Work continues apace on the ancient martian meteorite known as ALH84001. In this article, the author updates his survey of recent research on the question of ancient fossil life in the meteorite to help nonspecialists keep abreast of the debate. Each entry includes a first-paragraph capsule summary of the work followed by a few paragraphs of extended description. Finally, the author adds his own comments and perspectives in italic type.
Jull and coworkers measured the abundances of stable and radioactive isotopes of carbon in ALH 84001. Most of the carbon in ALH 84001 is from its carbonate mineral globules (as reported previously). Most of the remaining carbon is from Earth organic material, i.e., terrestrial contamination. A small fraction of the carbon (~8%) is too old to be Earth contamination, and is not (in chemistry and carbon isotopes) like carbon from the carbonate minerals. This carbon may be from organic material formed on Mars, or possibly a rare inorganic mineral (also from Mars).
Part of McKay et al.'s (1996) argument for traces of martian life in ALH 84001 is that the meteorite contains organic material, rich in PAH compounds, associated with its carbonate mineral globules. However, Becker et al. (1996) argued that this organic material is actually terrestrial contamination. To help resolve this issue, Jull and coworkers analyzed the isotopic composition of the carbon in the organic matter and the carbonate minerals of ALH 84001 (following Jull et al., 1997).
The principal clue used by Jull is the abundance of the radioactive isotope of carbon, carbon-14, in the organic material. Carbon-14 is used as an age-dating tool for archaeological and cultural artifacts (like the Shroud of Turin). Carbon-14 forms continuously and abundantly in the Earth's atmosphere. As soon as a carbon-bearing compound is isolated from the atmosphere (e.g., a tree dies and stops absorbing CO2 from the air), its carbon-14 starts decaying away with a half-life of 5730 years. Most of the organic matter in ALH 84001 contains significant amounts of carbon-14 which means that it is terrestrial contamination (there is no reasonable extraterrestrial source of so much carbon-14). Also, the carbon-14 gives an average age near 6000 years, which is approximately 7000 years after ALH 84001 fell to Earth. So, there is little doubt that most of the organic carbon in ALH 84001 is terrestrial contamination. In addition, the relative abundances of carbon-12 and carbon-13 (the d13C value) in the ALH 84001 organics are typical for carbon from living things on Earth.
The carbon in carbonate minerals in ALH 84001 is clearly not terrestrial it has little or no carbon-14, and a d13C value much higher than typical for Earth carbonates. Earlier, Jull et al. (1997) got similar results for carbonate minerals in a different sample of ALH 84001, although that sample had enough carbon-14 to suggest some chemical exchanges with Earth water.
However, a small part of the carbon in ALH 84001 might be martian organic material. This carbon was not dissolved away during acid treatment designed to remove carbonate minerals, so it is either organic or some (unknown) resistant mineral. This batch of carbon has no carbon-14, meaning that it is very old. Jull and coworkers take this ancient age to mean that this batch of carbon did not form on Earth it is martian.
This work appears to be carefully done, adequately documented, and carefully presented. It does not directly refute McKay et al.'s (1996) hypothesis of martian biological activity in ALH 84001, but it is not much of a confirmation, either. I have two comments about this work and possible evidence of martian biological activity in ALH 84001.
ALH 84001 contains hundreds of parts per million organic carbon, much more than other martian meteorites except EETA79001 (which Jull also analyzed in this paper). This high abundance of organic matter has been used to support claims of fossil martian biology in ALH 84001. However, ALH 84001 contains the same amount of organic carbon as do typical basalt meteorites from asteroids, even those found in Antarctica (Grady et al., 1997). Just as Jull and coworkers showed that most of the organic carbon in ALH 84001 is terrestrial contamination, Grady et al. (1997) showed that most of the carbon in asteroidal basalt meteorites is terrestrial contamination. In this way, ALH 84001 is quite average and was not contaminated any more than normal for a meteorite.
