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 ¹⁶O¹⁶O^2- 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 (SO₄^2-) from water and convert them to sulfide ions (S^2-) 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 ¹⁶O¹⁶O^2- 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.

P>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 D^{1 7}O 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). < /P>

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 ~ 550-600°C) and titanomagnetite (Curie T ~ 350-400°C). 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 350°C 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, Fe_1-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 angu