrogenetic relationship between carbonates and pyrite in martian meteorite ALH 84001 (abstract). Lunar Planet. Sci. XXVIII, 1293 -1294.


Turner G., Knott S.F., Ash R.D., and Gilmour J.D. (1997) Ar-Ar chronology of the martian meteorite ALH 84001: Evidence for the timing of the early bombardment of Mars. Geochim. Cosmochim. Acta 61, 3835-3850.

The authors studied the age of ALH 84001, using 39Ar-40 Ar (argon-argon) radio-isotope dating. For the `traces of ancient life' controversy, their most important result is a revision of the 3.6 billion-year-old age that McKay et al. (1996) used as the time of carbonate formation. Turner et al. have revised the age for this particular sample of carbonate to 3.83 + 0.15 billion years, within uncertainty of nearly all other 39Ar-40Ar ages for ALH 84001. This `carbonate' age may not be when the carbonates formed. It actually is the age of the feldspar-composition glass that is mixed with the carbonate minerals, which could be older, younger, or the same as the carbonates.

This paper represents an exhaustive study of the 39Ar-40Ar age of ALH 84001; this radioactive age-dating system is actually the potassium-argon (K-Ar) system, but some of the potassium is converted to 39Ar (in a nuclear reactor) so it can be measured at the same time as the 40Ar. Also, Turner and coworkers calculated the cosmic ray exposure age of ALH 84001 and its abundance of trapped martian atmosphere. Turner et al. studied three rock fragments by `stepped heating:' heating each sample up 100°C at a time and collecting all the argon that was released at each temperature. This method allowed them to tell what abundances of argon isotopes were released by each kind of mineral in the fragment. Turner et al. also analyzed 40 spots on thin sections (microscope slides) by vaporizing them with a laser beam and collecting the argon that was released.

After corrections for various sources of argon, including contamination from martian atmosphere, Turner et al. found that nearly all of the samples were consistent with an 39Ar-40Ar age of 3.97 billion years, possibly as old as 4.05 billion or as young as 3.8 billion. This 39Ar-40Ar age for ALH 84001 is essentially the same as determined by other research labs (Bogard and Garrison, 1997; Goswami et al., 1997). Two samples gave older ages, near 4.4 billion years; its is not clear if these ages are real.

The age of sample 110i, rich in carbonate minerals, was originally reported as 3.6 billion years (Knott et al., 1996); McKay et al. (1996) took that as the age of the carbonate globules and their possible signs of Martian life in ALH 84001. This ancient age was important, as it placed the possible signs of martian life in the distant past, when Mars was probably much wetter (and possibly much warmer) than it is now. This ancient `warm, wet' Mars would have been similar to the ancient Earth, and so a reasonable place for life to form and flourish.

However, Turner et al. have revised the age of sample 110i to 3.83 + 0.15 billion years, which is (within uncertainty) the same as nearly all other 39Ar-40Ar ages for ALH 84001. Further, sample 110i contains a LOT of potassium, much more than could have come from the carbonate minerals alone. The potassium in 110i probably came from silicate glass (like maskelynite) mixed with the carbonate, and so its 39Ar-40Ar age is the formation (or last heating event) of the silicate glass! So the age of spot 110i really does not limit the age when the carbonate formed.

In calculating the 39Ar-40Ar ages of their samples, Turner et al. had to determine the proportion of Earth and martian atmospheres in their samples, and also how long the samples were exposed to cosmic rays in interplanetary space. Some samples had significant proportions of Earth atmosphere, but most had relatively little martian atmosphere. On average, less than 5% of the 40Ar in the samples came from the present-day martian atmosphere; this 40Ar probably was forced into the glass in ALH 84001 when it was ejected from Mars. That probably happened approximately 14 million years ago, the cosmic ray exposure age (see the paper below by Eugster et al., 1997).

The 39Ar-40Ar age of approximately 4.0 billion years fits well with the ages of planetary bodies in the solar system. Most rocks from the Moon's highlands give 39Ar-40Ar ages from 3.8 to 4.0 billion; the oldest rocks on Earth formed at about 4.0 billion; many meteorites were shocked by impact between 4.1 and 3.5 billion years ago. Turner et al. suggest that the 39Ar-40Ar age of ALH 84001 represents an asteroid impact onto Mars (Treiman, 1995), and that impact was approximately at the same time as the large impact basins formed on the Moon. This correspondence seems to support the idea of a `lunar cataclysm' at about 4.0 billion years ago -- a time when the Moon's surface was especially hard hit by asteroids.

This paper was submitted for publication in August, 1996, back when ALH 84001 was most interesting as a sample of the ancient martian crust. That was the impetus for this study -- G. Turner and his group wanted to understand the age and impact history Mars, especially as it might relate to the Moon. Ar-Ar ages for moon rocks cluster at 4.1 - 3.8 billion years ago, which suggests to some people that this was a time of abundant large asteroid impacts on the Moon - the so-called `lunar cataclysm.' Other people have concluded that the Moon was hammered by asteroid collisions continuously from 4.5 billion years ago through 3.8 billion, but that older ages were erased by younger ones. The results here seem consistent with the notion of an impact `cataclysm' happening throughout the inner solar system.

For this study, Turner and colleagues had to know which event was actually being dated by the Ar-Ar system, they accepted Treiman's (1995) history as best fitting their data. Treiman (1995) proposed that ALH 84001 experienced two shock events: one that granulated and sheared the rock, and a second (after the carbonate globules formed) that produced shock glass with little deformation. Turner et al. assigned their age to the earlier event, and noted that production of shock glasses commonly does not reset Ar-Ar ages.

However, Turner et al. did not consider more recent, alternate histories for ALH 84001; they were proposed after this paper was written. Bradley et al. (1996, 1997) gave evidence that ALH 84001 was heated to above 500°C during formation of some magnetite grains, and (they infer) during formation of the carbonate globules. Scott et al. (1997) inferred that the carbonate globules and the feldspar-composition glass formed simultaneously in a single shock event. If either of these scenarios were true, they would most likely be recorded by the Ar-Ar age dates and could have happened 4.0 billion years ago.

Gleason J.D., Kring D.A., Hill D.H., and Boynton W.V. (1997) Petrography and bulk chemistry of Martian orthopyroxenite ALH 84001: Implications for the origin of secondary carbonates. Geochim. Cosmochim. Acta 61, 3503-3512.

Gleason and coworkers did a general study of ALH 84001, emphasizing microscope observations and chemical compositions of the rock and its minerals. Particularly, they examined the carbonate globules which McKay et al. (1996) suggested were formed by ancient martian life. Gleason and coworkers infer that the globules were deposited from liquid water, and so disagree with Harvey and McSween (1996) and Scott et al. (1997), who claimed that the carbonate minerals formed at high temperatures from molten carbonates.

However, Gleason saw no evidence that the carbonate globules were associated with life, and so do not support McKay et al. (1996). On the contrary, they noted that similar carbonate globules have formed in other meteorites and on Earth without any apparent biological influences.