The most intriguing part of Jull's work, at least to me, is the extraterrestrial organic (?) material they found in ALH 84001. They found this carbon in a sample of ALH 84001 that had been treated to remove all of its carbonate minerals. At lower temperatures (<450°C) this treated sample released the same terrestrial carbon (both 14C and d13C) as the untreated sample. But at higher temperatures, the treated sample released some carbon without any carbon-14, meaning it was pre-terrestrial. This high-temperature, noncarbonate carbon could be organic matter, or could possibly be a rare, acid-resistant, as-yet-unidentified inorganic mineral. Many different kinds of organics can be released at these higher temperatures, including material like kerogen, graphite, and PAHs. So, it is tempting to say that Jull and co-workers detected the same PAHs that McKay et al. found (and probably also other high-temperature organic compounds). But most meteorite basalts from asteroids also contain similar amounts of high-temperature carbon (10-30 parts per million; Grady et al., 1997). Could it be that basalts in the solar system just have this many high-temperature carbon compounds, whether or not life was present?
Becker L., Glavin D. P., and Bada J. L.
(1997) Polycycic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta, 61, 475-481.
Grady M. M., Wright I. P., and Pillinger
C. T. (1997) Carbon in howardite, eucrite, and diogenite basaltic achondrites. Meteoritics Planet. Sci., 32, 863-887.
Jull A. J. T., Eastoe C. J., and Cloudt S. (1997) Isotopic composition of carbonates in SNC meteorites, Allan Hills 84001 and Zagami. Jour. Geophys. Res., 102, 1663-1669.
Bada and coworkers analyzed ALH 84001 for amino acids, chemicals that are essential in life as we know it on Earth. In the meteorite's carbonate globules are small amounts of amino acids, which are nearly identical (in proportions of acid species and in their chemical handedness) to amino acids in Antarctic ice. So, Bada and coworkers conclude that (essentially) all the amino acids in ALH 84001 are terrestrial contamination, carried into the meteorite by melted Antarctic ice.
Part of McKay et al.'s (1996) argument for traces of martian life in ALH 84001 is that the meteorite contains organic material mixed with its carbonate mineral globules. Last year, Bada's research group claimed the organic material is terrestrial contamination (Becker et al., 1996). Continuing this work, Bada and coworkers analyzed ALH 84001 and its carbonate minerals for amino acids. Amino acids are small organic molecules, the building blocks of proteins and enzymes in all living things on Earth. Earth life only uses a few of the many possible amino acids in fairly characteristic relative abundances, and only uses the L form of those amino acids. With these distinctive characters, amino acids are a sensitive test for Earth organic contamination in meteorites.
To analyze for amino acids, Bada and coworkers used a very sensitive technique developed in their laboratory. McKay et al. suggested that the signs of ancient martian life were associated with carbonate minerals in ALH 84001, so Bada and coworkers used a chemical extraction to separate amino acids in the carbonate globules from those elsewhere. First, they rinsed the samples of ALH 84001 in distilled water, and that extracted no amino acids at all. Then, they reacted the samples with weak hydrochloric acid, which should dissolve the carbonate minerals in the rock and release any amino acids associated with them. This acid solution was dried and part of it analyzed for free amino acids (those not chemically bound to anything else). Another part of the solution was dried and treated to liberate amino acids that were bound to other molecules (for example, this treatment would break proteins into their constituent amino acids). And finally, they analyzed the remainder of the meteorite that was not dissolved in acid (including the pyroxene and chromite mineral grains) for bound amino acids.
Bada and coworkers found that the amino acids in ALH 84001 were most abundant as bound acids associated with the carbonate minerals. There were almost no amino acids in the distilled water wash, the acid-insoluble residue, or as free amino acids in the acid solution. The part of ALH 84001 that dissolved in acid contained about 10 parts per million total amino acids (almost all chemically bound), while the rest of the rock contained only 75-100 parts per billion of amino acids.
The amino acids in ALH 84001 are almost exactly in the same proportion as in the Antarctic ice the proportions of DL-serine to glycine to L-alanine are approximately 3:3:1. In addition, there is a little D-alanine in Antarctic ice and in ALH 84001 [author's note: possibly from micrometeorites in the ice?]. This similarity of terrestrial and ALH 84001 amino acids leaves little doubt that they are primarily terrestrial contamination, derived from amino acids in the ice that was around ALH 84001.
