Recent Scientific Research on ALH84001 Explained,
--with Insightful and Totally
by Allan Treiman,
I. After Science magazine published McKay et al.'s (1996a) article suggesting that they had recognized traces of ancient martian life in ALH 84001, many scientists wrote letters to Science disputing all or part of their results. Science collected these comments, and responses to them, as "Evaluating the Evidence for Past Life on Mars," Science, Vol. 274, pp. 2119-2125. These summaries and commentaries are in the order that Science presented the originals.
Anders E. (1996) Science, 274, 2119-2121.
After praising the quality and depth of their observations, Anders comments that McKay et al. (1996a) did not consider non-biological explanations of their discoveries: "For all these observations, an inorganic explanation is at least equally plausible, and, by Occam's Razor, preferable." Anders then suggests nonbiological explanations for most of the chemical evidence for martian life in McKay et al.
Anders raises two objections to the description of PAHs in ALH 84001 as implying biogenic activity. First, PAH molecules form as readily from nonbiological chemical compounds as from biological compounds. Given enough time and/or elevated temperature, PAHs form readily from other organic materials; this process is documented in nature and utilized in industry. Second, the spatial association of PAH molecules and the carbonate globules could have arisen without life. Formation of PAHs can be accelerated (i.e., catalyzed) by the mineral magnetite, and sub-microscopic grains of magnetite are abundant in the carbonate globules.
Anders also presented five objections to the arguments of McKay et al. concerning the minerals and chemical zoning of the carbonate globules.
1.The chemical zoning patterns in the carbonate globules could be a natural result of mineral solubilities, and need not imply the action of life.
2.The association of magnetite, iron sulfides (pyrrhotite), and carbonate minerals in ALH 84001 could form without the presence of life, as similar associations have formed in the carbonaceous chondrite meteorites.
3.The areas of partially dissolved carbonate minerals could form at normal temperatures and water compositions, without the action of life.
4.The greigite(?) iron sulfide mineral that McKay et al. found was not characterized well, and was not compared with non-biogenic greigite. Without this comparison, one cannot tell if the greigite(?) is actually relevant to the question of life.
5.Finally, the structure of the carbonate globules (claimed by McKay et al. to be evidence for a biological origin) was not compared to the structures of carbonate globules formed without assistance from life. Without this kind of comparison, one cannot tell if the structures of the carbonate globules are relevant or not.
Before the matter of ancient martian life in ALH 84001 is completely resolved, all of Anders' points will need to be studied. McKay et al. (1996b) and Clemett and Zare (1996) provide some answers in their responses to this comment.
The fundamental issue behind Anders' comment is scientific proof itself. Can the martian-life-in-ALH 84001 hypothesis be examined piece-by-piece, one line of evidence at a time? Or must all the evidence be considered together, as one complete package?
In natural sciences, it is rarely possible to prove that an idea is true--"proof" consists mostly of showing that an idea fits ("is consistent with") all the facts, and that all other ideas don't fit the facts or are too complicated. Most often, though, scientists can think up many different ideas that can fit all the facts. Then, scientists will commonly quote "Occam's Razor": The simplest idea is more likely than the complicated ideas. Unfortunately, what is simple to one scientist is needlessly complex to another. McKay et al.'s paper and Anders' comment use different ideas of simplicity, and so arrive at different preferred conclusions.
McKay et al. invoked Occam's Razor (without naming it) in justifying a biological origin for all their observations: "Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars." From this perspective, McKay et al. did not need to consider nonbiological explanations for each observation, only nonbiological explanations of all the observations at once. They did not find any nonbiological explanations, and so had to accept the idea of martian life.
On the other hand, Anders invoked Occam's Razor (quoted above) to justify nonbiological processes for each individual observation. Anders did not search for a single nonbiological explanation for all the evidence, and did not consider how likely it was that all his proposed processes could have affected small areas in a single rock.
To some extent, then, Anders and McKay et al. are not looking at the evidence in the same way; McKay et al. are "holists," and Anders is a "reductionist." For the possible martian fossils, it remains to be seen which view of the world is more useful.
Shearer C. K. and Papike J. J. (1996) Science, 274, 2121.