Gleason and coworkers inferred, from mineral textures, that the carbonate globules grew from water-rich fluid cooler that 300°C. The carbonate globules appear to have formed by replacing material with the composition of plagioclase feldspar. Treiman (1995) had inferred that this material was crystalline feldspar, but Gleason noted that carbonate replacing crystalline feldspar grows as crack filling and veinlets, NOT as globules. So, they conclude that the carbonates in ALH 84001 replaced feldspar glass, not crystals. If the feldspar glass ever been hotter than 300°C for a few hours even, it would have crystallized back to plagioclase again. Gleason inferred that the this feldspar glass formed at the same time as did the granular bands (crush zones) that criss-cross the meteorite.

Gleason and co-workers also observed is that the chemical composition of ALH 84001 varies a bit. They analyzed the chemical composition of two 1/3-gram fragments from different parts of ALH 84001. Some elements (like lanthanum) are five times less abundant in the fragment from a `crush zone' than the other fragment. Similar variability is apparent in other published chemical analyses. Gleason thinks this variability arose as some elements (like lanthanum) moved around in ALH 84001 before the carbonate globules grew.

Finally, Gleason noted that pyrite, an iron sulfide mineral, was associated with chromite. They did not mention finding any pyrrhotite, another iron sulfide mineral. The significance of these observations is discussed below.

Gleason and co-workers have provided a wealth of new chemical data on ALH 84001, and their excellent microscope observations (although important) do not resolve the issue of ancient life in ALH 84001. Rather, their work serves to emphasize the depth of disagreement about ALH 84001, and how much remains to be learned about the rock.

As for the carbonate globules, Gleason and co-workers support the low-temperature position of Romanek et al. (1994), Treiman (1995) and Valley et al. (1997); low-temperature here means < 300°C, which could still be much too hot for life as we know it. Gleason sees no evidence for the very high temperatures (> 500°C) inferred by Harvey and McSween (1996), Bradley et al. (1996), and Scott et al. (1997). Gleason and co-workers do not have proof that the carbonates formed without life, just their reasoned judgment that life is not absolutely required to produce the structures and compositions they found.

Their inference that the carbonate globules replaced glass rather than crystalline plagioclase is intriguing, and seems to be more realistic than my 1995 suggestion that the carbonates replaced crystalline plagioclase. However, there is no general agreement on how the carbonate globules formed; others have claimed that they replace pyroxenes or that they filled cracks and bubbles in the rock.

The variability of the chemical composition of ALH 84001 is not surprising. ALH formed when crystals of the mineral orthopyroxene grew in a mass of basalt magma, and settled out to the bottom of the mass. Elements like lanthanum would have been concentrated in the magma among the settled crystals. So the amount of lanthanum in a piece of ALH 84001 would represent how much magma was caught among the orthopyroxene crystals. And the amount of magma might vary simply because the crystals were packed together tighter is some spots. On the other hand, the low-lanthanum sample is from a `crushed zone,' and it is possible that the crushing managed to squeeze some lanthanum-bearing mineral (like plagioclase glass) out of that area.

Finally, the observations here remind us of problems with the sulfide minerals in ALH 84001. First, Gleason and coworkers found pyrite (FeS2) associated with chromite rather than with the carbonate globules as reported by most other workers. The chromite has nothing to do with the hypothesis of fossil life in ALH 84001, while (of course) the carbonate globules do. Now, the sulfur isotope ratio (34S/32S) in the pyrite does not look those in Earth life, and so seemed to mean that the carbonate globules could not be associated with life (Shearer et al., 1996; Greenwood et al., 1997; Shearer, 1997; Shearer and Papike, 1996, 1997). However, if the pyrite did not form with the carbonate globules, its sulfur isotope ratio is not relevant to the hypothesis of life. Second, Gleason and co-workers did not mention finding any pyrrhotite (Fe1-xS) in ALH 84001; in fact, no pyrrhotite has been seen in thin sections. This absence is peculiar, as Kirschvink et al (1997) found that the magnetism in ALH 84001 was trapped in pyrrhotite! Where is the pyrrhotite, or could the magnetic signature be from some other mineral?

Eugster O., Weigel A., and Polnau E. (1997) Ejection times of Martian meteorites. Geochim. Cosmochim. Acta 61, 2749-2757.

The authors used abundances of `cosmogenic nuclides,' produced when a meteorite is exposed to cosmic rays, to measure how long four martian meteorites were in interplanetary space. ALH 84001 was exposed to cosmic rays for 14.4 + 0.7 million years, which probably is the time when ALH 84001 was blasted of f Mars. This cosmic ray exposure age for ALH 84001 is similar to ages found by other researchers (e.g., Goswami et al., 1997). None of the other martian meteorites were exposed in interplanetary space for so long, so it seems fairly certain that ALH 84001 did not come from the same site on Mars (impact crater on Mars) as any other martian meteorite.

Hutchins K.S. and Jakosky B.M. (1997) Carbonates in martian meteorite ALH84001: A planetary perspective on formation temperature. Geophys. Res. Lett. 24, 819-822.

The possible traces of life in ALH 84001 are all associated with its carbonate mineral globules, and so the formation of the globules is very important. If the globules formed was hotter than about 150C, a biological origin seems quite unlikely. Low formation temperatures, less than 80C, have been derived from the abundances of oxygen isotopes (16O and 18O) in the carbonates by Romanek et al. (1994).

Here, Hutchins and Jakosky suggest that Romanek et al.s temperature estimate was too low. Romanek et al. used oxygen isotope ratios as a thermometer, by comparing the oxygen isotope ratios (18O/16O) of a mineral and the liquid it grew from. The greater the difference in 18O/16O between the mineral and liquid, the lower the temperature would have been. None of the liquid is trapped in ALH 84001, so Romanek et al. had to estimate its oxygen isotope ratio as something like normal waters on Earth. Hutchins and Jakosky point out that oxygen and carbon in the martian atmosphere are much richer heavy isotopes of oxygen and carbon (18O and 13C) than in the Earths atmosphere, and so Mars' water is also likely to have a relatively high 18O/16O and 13C/12C ratios. When put into the oxygen isotope thermometer, this difference means that the ALH 84001 carbonates probably formed between 40C and 250C.

This paper emphasizes yet another uncertainty in determining the temperature of formation of the carbonate globules in ALH 84001. Whether Romanek et al. or Hutchins and Jakosky are more correct depends on two questions.

First, did Mars' atmosphere have its current high 18O/16O and 13C/12C ratio before the carbonates formed? If not, Hutchins and Jakosky's argument is not valid. Today, Mars' atmosphere has a significantly higher 18O/16O and 13C/12C than martian rocks (the meteorites), and this difference means that the atmosphere somehow lost much of its original 16O and 12C to space. How the atmosphere lost these light isotopes is not certain, but Mars' low gravity (compared to Earth) and weaker magnetic field were probably important. When the light isotopes left Mars' atmosphere is not known (Jakosky and Jones, 1997); unfortunately, when the carbonates were deposited is not really known either.