The amino acids that Bada and coworkers found in ALH 84001 are from the Antarctic ice. But this fact is not a deathblow to the hypothesis that ALH 84001 contains traces of ancient martian life (McKay et al. 1996). Despite an exuberant press release from Scripps Oceanographic Institution, Bada's work is not a conclusive test of McKay's hypothesis. McKay et al. (1996) did not talk about amino acids, so the absence of preterrestrial amino acids does not refute their hypothesis at all. Of course, if Bada and coworkers had found abundant preterrestrial amino acids, it would have been strong support for McKay et al.'s hypothesis.
Two aspects of Bada's experiments are puzzling (although probably not very important). First, their acid treatment was designed to dissolve carbonate minerals, but it dissolved 20% of their carbonate-free sample of lunar rock. What actually dissolved from the lunar rock? Possibly feldspar? Could feldspar (or whatever) also have dissolved from ALH 84001, and would this change the conclusions? Second, their acid treatment seems to have increased the masses of their samples. For instance, sample 2 of ALH 84001 started at 463 milligrams, and ended up as 472.5 milligrams (text and Table 1). What is this extra mass? Could it be lab contamination that might carry amino acids?
Becker L., Glavin D. P., and Bada J. L. (1997)
Polycycic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice.
Geochim. Cosmochim. Acta, 61, 475-481.
McKay D. S., Gibson E. K. Jr., Thomas-Keprta K., and Vali H. (1997)
Reply to "No `nanofossils' in martian meteorite orthopyroxenite."
Nature, 390, 455-456.
Bradley et al. claim that the possible nanofossils found by McKay et al. (1996) in martian meteorite ALH 84001 are actually irregularities in the surfaces of mineral grains. These irregularities were accentuated by the metal coating that had to be put on the samples for electron microscope examination. So, Bradley and coworkers reject the hypothesis that ALH 84001 carries nanofossils of ancient martian life. In response, McKay et al. say that they also found the same surface irregularities, and that they are not possible martian nanofossils. The metal coating on the samples did not interfere with their identification of objects as nanofossils, because they did control experiments on metal coatings and know what the coating does. (G. J. Taylor has posted a nice summary of these letters on the World Wide Web at http://www.soest.hawaii.edu/PSRdiscoveries/Dec97/LifeonMarsUpdate2.html.)
Bradley and coworkers examined fracture surfaces of ALH 84001 using nearly the same methods that McKay et al. (1996) used. They found sausage-shaped surface features, approximately 100-400 nanometers (billionths of a meter) long, that looked (to them) similar to the possible nanofossils in the McKay et al. (1996) paper and in later magazine articles and press briefings. Bradley found these sausage-shaped features on the carbonate minerals (as McKay's "nanofossils" were) and also on the host silicate minerals. By observing the sample from many angles (in their electron microscope), Bradley found that the "sausages" were not sitting on the host minerals, but were actually ridges poking out of the host minerals.
Bradley also did a few experiments on how the metal coating on the samples changes the shapes of surface features. They found (as have others) that metal coatings tend to make surface features look segmented (the thicker, the more segmented) an appearance that McKay's group had suggested might reflect cell boundaries.
McKay et al. respond that they have also seen ridges on minerals' surfaces that Bradley et al. found same sizes, shapes, and textures. McKay and coworkers suggest that the ridges are grains of clay minerals formed during "incipient" alteration of the host minerals. But these surface ridges, say McKay et al., are not the possible nanofossils they described in 1996 and subsequently. Their possible nanofossils differ from the Bradley ridges by not being parallel with each other, by intersecting with each other at distinct angles, by being curved, and by being rather isolated from each other.
McKay and colleagues also dispute that their identifications of possible nanofossils (here and earlier) were compromised by metal coatings on the samples. They reiterate that they did control experiments on the effects of metal coatings, and that the nanofossil morphologies do not result from coating. McKay also notes that some of Bradley's samples were coated with gold alone, rather than a gold-palladium alloy, and that gold coatings are known to make larger artifacts (artificial structures) than are gold-palladium.