Here, the authors summarize their sulfur isotope measurements, which were reported earlier in Shearer et al. (1996) and are described below. Shearer and Papike emphasize that the pyrite mineral grains that they analyzed earlier are related to the carbonate globules, and that the sulfur in the pyrite is enriched in the stable isotope 34S compared to the solar system average. Sulfur-eating bacteria on Earth produce minerals such as pyrite that are strongly depleted in 34S, so it is unlikely that the pyrite in ALH 84001 was made by Earth-type bacteria. Martian bacteria could still be involved, however, if Mars itself was much richer in 34S than the Earth is, or if martian bacteria process sulfur differently than Earth bacteria. For more detail, see the discussion of Shearer et al. (1996) below.
Gibson et al. (1996) respond directly to this comment. McKay et al. (1996a) did not claim that the pyrite in ALH 84001 was biogenic, so, strictly speaking, this report by Shearer and Papike is not relevant to the current hypothesis of ancient martian fossils in ALH 84001. However, the pyrite crystals are spatially associated with the carbonate globules, and it would have seemed reasonable that the pyrite and the carbonates grew from the same fluids with the same sulfur isotope abundances. On the other hand, if the pyrite had a deficiency of 34S (such as might be expected from biogenic pyrite on Earth), it might possibly have been cited by Gibson et al. (1996) as further evidence of biogenic activity in ALH 84001.
This work has continued in Shearer (1997) and Shearer and Papike (1997).
Bell J. F. (1996) Science, 274, 2121-2122.
Bell's comment centers on the PAH organic molecules found in ALH 84001 by McKay et al. (1996); Bell accepts that these PAHs are martian, but not that they imply martian life. He suggests that the PAHs may have come from meteorites falling onto Mars, just as a few percent of the Moon's soil is made of meteorite debris. Specifically, Bell suggests that the PAHs in ALH 84001 came from material like the C2 carbonaceous chondrite meteorites, and suggests that the sources of this C2 material included the moons of Mars, Phobos and Deimos.
McKay et al. (1996a) and Becker et al. (1997) agree with Bell that the PAHs in ALH 84001 are similar to those in the C2 carbonaceous chondrites. The PAHs in these meteorites are not identical, but are they similar enough to suggest a common origin? Bell and Becker say "yes," McKay et al. say "no, especially in light of the associated evidence." Bell is correct that a few percent of the Moon's soil is made of meteoritic material like C2 carbonaceous chondrites (a point I mistakenly disputed in earlier versions of this commentary). Although most meteorites are not carbonaceous, most of the mass that lands on planets is in dust (so-called interplanetary dust), and has a composition like that of carbonaceous chondrites. The moons of Mars are very dark; their darkness might be from the carbon in carbonaceous chondrite material, but their darkness might have other causes (Murchie et al., 1991).
Clemett S. J. and Zare R. N. (1996) Science, 274, 2122-2123.
Clemett and Zare are among the authors in the original McKay et al. paper, and they respond to comments of Anders and Bell related to PAHs, the organic molecules called polycyclic aromatic hydrocarbons. Clemett and Zare emphasize that the PAHs they found in ALH 84001 are not laboratory contaminants, and are apparently only a small part of all the organic materials in ALH 84001. They agree with Anders (1996) that some of the PAHs in ALH 84001, the low-mass ones, could have formed by inorganic processes at high temperature. The high-mass PAHs, although less abundant, are very similar to the break-down products of kerogen, a variety of solid organic material that is common on Earth and in the carbonaceous chondrite meteorites. Earth kerogen formed from living matter, and meteorite kerogen did not. Clemett and Zare leave two questions: How could nonbiologic kerogen get into an igneous rock, one that solidified from molten lava; and how could nonbiologic kerogens (or PAHs) come to be associated only with the carbonate globules in ALH 84001?
As an aside, Clemett and Zare also reply to comments from Simoneit and Hites and from Requejo and Sassen, but neither of these comments was printed in Science.