Second, did the liquid that deposited the carbonates come (eventually) from Mars' atmosphere? Hutchins and Jakosky's paper works from the idea that the liquid came from the atmosphere, and shared its high 18O/16O and 13C/12C ratios. But it is possible that the liquid came from deep inside Mars (in the jargon, 'juvenile water'), and never contacted the atmosphere. In that case, the high 18O/16O and 13C/12C of the ALH 84001 carbonates came entirely from a low formation temperature.


Scott E.R.D., Yamaguchi A. and Krot A.N. (1997) Petrological evidence for shock melting of carbonates in the martian meteorite ALH84001. Nature 387, 366-379.

Here's a new theory of the origin of the carbonate globules in ALH 84001: they formed at very high temperature, during an asteroid impact on Mars, from carbonate rich melt. If the globules formed this way, they could not have been hosts to ancient martian life forms (McKay et al., 1996).

The authors' argument is in four parts: 1) the clear silicate glass in ALH 84001 was melted during an impact shock (presumably an asteroid hitting Mars); 2) all the shock features in ALH 84001 formed in this same shock event; 3) the small dispersed grains of carbonate minerals were once molten, like the glass, because they all share similar structures and textures; and 4) and the structures and textures of the large carbonate globules also fit with once being molten.

First, the authors show that the clear glass was once molten, a liquid. This glass had been called 'maskelynite,' which forms from feldspar minerals during shock without melting. The authors here show that the glass was molten because: its shapes were modified by shock, veinlets of the glass were injected into other minerals, it contains flow features, and it contains bubbles. Further, the chemical composition of the glass is not just the same as feldspar minerals; in addition to feldspar, the glass contains extra silica and sometimes extra chromium (from the mineral chromite). [The authors do not give a melting temperature, but it was much more than 1000°C!]

Second, the authors suggest that all the shock features in ALH 84001 formed in the same shock event that melted the glass. They note that single impact events can produce lots of different shock effects in a single rock, and the effects can cut across each other. They infer that the 'crush zones' or granular bands that criss-cross the rock were the first shock effect, and that the glass formed next [probably within seconds or a minute].

Third, the authors see that the glass and the small carbonate grains have similar shapes, and infer that both formed in the same way. The glass and small carbonates both enclose pyroxene grains in rounded shapes, and fill cracks in grains. Some cracks contain both carbonate minerals and the glass, which suggests to the authors that the cracks (and the "crush zones") formed at the same time as both the glass and the carbonates. So, the authors suggest that the glass and the carbonate melted at the same time and squirted into and around other minerals in ALH 84001. Carbonate melts are very runny, so they would squirt more easily into cracks; there is more carbonate than glass in the cracks. The authors looked for evidence in support of other proposed origins for the carbonates, and found none to their satisfaction.

Fourth, the authors demonstrate that the carbonate globules could have formed as melt droplets, just like the small carbonate grains. The small carbonate grains cover the same range of chemical compositions as the large grains, suggesting that they formed at the same time in the same process (as impact melts). The shapes of the globules in the glass are like liquids that don't mix (like oil droplets in water); carbonate melts do not mix with silica-rich melts, and can form rounded shapes like the globules in ALH 84001. The authors also cite cases on Earth where carbonate minerals have been melted and moved around during impact shocks.

So, all the carbonates now in ALH 84001 formed at very high temperatures. This theory is completely inconsistent with the inferences of McKay et al. (1996) that the carbonate globules contain evidence for martian life. ALH 84001 must have contained carbonate minerals before they were shock melted, but the origin of these ancestral carbonates is not known.

In my opinion, this paper does not refute McKay et al. (1996), because it doesn't prove that the carbonate globules formed at a high temperature. The actual observations here are new and convincing, and it seems certain that that the clear glasses in ALH 84001 were once molten. There remain (to me) some stumbling blocks between this conclusion and the claim that all the carbonate globules formed at the same high temperature as the clear glass.

The biggest doubt is whether the carbonate globules were ever molten, whether they actually were rapidly cooled droplets of carbonate melt. These observations, among others, seem difficult to explain if the carbonate globules were once molten.

There are other problems here too, and they will be explored at length. First, the evidence that all the shock features in ALH 84001 formed in a single impact event is not (to me) very convincing, compared to evidence for multiple impacts (Treiman, 1995). Second, McKay and Lofgren (1996) showed a picture of a the glass cutting across the Ca-Fe-Mg layering and 'oreo cookie' rim of a carbonate globule. This structure seems difficult to make if the glass and carbonate were liquid at the same time. And third, the zoning in oxygen isotopes from core to rim in the globules (Valley et al., 1997; Leshin et al., 1997; Saxton et al., 1997) may be impossible to produce at the high temperatures needed to melt these carbonates.

For more about this paper, check out the University of Hawaii's Planetary Science Research Discoveries webzine.


Kirschvink, J. L., Maine A. T., and Vali H. (1997) Paleomagnetic evidence of a low-temperature origin of carbonate in the martian meteorite ALH 84001. Science, 275, 1629-1633.

When a rock forms or cools down, it can trap some of the local magnetic field; magnetic minerals in the rock become little bar magnets, aligned with the planet's magnetic field. This trapped magnetic field, called natural remnant magnetism or NRM, can stay in the rock indefinitely, and can be used to unravel the history of the magnetic minerals and the rock. The strength of the trapped magnetic field can tell how strong the planet's field was. If the rock is broken or bent, the magnetic field trapped in it will point in a different direction from the original field. If the rock gets heated above a critical temperature, the old trapped magnetic field is lost, and a new one is trapped when it cools down again.

For the McKay et al. (1996) hypothesis of fossil martian life in ALH 84001, the most important result from Kirschvink et al. is that the carbonate globules formed below 325°C, and probably below ~110°C. McKay et al. require a low formation temperature to permit bacterial growth, and many types of Earth bacteria and archaea can live and prosper at 110°C! The upper temperature limit is too high for known Earth life, but is an UPPER LIMIT, and is still better for McKay et al. than the 500°-700°C temperatures estimated by other groups.

The argument for carbonate formation below 325°C is indirect, but fairly clear. Kirschvink et al. measured the trapped magnetic fields (NRM) in two adjacent fragments of ALH 84001 from the fracture zone where McKay et al. found the most carbonate globules. The trapped fields in the two fragments were strong, equally strong, but in different orientations; the "bar-magnets" of the magnetic minerals were pointed in different directions. This meant that the two fragments had probably trapped the same original field, but had been rotated or jostled when the fracture between them formed. If ALH 84001 had ever been hotter than 325°C since the fragments were jostled, they would have lost their original magnetic fields; when they cooled, the fragments would have trapped the new magnetic field, with the same direction in both fragments! Because the fragments do have magnetic fields in different directions, ALH 84001 could not have been hotter that 325°C at any time after the fractures formed. Now, the carbonate globules are in these same fractures, and must have formed after the fractures did, and so must not have formed at temperatures hotter than 325°C (otherwise the rock fragments would have their trapped magnetic fields pointing in the same direction)!