This exchange focuses on two important issues about the possible martian nanofossils in ALH 84001: (1) How can you recognize a nanofossil? and (2) How does laboratory preparation change the surfaces of the samples? Unfortunately, short "correspondence and reply" tidbits cannot carry enough scientific "meat" to resolve these issues.
(1) How can you recognize that a shape in ALH 84001 is a martian nanofossil? In 1996, McKay et al. cited ". . .regularly shaped ovoid and elongate forms ranging from 20 to 100 nanometers in longest dimension" as possible nanofossils (their Figure 6 and Kerr, 1996). At their big NASA press conference, McKay and colleagues also presented an image of aligned sausage-shaped objects in a grid formation as being possible nanofossils (http://www.lpi.usra.edu/lpi/meteorites/s96-1229.gif). Bradley et al. found features that matched these characteristics and showed that they were not biological.
Here, McKay et al. seem to have changed their definition of martian nanofossils. Nanofossils are still elongate and ovoid. Now, however, they do not appear in parallel, but display "intersecting alignments"; they are relatively isolated from each other; they are significantly curved (their Fig. 2c); and they are much larger, up to 750 nanometers long. With these new criteria, many of McKay's own objects may not qualify as nanofossils: the ovoids of Figure 6a in McKay et al. (1996); the famous segmented worm shape (http://www.lpi.usra.edu/lpi/meteorites/Photomicrograph.gif; Kerr, 1996); and the aligned sausage-shaped objects (http://www.lpi.usra.edu/lpi/meteorites/s96-1229.gif).
(2) How does the metal coating (for electron microscopy) affect the surfaces of minerals in ALH 84001? This question has been argued, mostly in private, since McKay et al. (1996) was published. In other words, are some of the "nanofossils" in ALH 84001 completely artificial, made during metal coating, and completely irrelevant to life on Mars? Believable answers to these questions will only come from carefully controlled experiments, where fragments of ALH 84001 are coated with various thicknesses of different metals and alloys. Bradley et al. report that they did a few experiments in this program; McKay et al. report that they did a series of experiments on a different sample (a lunar glass). Unfortunately, neither set of experiments has been reported in any detail, and I am still not sure of what metal coatings (gold or gold/palladium) do to surface morphology at these very small sizes.
Kerr R. A. (1996)
Ancient life on Mars?
Science, 273, 864-866.
Murty S. V. S. and Mohapatra R. K. (1997)
Nitrogen and heavy noble gases in ALH 84001: Signatures of ancient Martian atmosphere.
Geochim. Cosmochim. Acta, 61, 5417-5428.
About 4 billion years ago, traces of noble gases and nitrogen from the martian atmosphere were trapped in ALH 84001. The isotopic compositions and relative abundances of the heavy noble gases xenon (Xe) and krypton (Kr) are similar to the present-day martian atmosphere. So, Mars' unusual Xe and Kr compositions and abundances were set earlier than 4.0 billion years ago. Argon trapped in ALH 84001 has less 40Ar from radioactive 40K (potassium) than Mars' present-day atmosphere, suggesting that it has continued to gain 40Ar over time [author's note: e.g., by volcanic outgassing]. Nitrogen trapped in ALH 84001 has much less of the heavy isotope 15N, consistent with loss of the light isotope 14N (and other lightweight gases) from Mars' atmosphere over the last 4 billion years.
The elemental and isotopic composition of the martian atmosphere has been a real puzzle. It is greatly depleted in the light stable isotopes of all gas elements, from hydrogen to xenon. For instance, the abundance ratios of light to heavy xenon isotopes (e.g., 128Xe/136Xe) are approximately 0.7 times that in the Sun (Zahnle, 1993). It is a mystery how and when the lightweight isotopes were removed, but a separate process must have acted for each element (Pepin, 1994). Any process strong enough to remove a lot of, say, 128Xe compared to 136Xe, would certainly have removed all of the lighter gaseous elements completely (like krypton, argon, and nitrogen). Similarly, any process capable of separating 36Ar from 38Ar to the extent seen in the martian atmosphere would have removed essentially all of its nitrogen.
One way to help understand the martian atmosphere would be to learn how its composition has changed through time. Its present-day atmosphere (analyzed by Earth telescopes and the Viking landers) is the same as the atmosphere of 180 million years ago, as trapped in some martian meteorites (most notably EETA79001). Recognizing that ALH 84001 has retained noble gases (like argon) for 4 billion years, Murty and Mohapatra investigated whether it might contain trapped martian atmosphere from that time. They used standard techniques separating the meteorite into its minerals by their density, heating the samples up in steps of 200°C (or more) to 1600°C, and collecting the gases given off by each sample in each temperature step. The gases were separated, and the isotopic composition of each element was measured with a mass spectrometer.
Murty and Mohapatra found that ALH 84001 contains significant quantities of nitrogen, argon, krypton, and xenon gases. Most gases (xenon, krypton, nitrogen, and argon36) all were released by the samples at nearly the same temperatures, suggesting that they are from the same trapped atmosphere component. ALH 84001 contains a nitrogen component comparable to Mars "mantle"(the Chassigny meteorite) and a trapped component with d15N ³ +85; the current Mars atmosphere has d15N » +620. From the isotopic composition of the argon (in mineral and temperature and temperature separates), the authors estimate that the trapped gas has 40Ar/36Ar £ 1400, while the current Mars atmosphere has a value of 2400. The trapped gas in ALH 84001 has 14N/36Ar, about 60 times the value for the current Mars atmosphere. The Kr and Xe isotope compositions of most of the trapped gas are similar to the current martian atmosphere, or the current atmosphere as modified by groundwater processes.
Murty and Mohapatra infer that this trapped gas component is a sample of the martian atmosphere from 4 billion years ago, the age when argon gas was last lost from ALH 84001. The ancient and modern atmospheres have similar isotopes and relative abundances of xenon and krypton (the heaviest noble gases), which means that the hydrodynamic escape processes that set these abundances (Pepin, 1994) were complete by 4 billion years ago. The higher 40Ar/36Ar in the current atmosphere reflects production of 40Ar from potassium over the history of Mars. And the decrease in 14N/36Ar may reflect loss of nitrogen (through sputtering) into space over the last 4 billion years.
This work is not directly related to the "life in ALH 84001" folderol. It is part of the long-term effort to learn about Mars' ancient environments through clues in the martian meteorites. The noble gases and nitrogen hold great promise in unraveling the evolution of Mars' atmosphere, particularly why it is so thin now (surface pressure of ~1/200 that of Earth) and where its water has gone. But this work, no matter how good, is not likely to be the final word on ALH 84001. The uncertainty here is not from Murty and Mohapatra's analyses, but in the inherent variability of samples of ALH 84001 and the many assumptions that must be made to unravel the noble gas story.
First, it appears that different samples of ALH 84001 contain different quantities, proportions, and isotope compositions of the noble gases and nitrogen. This is perhaps not too surprising, as the mineral proportions and chemical composition of ALH 84001 are rather variable, for instance potassium abundances (108 vs. 200 parts per million: Mittlefehldt, 1994b; Dreibus et al., 1994). For the noble gases, this variability can appear as differences in the proportion of 40Ar that comes from radioactive potassium (this paper; Turner et al., 1997), and as differences in xenon isotope ratios (Fig. 9 of this paper vs. Fig. 2 of Swindle et al., 1995, and Fig. 3 of Miura et al., 1995). Variabilities like these in elemental and isotopic abundances suggests that the gases in ALH 84001 came from many different sources and were not mixed well. It will be possible, eventually, to sort out the different sources (or components) of gas; now, it seems to be a muddle.
Second, interpretation of noble gas and nitrogen abundances is not simple and relies on some (fairly complex) correction schemes and underlying assumptions. Different research groups have not treated their data the same way, so when their results appear in conflict, it may be difficult for a nonspecialist (like me) to understand why. For instance, all groups so far have agreed that some of the argon in ALH 84001 comes from atmosphere trapped in the mineral grains. Turner et al. (1997) present evidence that this trapped gas is like argon from the Earth's atmosphere: 40Ar/36Ar = 295. Murty and Mohapatra infer that the trapped argon is ancient martian, with 40Ar/36Ar £ 1410. Miura et al. (1995) and Goswami et al. (1997) use the current martian atmosphere value of 40Ar/36Ar » 2400. Swindle et al. (1995) do not infer a specific 40Ar/36Ar for the trapped component. Is each group justified, given their data and the intrinsic variability of ALH 84001, or have some (or all) of them made unjustified simplifications in their data treatment?