The argument for carbonate formation below ~110°C depends on the details of how the trapped magnetic field changes as the rock is heated. In ALH 84001, the trapped magnetic field is in the iron sulfide mineral pyrrhotite. When pyrrhotite is heated to temperatures below its critical temperature of 325°C, its trapped magnetic field fades away somewhat. But Kirschvink et al. found no hint of this fading in ALH 84001's trapped magnetic field. The 110°C temperature actually comes from their sample preparation, not anything inside the rock. They had to heat their samples to 110°C to allow their glue to cure. If ALH 84001 had been heated to >110°C on Mars, any magnetic effects would have been erased as the glue cured.

The results of this paper are a strong challenge to "anti-life in ALH 84001" scientists. However, the results are not (yet) proof of a low-temperature origin and certainly not proof of life on Mars. Although I am not an expert on magnetism, I see two issues in this work as it relates to McKay et al.'s hypothesis that ALH 84001 contains traces of ancient martian life.

The first issue is the timing of fracturing of ALH 84001 compared to the timing of carbonate formation. ALH 84001 was fractured at least twice, before and after the carbonate globules formed. Many carbonate globules sit in fractures, so these fractures must have been there first (McKay et al., 1996). The carbonate globules are themselves sliced and broken along fractures, which must have come later (Mittlefehldt, 1994; Treiman, 1995; McKay et al., 1997). So, could Kirschvink's two rock fragments have separated by a late fracture, rather than an early fracture? If this particular fracture formed after the carbonate globules were deposited, Kirschvink's results here would say nothing about formation of the carbonate globules.

The second issue is the absolute age of the carbonate globules, which should be ancient (3.6 billion years old) according to McKay et al. (1996). The problem here is that the tiny magnetite grains in the carbonate globules have not trapped any detectable magnetic field themselves. The magnetites do contribute to other magnetic properties of the rock, just not the trapped field (the NRM). Could this mean that the magnetite grains grew when there was no field, and so are fairly young (Wadhwa and Lugmair, 1996)? Or could it mean that Kirschvink's sample had so few carbonate globules that their trapped magnetic field could not be detected?

The most important result from this paper, particularly for life on Mars, is the evidence that Mars had a strong magnetic field! Mars now has no detectable magnetic field, and had hardly any field 1.3 billion years ago, when many of the martian meteorites formed. Kirschvink et al. have demonstrated that Mars had a strong magnetic field (possibly as strong as the Earth's is now) about 4.0 billion years ago, when ALH 84001 cooled.

First, a strong magnetic field would have protected Mars' surface from much deadly radiation from space. Its magnetic field would have deflected radiation like electrons and protons from the Sun, just as the Earth's magnetic field protects us now.

Second, and perhaps more important, a magnetic field early in Mars' history would have protected its atmosphere. Mars' atmosphere is now quite thin, about 1/200 as thick as the Earth's. Without a thick atmosphere, Mars' surface could never have been warm enough to permit liquid water, and there is very good geologic evidence that liquid water was once abundant on the surface of Mars. What happened to Mars' atmosphere? Much of it was swept away by the solar wind, the continual stream of electron and protons that shoot off the Sun. But a strong magnetic field would have protected Mars' atmosphere, possibly letting Mars' surface be warm and wet enough for life to develop.


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 meteorite ALH 84001: Evidence from stable isotopes and mineralogy. Science, 275, 1633-1638.

The temperature of formation of carbonate globules in ALH 84001 is important because the globules are hosts to the possible traces of ancient martian life (McKay et al., 1996). The first estimates of the globules' formation temperature, <320°C, relied on oxygen isotope measurements (Romanek et al., 1994); here, Valley et al. revisit the oxygen isotope measurements with a new improved analytical method and confirm the low formation temperature.

Valley et al. used an ion microprobe to determine oxygen isotope abundances in the carbonate globules and other minerals in ALH 84001. The ion microprobe can produce analyses from very small spots, about 20 micrometers (m) in diameter, which is important because the carbonate globules are <200 µm in diameter. Valley et al. analyzed oxygen isotope ratios in carbonates from two separate ellipsoids, one of which was a composite of two smaller carbonate bodies. To help calibrate the ion microprobe measurements, Valley et al. also obtained chemical analyses of these and nearby spots in ALH 84001 using an electron microprobe.

Valley's results are consistent with, and expand on, the earlier work of Romanek et al. (1994). They found that the carbonate minerals were variably enriched in the heavy oxygen isotope 18O, with enrichments ranging from d18O = 9.5 to 20.5 "per mil" (or parts per thousand). Carbonate near the globule rims was much richer in 18O than carbonate from the cores, and the different globules had different 18O enrichments in their cores.

Valley et al. inferred that the carbonate globules formed at low temperatures because their chemical and isotopic variations could not have been preserved, if they had formed at high temperatures. Valley et al. estimate that the carbonate globules formed at <100°C. An absolute upper temperature limit from their results comes from assuming that the carbonates were in oxygen isotopic equilibrium with the surrounding pyroxene. This upper limit on temperature is ~300°C; the temperature had to have been lower because the pyroxene and carbonate were not in chemical equilibrium.

Valley et al. also made some interesting discoveries and observations during their work: (1) They also analyzed the carbonates for carbon isotope composition, and found some evidence for an organic carbon component that has relatively little of the heavy carbon isotope 13C. This finding is one of a number of hints now of very "light," possibly organic, carbon in ALH 84001. (2) Valley et al. found a veinlet of silica that cut across one of the carbonate globules. This indicates that silicate minerals were mobile after the carbonate veinlets formed, and similar evidence was presented by other groups at the 28th Lunar and Planetary Science Conference. (3) Valley et al. note that the near-absence of hydrous minerals in ALH 84001, long cited as a problem for a low-temperature origin of the carbonates, is not actually a problem at all. There are many instances on Earth where low-temperature carbonate veins cut silicate rocks without formation of hydrous silicate minerals.

The oxygen isotope abundance ratios measured by Valley et al. have been confirmed and extended by two other groups using similar ion microprobe techniques: L. Leshin et al. (1997) and J. Saxton et al. (1997). Although there are still some problems with calibrations and interlaboratory biases, it seems indisputable that the carbonates in ALH 84001 contain relatively heavy oxygen (high d18O) and that they are strongly zoned in oxygen isotope ratios from core to rim.

However, the meaning of this zoning is quite disputable. Valley et al. have interpreted the zoning as most consistent with carbonate minerals growing, at low temperature, from a fluid that changed composition over time. Their low temperature is consistent with, but not proof of, microbial life. Leshin et al., on the other hand, interpret the oxygen isotope zoning as forming at higher temperatures in a closed system. Higher temperatures here means 250°C, too high for known Earth bacteria, but a far cry from the 500°-700°C suggested by some other investigators.


Jull A. J. T., Eastoe C. J., and Cloudt S. (1997) Isotopic composition of carbonates in the SNC meteorites Allan Hills 84001 and Zagami. J. Geophys. Res., 102, 1663-1669.