Dreibus G., Burghele A., Jochum K. P., Spettel B., Wlotzka F., and Wänke H. (1994)
Chemical and mineral composition of ALH 84001: A martian orthopyroxenite (abstract).
Meteoritics, 29, 461.
Goswami J. N., Sinha N., Murty S. V. S., Mohapatra R. K., and Clement C. J. (1997)
Nuclear tracks and light noble gases in Allan Hills 84001: Pre-atmospheric size, fall characteristics, cosmic ray exposure duration, and formation age.
Meteoritics Planet. Sci., 32, 91-96.
Mittlefehldt D. W. (1994)
Meteoritics, 29, 900.
Miura Y. N., Nagao K., Sugiura N., Sagawa H., and Matsubara K. (1995)
Orthopyroxenite ALH84001 and shergottite ALHA77005: Additional evidence for a martian origin from noble gases.
Geochim. Cosmochim. Acta, 59, 2105-2113.
Pepin R. O. (1994)
Evolution of the martian atmosphere.
Icarus, 111, 289-304.
Swindle T. D., Grier J. A., and Burkland M. K. (1995)
Noble gases in orthopyroxenite ALH84001: A different kind of martian meteorite with an atmospheric signature.
Geochim. Cosmochim. Acta, 59, 793-801.
Turner G., Knott S. F., Ash R. D., and Gilmour J. D. (1997)
Ar-Ar chronology of the Martian meteorite ALH84001: Evidence for the timing of the early bombardment of Mars.
Geochim. Cosmochim. Acta, 61, 3835-3850.
Zahnle K. J. (1993)
Xenonological constraints on the impact erosion of the early martian atmosphere.
Jour. Geophys. Res., 98, 10899-10913.
Greenwood J. P., Riciputi L. R., and McSween H. Y. Jr. (1997)
Sulfide isotopic compositions in shergottites and ALH 84001, and possible implications for life on Mars.
Geochim. Cosmochim. Acta, 61, 4449-4453.
The authors measured the abundance ratio of sulfur isotopes (34S/32S) in minerals of martian meteorites to see if the sulfur in ALH 84001 had been processed by sulfate-reducing bacteria, as implied by McKay et al. (1996). They found no evidence for the action of sulfate-reducing bacteria in ALH 84001, and so reject the McKay et al. (1996) hypothesis that ALH 84001 contains traces of ancient martian life.
The element sulfur occurs as two stable (not radioactive) isotopes with masses of 32 and 34, 32S and 34S. Most sources of sulfur have abundance ratios of 34S/32S that are very similar to the average in the solar system. However, sulfur that has been processed by bacteria (or other life forms) can have distinctly different abundances of these isotopes. The greatest changes in S isotopes come from sulfate-reducing bacteria, which take sulfate ions (SO42-) from water and convert them to sulfide ions (S2-) in water or as solid sulfide minerals. Sulfate-reducing bacteria, when they have lots of sulfate in water around them, can form sulfide minerals with ~5% less 34S than the sulfate in the water. This difference is easily detected, and has been used (on Earth) as a guide to the action of these bacteria.
To estimate the sulfur isotope ratio for bulk Mars, Greenwood et al. measured sulfur isotope ratios in the martian basalt meteorites (Shergotty, Zagami, EETA 79001, LEW 88516, and QUE 94201). The sulfur isotope ratios for these meteorites are within 0.3% of the solar system average. In ALH 84001, they first measured sulfur isotopes in millimeter-sized grains of pyrite (FeS2), which are not associated with the possible traces of ancient martian life (Gibson et al., 1996; but see Shearer et al., 1997). The pyrite had variable and slightly "heavier" sulfur than the other martian meteorites, with 34S/32S from approximately 0.2 to 0.75% larger than the solar system average; this agrees with earlier work of Shearer et al. (1996). Finally, they analyzed the sulfur-rich outer zone of a single carbonate globule from ALH 84001 iron sulfide minerals in the carbonate globules were claimed by McKay et al. (1996) to have formed through the action of martian biological organisms. The outer parts of the carbonate globules contain carbonate and oxide minerals in addition to the sulfides, so Greenwood et al. did not get as precise a result here as for the pure sulfide minerals. Also, they had to apply a small correction for pairs of oxygen atoms masquerading as sulfur. But the 34S/32S for the sulfide-rich region of the carbonate globule is identical to the nonbiological pyrite in ALH 84001: 0.6% larger than the solar system average.