The authors investigated the sources of the carbon in ALH 84001 (and other martian meteorites), especially using radioactive carbon-14 (14C) as a marker for carbonates that formed on Earth. Radioactive 14C forms continuously in the Earth's atmosphere (and from nuclear bomb tests) and forms only sparingly in space, so the abundance of 14C in the carbonates is a clue to how much they have reacted with carbon from Earth. The authors find that most of the carbonate in ALH 84001 contains 14C, so much 14C that it must have either formed on Earth or traded some of its martian carbon for Earth carbon. The carbon in ALH 84001 with the least 14C is also the richest in the stable carbon isotope 13C, and its 13C abundance is the same as measured for martian carbonates in ALH 84001 and other martian meteorites.
This work and Jull et al. (1995) are important for understanding terrestrial contamination in ALH 84001. The authors argue that a great proportion of the carbon and oxygen in the ALH 84001 carbonates originated on Earth, and then diffused into the carbonate mineral grains in the meteorite. This argument, if true, lends plausibility to the idea that the PAHs in ALH 84001 are also terrestrial (Becker et al., 1997). However, Wright et al. (1997) suggest that the amount of 14C found here could also mean only limited contamination by Earth carbon.

Goswami J. N., Sinha N., Murty S. V. S., Mohapatra R. K., and Clement C. J. (1997) Nuclear tracks and light noble gases in Allan Hills 84001:  Pre-atmospheric size, fall characteristics, cosmic ray exposure duration and formation age. Meteor. Planet Sci., 32, 91-96.

As ALH 84001 traveled between Mars and the Earth, it was bombarded by cosmic rays, high-energy particles from the Sun and the galaxy. Interactions of cosmic-ray particles with meteorites leave characteristic signatures like the nuclear tracks produced by cosmic-ray heavy nuclei and trace abundances of the noble elements (e.g., neon and argon) resulting from nuclear interactions of cosmic ray protons with meteoritic matter. Here the authors investigated the evidence for cosmic-ray bombardment in ALH 84001 to understand what happened to this meteoroid after it left Mars and before it landed in Antarctica. They found that ALH 84001 formed approximately 4 billion years ago, and spent approximately 17 million years exposed to cosmic rays; these numbers are consistent with results from many other groups. In addition, the authors here deduce that ALH 84001 was approximately 20 centimeters in diameter before it encountered the Earth, and that ~85% of it burnt up as it passed through the Earth's atmosphere. They also suggest that ALH 84001 did not break up into multiple fragments as it fell through the Earth's atmosphere, and so it is also unlikely that additional fragments of this meteorite exist.
There may be calls for the Antarctic Search for Meteorites Program, ANSMET, to return to the Allan Hills area of Antarctica to search for more fragments of ALH 84001 rock. The results in this paper suggest that returning to the Allan Hills for martian meteorites would be no more fruitful than collecting elsewhere in Antarctica. In fact, ANSMET field parties have gathered meteorites from the Allan Hills area many times since their first visit in 1976. In that time, only two martian meteorites have been found in the Allan Hills:  ALHA 77005 and ALH 84001. These two meteorites are quite different, and could not be separate fragments from a single meteorite fall.

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.

McKay et al. (1996) discovered that ALH 84001 contains polycyclic aromatic hydrocarbon molecules (PAHs) in moderate abundance, found that these PAHs were distinct from meteoritic and terrestrial PAHs, and found that the PAHs in ALH 84001 were intimately associated with the carbonate minerals that host other possible indications of fossil life. Here, the authors evaluate whether the PAHs in ALH 84001 might be contaminants.

To see if the association of PAHs and carbonate minerals in ALH 84001 really suggests that they formed together, the authors put carbonate mineral grains in water samples that contained PAHs--a standard--and a sample of Antarctic ice from the Allan Hills. In both cases, the PAHs in the water attached themselves to the carbonate mineral grains. From this result, the authors infer that the PAHs in ALH 84001 might have become associated with the carbonate minerals without any biologic action.

To see if the PAHs in ALH 84001 were actually different from those in other sources, the authors analyzed PAHs in the martian meteorite EETA 79001 (both carbonate minerals and bulk rock), in two carbonaceous chondrite meteorites, and in Antarctic ice from the Allan Hills. The PAHs from these other samples are all similar to those in ALH 84001, especially in having strong signals from the few simplest PAHs (called parent or nonalkylated molecules). The ALH 84001 PAHs are most similar to PAHs in carbonate minerals in the EETA 79001; both meteorites have similar simple PAHs and in similar small amounts of big complex PAHs. The carbonates in EETA 79001 are known to be contaminated with carbon and organic molecules from Earth (Jull et al., 1995; McDonald and Bada, 1995), and so probably contaminated with Earth PAHs. So, Becker et al. conclude that the PAHs in ALH 84001 are probably a mixture of PAHs from Antarctic ice and PAHs from carbonaceous meteorites or interplanetary dust, which could have entered ALH 84001 either on Earth or on Mars. They see no clear evidence in the PAHs for a biological origin on Mars, and suggest that amino acids would be better biomarkers than PAHs.

This article is important for characterizing the PAHs from Earth that are likely to collect on meteorites as they sit in Antarctica, and would seem to weaken McKay et al.'s case for traces of martian fossils in ALH 84001. But many questions are not yet answered.
  1. The PAHs in ALH 84001 are not merely a mixture of PAHs from CM chondrites and from the Allan Hills ice. The ice contains strong signals from the PAHs naphthalene (mass 128) and coronene (mass 300), while carbonates in ALH 84001 contain neither (their Table 1). Other differences are apparent in the relative strengths of some PAH signals, and in the presence or absence of signals from some less-abundant PAHs. Are these differences artificial, for instance because Becker et al. and McKay et al. used slightly different analytical techniques? Or could the differences be real and significant for the origin of the PAHs?
  2. The authors here showed that PAHs in water stick strongly to a calcium carbonate mineral, but is this relevant to ALH 84001? Calcium-rich carbonate minerals are rare in ALH 84001; most of its carbonate is rich in magnesium and iron. Further, the calcium carbonate used in the experiments was not characterized, and may not have the same crystal structure as the carbonates in ALH 84001 (calcite vs. aragonite vs. vaterite structure types); PAHs may bond differently to different carbonate mineral structures.
  3. Becker et al. suggest that the PAHs in ALH 84001 are associated with the carbonate minerals because their experiment showed that PAHs in water stick strongly to a carbonate mineral. But do PAHs prefer to stick to carbonates compared to the other minerals ALH 84001? The experiments of Becker et al. shed no light on this question.

Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1996) Magnetite whiskers and platelets in ALH 84001 Martian meteorite:  Evidence of vapor phase growth. Geochim. Cosmochim. Acta, 60, 5149-5155.

McKay et al. (1996) found that submicroscopic magnetite grains in the ALH 84001 carbonate globules are ". . . cuboid, teardrop, and irregular in shape" and have ". . . no structural defects." These magnetite crystals are similar to crystals produced by bacteria on Earth, and so McKay et al. suggested that the magnetites in ALH 84001 could have been made by martian bacteria.

The authors here show that the submicroscopic magnetite grains also occur in other shapes and with structural defects. Using transmission electron microscopy, the authors discovered whisker-shaped magnetite crystals, five times as long as they are wide (10 millionths of a millimeter by 50 millionths of a millimeter). Many of these magnetite whiskers contain a common kind of structural defect, a screw dislocation. The authors also discovered blade- and plate-shaped crystals of magnetite, and many of them contain a structural defect called twinning.

On searching through other technical papers, the authors found that magnetite (and similar substances) grow in whisker shapes only from hot gases, hotter than 500°C. Hot gas like this occurs in nature near volcanos, in structures called fumaroles, where the hot gases from a volcano or lava flow escape into the air. In fact, whisker-shaped magnetite crystals were reported from a fumarole deposit in Indonesia by Symonds (1993). Also, Bradley et al. could find no descriptions of bacterial magnetites that were blade shaped, plate shaped, whisker shaped, or that contained structural defects.

The authors conclude that the magnetites in the ALH 84001 carbonate globules formed at high temperatures, and not from biological processes. In addition, they note that the magnetite whiskers are approximately the same sizes and shapes as some of the possible fossilized bacteria shown in the McKay et al. (1996) paper.

This work can be viewed in two ways:  as a refutation of McKay et al.'s claims that the magnetites were made by microorganisms; or as an ambiguous result that merely shows that McKay et al. were a bit exuberant in claiming that all the magnetite crystals were structurally perfect.

In the first view, it is clear that some of the magnetite crystals in the ALH 84001 carbonates do not have the shapes and structures of common biogenic magnetites. This fact alone can be seen as a refutation of part of the McKay et al. hypothesis. Because magnetite has a cubic crystal structure, it almost always grows as cubes, octahedra, or other compact shapes. Elongated magnetite crystals are known to grow only from high-temperature gases, whether in nature or in the laboratory. And this inference of high temperature, while not conclusive, is certainly inconsistent with life.

In the second view, most of the arguments in Bradley et al. (1996) are ambiguous. While they all are interesting observations, none of them invalidates the hypothesis of McKay et al.

  1. From the description in their paper, it is not clear that Bradley's magnetites are from the same layers and veins as the magnetites studied by McKay et al.
  2. Although Bradley et al. did find structurally imperfect whisker-shaped magnetites, it would still appear that most of the magnetite crystals in the ALH 84001 carbonates are structurally perfect cuboids (and similar shapes). So far, there is no proof that the whisker and cuboid magnetites formed at the same temperature.
  3. To support a high-temperature origin for the ALH 84001 magnetites, Bradley et al. refer to Symonds (1993), who found that whisker-shaped magnetite crystals grew from the hot gases given off by a volcano. But Symonds suggested that temperature alone did not control whether the magnetite crystals grew as cubes or whiskers. In fact, the highest-temperature magnetites grew as cubes, while the whisker-shaped crystals formed at lower temperatures where they grew very quickly (i.e., the gas was very supersaturated). Whisker-shaped magnetites apparently have not been reported in low-temperature carbonate deposits, but it is quite possible that no one has looked carefully.

Bradley et al. (1997) will present these results and more at the Lunar and Planetary Science conference this week. Thomas-Keprta et al. (1997) will counter with information that some bacteria do produce elongated magnetite crystals.


Shearer C. K., Layne G. D., Papike J. J., and Spilde M. N. (1996) Sulfur isotope systematics in alteration assemblages in martian meteorite ALH 84001. Geochim. Cosmochim. Acta, 60, 2921-2926.

The element sulfur has two stable (not radioactive) isotopes, 32S and 34S. The relative abundances of these sulfur isotopes, called the sulfur isotope ratio, can be affected by chemical processes, including metabolism by bacteria. Many Earth bacteria can "eat" sulfur compounds and use them as fuel for growth. Sulfur processed this way by bacteria is typically very depleted in 34S compared to the starting sulfur. Nonbiological processes can enrich or deplete sulfur in 34S, but usually not so much as biological processes.

The authors here analyzed the isotopic composition of three pyrite grains associated with the carbonate globules of ALH 84001. The pyrite grains all were enriched 34S compared to the solar system average; in the jargon, they had d34S (pronounced "delta thirty-four S") between +5 and +8 "per mil" (or parts per thousand). These enrichments in 34S suggest that the pyrite (and also the carbonate globules) formed at "low" temperatures, and that the sulfur in the pyrite was probably never processed by bacteria like those on Earth.

However, these results are ambiguous because the isotope ratio in Mars' starting sulfur is not known well. If Mars' starting sulfur has d34S near zero (the solar system average), the high d34S of the pyrites could not come from biological processing, at least by bacteria like those on Earth. Nor could the high d34S develop during high-temperature chemical processes. More likely, the pyrites grew from alkaline, oxygen-poor water at less than 150°C. Lunar soils also have a high d34S, which develops as meteorite impacts vaporize some of the soils.

On the other hand, if Mars' starting sulfur was rich in 34S (had a high value of d34S), the isotopic composition of the sulfur could be consistent with either a high temperature or a biogenic origin. At high temperatures, inorganic processes do not separate the sulfur isotopes well, so a fluid rich in 34S would deposit pyrite rich in 34S. Acidic waters at low temperature also would not separate sulfur isotopes well, so a fluid rich in 34S would deposit pyrite rich in 34S. If Earth-type sulfur-eating bacteria were fed sulfur that was very rich in 34S, they would accumulate in them sulfur that was not so rich in 34S, perhaps similar to the sulfur in the pyrites. Of course, if martian bacteria process sulfur differently from Earth bacteria, all bets are off.

It is easy to think that a low formation temperature for the carbonates in ALH 84001 means that they formed from martian life. But temperature and biology are separate issues. Here, Shearer et al. infer that the pyrite and carbonates in ALH 84001 formed at low temperature without life!

Since this work was published, Greenwood et al. (1997) have also analyzed the isotopic composition of sulfur in ALH 84001, and in martian meteorites that have no known or suspected signs of life in them. For ALH 84001, Greenwood et al. got essentially the same sulfur isotope values as this paper; for the other martian meteorites, Greenwood et al. got d34S values between about +3 and -3. These low numbers, so close to the average for the solar system, suggest that Mars' original sulfur was not very different from the solar system average, and so support Shearer's inference of a nonbiological origin for the pyrite. The temperature of pyrite formation is not clear yet: Shearer et al. suggest low temperature, while Greenwood et al. suggest high temperature. This work is continued in Shearer and Papike (1997) and Shearer (1997).

  • After Science magazine published McKay et al.'s (1996a) article suggesting that they had recognized traces of ancient martian life in ALH 84001, many scientists wrote letters to Science disputing all or part of their results. Science collected these comments and responses to them as "Evaluating the evidence for past life on Mars," Science, 274, pp. 2119-2125. These summaries and commentaries are in the order that Science presented the originals.