The nonbiological and possibly biological sulfide minerals in ALH 84001 have nearly identical 34S/32S ratios. Greenwood et al. take this similarity to suggest that sulfur (in the possibly biological sulfides) in the carbonate globules was not processed by sulfate-reducing bacteria that the McKay et al. (1996) hypothesis is wrong. Rather, they suggest that all the sulfides in ALH 84001 formed from a high-temperature fluid (too hot for life as we know it), probably generated by an asteroid impact onto Mars. The variations in sulfur isotope ratios suggest mixing of "light" and "heavy" sulfur, the former perhaps from igneous rocks, the latter perhaps from Mars' surface.
This paper is much weaker than it could have been because the authors did not document their experiments adequately. The analyses of sulfur isotopes in the pure sulfide minerals (pyrite and pyrrhotite) seem superb; they follow carefully described procedures, are based on good standards, and are repeatable. But the analysis of sulfur isotopes in the carbonate globule, the critical analysis for evaluating the hypothesis of ancient martian life (McKay et al., 1996), will be suspect until Greenwood et al. document it fully.
The problem with Greenwood's analysis for sulfur isotopes in the carbonate globule is that they did not analyze only sulfide minerals. Their instrument, an ion microprobe, shoots cesium ions at the sample, and collects ions from the sample that are sputtered off by the cesium. Sulfur come off as S2- ions, both as the "light" 32S2- and the "heavy" 34S2-. Two problems are possible when the sulfur is present as sulfides among other minerals, like carbonates and oxides.
If the sulfide minerals are mixed with oxide and carbonate minerals, the ion 16O16O 2- might be formed in abundance (from the carbonates and oxides) and might pass as 32S2-, as both ions have the same masses and charges. If there were lots of 16O16 O 2- passing for 32S2-, the sulfur would appear "lighter" than it really is.
It is also possible that having sulfur-bearing minerals among other minerals influences the way that the sulfur sputters off the sample and into the analyzer. For instance, sulfur in sulfides mixed with carbonates and oxides might sputter more like a sulfate than a sulfide, and require a different corrective procedure.
Greenwood et al. were aware of these potential problems, and reported that they (1) corrected for the presence of 16O16O 2- (less than 0.2% in their value of 34S/32S), and (2) did experiments to show that their sulfur isotope corrective procedures gave consistent results for 34S/32S with or without admixed carbonates and oxides. But they gave no details on the 16O16O 2- correction, and no results for the experiments on mixtures. Since we cannot see the details of their corrections, and the results of their experiments, we are really asked to take on faith that Greenwood did both properly. Some scientists, trusting the authors implicitly, will take their work on faith. Others, who do not accept the conclusions of Greenwood et al., will point to these problems as cause for discounting the paper entirely. And those who wish to "trust, but verify" will merely be disappointed.
Gibson E. K. Jr., McKay D. S., Thomas-Keprta K. L., and Romanek C. S. (1996)
Evaluating the evidence for past life on Mars (letter).
Science, 274, 2125.
Shearer C. K., Layne G. D., Papike J. J., and Spilde M. N. (1996)
Sulfur isotope systematics in alteration assemblages in martian meteorite ALH84001.
Geochim. Cosmochim. Acta, 60, 2921-2926.
Shearer C. K., Spilde M. N., Wiedenbeck M., and Papike J. J. (1997)
The petrogenetic relationship between carbonates and pyrite in martian meteorite ALH 84001 (abstract).
Lunar Planet. Sci. XXVIII, 1293-1294.