    Anders E. (1996) Science, 274, 2119-2121.

    After praising the quality and depth of their observations, Anders comments that McKay et al. (1996a) did not consider nonbiological explanations for their discoveries: "For all these observations, an inorganic explanation is at least equally plausible, and, by Occam's Razor, preferable." Anders then suggests nonbiological explanations for most of the chemical evidence for martian life in McKay et al.

    Anders raises two objections to the description of PAHs in ALH 84001 as implying biogenic activity. First, PAH molecules form as readily from nonbiological chemical compounds as from biological compounds. Given enough time and/or an elevated temperature, PAHs form readily from other organic materials; this process is documented in nature and utilized in industry. Second, the spatial association of PAH molecules and the carbonate globules could have arisen without life. Formation of PAHs can be accelerated (i.e., catalyzed) by the mineral magnetite, and submicroscopic grains of magnetite are abundant in the carbonate globules.

    Anders also presented five objections to the arguments of McKay et al. concerning the minerals and chemical zoning of the carbonate globules.

    1. The chemical zoning patterns in the carbonate globules could be a natural result of mineral solubilities, and need not imply the action of life.
    2. The association of magnetite, iron sulfides (pyrrhotite), and carbonate minerals in ALH 84001 could form without the presence of life, as similar associations have formed in the carbonaceous chondrite meteorites.
    3. The areas of partially dissolved carbonate minerals could form at normal temperatures and water compositions, without the action of life.
    4. The greigite(?) iron sulfide mineral that McKay et al. found was not characterized well, and was not compared with nonbiogenic greigite. Without this comparison, one cannot tell if the greigite(?) is actually relevant to the question of life.
    5. Finally, the structure of the carbonate globules (claimed by McKay et al. to be evidence for a biological origin) was not compared to the structures of carbonate globules formed without assistance from life. Without this kind of comparison, one cannot tell if the structures of the carbonate globules are relevant or not.
    Before the matter of ancient martian life in ALH 84001 is completely resolved, all of Anders' points will need to be studied. McKay et al. (1996b) and Clemett and Zare (1996) provide some answers in their responses to this comment.

    The fundamental issue behind Anders' comment is scientific proof itself. Can the martian-life-in-ALH-84001 hypothesis be examined piece by piece, one line of evidence at a time? Or must all the evidence be considered together, as one complete package?

    In natural sciences, it is rarely possible to prove that an idea is true--"proof" consists mostly of showing that an idea fits ("is consistent with") all the facts, and that all other ideas don't fit the facts or are too complicated. Most often, though, scientists can think up many different ideas that can fit all the facts. Then, they will commonly quote "Occam's Razor," which states that the simplest idea is more likely than the complicated ideas. Unfortunately, what is simple to one scientist is needlessly complex to another. McKay et al.'s paper and Anders' comment use different ideas of simplicity, and so arrive at different preferred conclusions.

    McKay et al. invoked Occam's Razor (without naming it) in justifying a biological origin for all their observations: "Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars." From this perspective, McKay et al. did not need to consider nonbiological explanations for each observation, only nonbiological explanations of the all of the observations at once. They did not find any nonbiological explanations, and so had to accept the idea of martian life.

    On the other hand, Anders invoked Occam's Razor (quoted above) to justify nonbiological processes for each individual observation. Anders did not search for a single nonbiological explanation for all the evidence, and did not consider how likely it was that all of his proposed processes could have affected small areas in a single rock.

    To some extent, then, Anders and McKay et al. are not looking at the evidence in the same way; McKay et al. are "holists," and Anders is a "reductionist." For the possible martian fossils, it remains to be seen which view of the world* is more useful.

    * "Weltanschauung" to the philosophers.


    Shearer C. K. and Papike J. J. (1996) Science, 274, 2121.

    Here, the authors summarize their sulfur isotope measurements that were reported earlier in Shearer et al. (1996), which are described below. Shearer and Papike emphasize that the pyrite mineral grains that they analyzed earlier are related to the carbonate globules, and that the sulfur in the pyrite is enriched in the stable isotope 34S compared to the solar system average. Sulfur-eating bacteria on Earth produce mineral-like pyrite that is strongly depleted in 34S, so it is unlikely that the pyrite in ALH 84001 was made by Earth-type bacteria. Martian bacteria could still be involved, however, if Mars itself was much richer in 34S than the Earth is, or if martian bacteria process sulfur differently from Earth bacteria. For more detail, see the discussion of Shearer et al. (1996) below.
    Gibson et al. (1996) respond directly to this comment. McKay et al. (1996a) did not claim that the pyrite in ALH 84001 was biogenic, so, strictly speaking, this report by Shearer and Papike is not relevant to the current hypothesis of ancient martian fossils in ALH 84001. However, the pyrite crystals are spatially associated with the carbonate globules, and it would have seemed reasonable that the pyrite and the carbonates grew from the same fluids with the same sulfur isotope abundances. On the other hand, if the pyrite had a deficiency of 34S (such as might be expected from biogenic pyrite on Earth), it might possibly have been cited by Gibson et al. (1996) as further evidence of biogenic activity in ALH 84001.

    This work has continued in Shearer (1997) and Shearer and Papike (1997).


    Bell J. F. (1996) Science, 274, 2121-2122.

    Bell's comment centers on the PAH organic molecules found in ALH 84001 by McKay et al. (1996); Bell accepts that these PAHs are martian, but not that they imply martian life. He suggests that the PAHs may have come from meteorites falling onto Mars, just as a few percent of the Moon's soil is made of meteorite debris. Specifically, Bell suggests that the PAHs in ALH 84001 came from material like the C2 carbonaceous chondrite meteorites, and suggests that the sources of this C2 material included the moons of Mars, Phobos and Deimos.
    McKay et al. (1996a) and Becker et al. (1997) agree with Bell that the PAHs in ALH 84001 are similar to those in the C2 carbonaceous chondrites. The PAHs in these meteorites are not identical, but are they similar enough to suggest a common origin? Bell and Becker say "yes," McKay et al. say "no, especially in light of the associated evidence." Bell is correct that a few percent of the lunar soil is made of meteoritic material like C2 carbonaceous chondrites (a point I mistakenly disputed in earlier versions of this commentary). Although few meteorites are carbonaceous, the vast majority of interplanetary dust is like C2 carbonaceous chondrites, and that dust makes up most of the mass that falls onto planets. The moons of Mars are very dark; their darkness might be from the carbon in carbonaceous chondrite material, but their darkness might have other causes (Murchie et al., 1991).

    Clemett S. J. and Zare R. N. (1996) Science, 274, p. 2122-2123.

    Clemett and Zare are among the authors in the original McKay et al. paper, and they respond to comments of Anders and Bell related to PAHs, the organic molecules called polycyclic aromatic hydrocarbons. Clemett and Zare emphasize that the PAHs they found in ALH 84001 are not laboratory contaminants, and are apparently only a small part of all the organic materials in ALH 84001. They agree with Anders (1996) that some of the PAHs in ALH 84001, the low-mass ones, could have formed by inorganic processes at high temperature. The high-mass PAHs, although less abundant, are very similar to the break-down products of kerogen, a variety of solid organic material that is common on Earth and in the carbonaceous chondrite meteorites. Earth kerogen formed from living matter, and meteorite kerogen did not. Clemett and Zare leave with two questions:  how could nonbiologic kerogen get into an igneous rock, one that solidified from molten lava; and how could nonbiologic kerogens (or PAHs) come to be associated only with the carbonate globules in ALH 84001?

    As an aside, Clemett and Zare also reply to comments from Simoneit and Hites and from Requejo and Sassen, but neither of these comments was printed in Science.

    Clemett and Zare agree that some nonbiological processes could have produced the distribution and abundances of PAHs that they observed in ALH 84001:  the low-mass PAHs could be the product of inorganic reaction at high temperature, and the high-mass PAHs could form by the low-temperature reaction of inorganic kerogen. But the issue is whether the PAHs in ALH 84001, in their association with the carbonate globules, are more easily explained by biological or nonbiological mechanisms. A nonbiological scenario would have to start with carbon-rich gas reacting at high temperature to form the low-mass PAHs. Then, nonbiologic kerogen, from some other source, would have to decompose at low temperature into high-mass PAHs. Either the kerogen or the high-mass PAHs would have to infiltrate ALH 84001, adhere only to the carbonates, and not displace the low-mass PAHs already in place. Is this sequence of events actually simpler and more believable than the growth, death, and decomposition of martian bacteria?

    McKay D. S., Thomas-Keprta K. L., Romanek C. S., Gibson E. K. Jr., and Vali H. (1996b) Science, 274, 2123-2125.

    Here, McKay et al. respond directly to Anders' (1996) comments about minerals in the carbonate globules and about the morphology of possible fossil shapes in ALH 84001; Clemett and Zare responded to Anders' comments on PAHs. Anders' comments stressed the similarity of the carbonate globules and their minerals to some grains in the CI carbonaceous chondrite meteorites. McKay et al. agree that similarities are present, but emphasize the significant differences between ALH 84001 and the CI carbonaceous chondrites. In particular, ALH 84001 is an igneous rock, while the CIs have been altered at low temperatures to clays, serpentine, and similar water-bearing silicate minerals. McKay's responses to Anders' comments are keyed to Anders' points (as above).
    1. McKay et al. agree with Anders that the chemical zoning pattern in the carbonate globules could have been produced by inorganic crystallization. They stress, however, that the repetitive (oscillatory) zoning pattern and composition difference between one globule and another can only arise from complex inorganic processes.
    2. Anders compared the carbonate-magnetite-sulfide minerals in ALH 84001 to those in CI carbonaceous chondrite meteorites. McKay et al. respond that, in effect, the CIs are not good analogies. Magnetite grains in carbonate minerals are much larger in CIs than in ALH 84001. And magnetite grains in carbonate minerals in CIs do not have cuboid shapes as they do in ALH 84001.
    3. McKay et al. agree with Anders that the partially dissolved carbonate grains in the carbonate globules could have formed in nearly neutral (nonacidic or alkaline) water, and do not require the moderate acidity invoked in McKay et al. (1996a). McKay et al. restate that the globular morphology of the ALH 84001 carbonates is similar to those formed by bacteria on Earth, and unlike the carbonate areas formed inorganically in the CI carbonaceous chondrites. They stress, however, that no matter what the exact water composition, no simple inorganic process can form all the observed structures and minerals in the carbonate ellipsoids.
    4. On the matter of greigite(?) in ALH 84001, Anders had hoped to see it compared to nonbiogenic greigite. McKay et al. respond that life seemed to be involved with the formation of all greigite on Earth, at least all the greigite that they were aware of. Living organisms either produce greigite directly themselves, or produce the hydrogen sulfide gas that goes to form greigite.
    5. Anders commented that the structures of the carbonate globules should have been compared directly to carbonates that grew without assistance from life. McKay et al. respond that the shape of possible fossil forms is not yet definitive proof that they are real fossils, that similar shapes have not been found in lunar or asteroidal meteorite samples, and that more work is needed. They also agree with Anders that a proof that the fossil shapes actually are fossils would make all the other arguments irrelevant.
      I see two underlying themes in this response:  that ALH 84001 is unique, and that the minerals and structures of the carbonate globules are too complex for any simple inorganic processes. There is, of course, no doubt that ALH 84001 is unique. But Anders and McKay et al. disagree on whether the carbonate globules in ALH 84001 are so unusual that seemingly similar structures in the CI carbonaceous chondrites are not relevant. It has been suggested that the CI carbonaceous chondrites are from Mars (Brandenburg, 1996), but most evidence seems to suggest otherwise (Treiman, 1996).

      McKay et al. emphasize the complexity of the carbonate globules, both in the chemical zoning of their carbonate minerals and in the groupings of minor minerals in the carbonates. The complexity alone suggests to them the action of complex biological systems, and they want to consider all the evidence in McKay et al. (1996a) as a systematic whole, and not as a set of separate pieces. Quoting from their response, the formation of the carbonate globules ". . . cannot be simple equilibrium . . . , and must include changing conditions and kinetic effects. Whether such models are more plausible than biogenic models is a matter of judgment."

      As an aside, the fifth point of Anders' comments seems to refer to the shapes and structures of the carbonate globules, while McKay et al. here responded about the sausage-shaped things that might be fossil bacteria. Some critical sentence or idea may have been lost.


    Gibson E. K. Jr., McKay D. S., Thomas-Keprta K. L., and Romanek C. S. (1996) Science, 274, 2125.

    The authors respond directly to Shearer and Papike's (1996) claim that sulfur isotope ratios on pyrites near the carbonate globules probably mean that they formed without help from bacteria. The authors note that the pyrite grains may not be relevant to McKay et al.'s hypothesis because pyrite is not in the carbonate globules, it did not grow with the structurally flawless submicroscopic magnetites, and is not associated with the PAHs. The submicroscopic sulfur-bearing minerals, pyrrhotite and greigite, which are part of McKay et al.'s hypothesis, will be very difficult to analyze for sulfur isotope ratios. These sulfur-bearing grains are so small that the carbonate and magnetite grains around them would also end up being analyzed for sulfur. The carbonate and magnetite grains don't contain sulfur, but they do contain lots of oxygen, and oxygen molecules can masquerade as sulfur atoms in these isotope analyses. Sulfur atoms with mass 32, 32S, can be mimicked by the oxygen molecule 16O16O; and sulfur atoms with mass 34, 34S, can be mimicked by the oxygen molecule 16O18O.
    Greenwood et al. (1997) report that the isotope ratio for sulfur from the carbonate globules, presumably from pyrrhotite, is nearly the same as for the pyrite grains. Their sulfur isotope of the pyrrhotite analyses are quite imprecise (d34S somewhere between +12 and -1), and it is not clear if they considered the possible interferences from molecular oxygen.

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