Clemett and Zare agree that some nonbiological processes could have produced the distribution and abundances of PAHs that they observed in ALH 84001: The low-mass PAHs could be the product of inorganic reaction at high temperature, and the high-mass PAHs could form by the low-temperature reaction of inorganic kerogen. But the issue is whether the PAHs in ALH 84001, in their association with the carbonate globules, are more easily explained by biological or nonbiological mechanisms. A nonbiological scenario would have to start with carbon-rich gas reacting at high temperature to form the low-mass PAHs. Then, nonbiologic kerogen, from some other source, would have to decompose at low temperature into high-mass PAHs. Either the kerogen or the high-mass PAHs would have to infiltrate ALH 84001, adhere only to the carbonates, and not displace the low-mass PAHs already in place. Is this sequence of events actually simpler and more believable than the growth, death, and decomposition of martian bacteria?
McKay D. S., Thomas-Keprta K. L., Romanek C. S., Gibson E. K. Jr., and Vali H. (1996b) Science, 274, 2123-2125.
Here, McKay et al. respond directly to Anders' (1996) comments about minerals in the carbonate globules and about the morphology of possible fossil shapes in ALH 84001; Clemett and Zare responded to Anders' comments on PAHs.
Anders' comments stressed the similarity of the carbonate globules and their minerals to some grains in the CI carbonaceous chondrite meteorites. McKay et al. agree that similarities are present, but emphasize the significant differences between ALH 84001 and the CI carbonaceous chondrites. In particular, ALH 84001 is an igneous rock, while the CIs have been altered at low temperatures to clays, serpentine, and similar water-bearing silicate minerals. McKay's responses to Anders' comments are keyed to Anders' points (as above).
1.McKay et al. agree with Anders that the chemical zoning pattern in the carbonate globules could have been produced by inorganic crystallization. They stress, however, that the repetitive (oscillatory) zoning pattern and composition difference between one globule and another can only arise from complex inorganic processes.
2.Anders compared the carbonate-magnetite-sulfide minerals in ALH 84001 to those in CI carbonaceous chondrite meteorites; McKay et al. respond that, in effect, the CIs are not good analogies. Magnetite grains in carbonate minerals are much larger in CIs than in ALH 84001. And magnetite grains in carbonate minerals in CIs do not have cuboid shapes as they do in ALH 84001.
3.McKay et al. agree with Anders that the partially dissolved carbonate grains in the carbonate globules could have formed in nearly neutral (nonacidic or alkaline) water, and do not require the moderate acidity invoked in McKay et al. (1996a). McKay et al. re-state that the globular morphology of the ALH 84001 carbonates is similar to those formed by bacteria on Earth, and unlike the carbonate areas formed inorganically in the CI carbonaceous chondrites. They stress, however, that no matter what the exact water composition, no simple inorganic process can form all the observed structures and minerals in the carbonate ellipsoids.
4.On the matter of greigite(?) in ALH 84001, Anders had hoped to see it compared to non-biogenic greigite. McKay et al. respond that life seemed to be involved with formation of all greigite on Earth, at least all the greigite that they were aware of. Living organisms either produce greigite directly themselves, or produce the hydrogen sulfide gas that goes to form greigite.
5.Anders commented that the structures of the carbonate globules should have been compared directly to carbonates that grew without assistance from life. McKay et al. respond that the shapes of possible fossil forms is not yet definitive proof that they are real fossils, that similar shapes have not been found in lunar or asteroidal meteorite samples, and that more work is needed. They also agree with Anders that a proof that the fossil shapes actually are fossils would make all the other arguments irrelevant.
I see two underlying themes in this response: that ALH 84001 is unique, and that the minerals and structures of the carbonate globules are too complex for any simple inorganic processes. There is, of course, no doubt that ALH 84001 is unique. But Anders and McKay et al. disagree on whether the carbonate globules in ALH 84001 are so unusual that seemingly similar structures in the CI carbonaceous are not relevant. It has been suggested that the CI carbonaceous chondrites are from Mars (Brandenburg, 1996), but most evidence seems to suggest otherwise (Treiman, 1996).
McKay et al. emphasize the complexity of the carbonate globules, both in the chemical zoning of their carbonate minerals and in the groupings of minor minerals in the carbonates. The complexity alone suggests to them the action of complex biological systems, and want to consider all the evidence in McKay et al. (1996a) as a systematic whole, and not as a set of separate pieces. Quoting from their response, the formation of the carbonate globules
". . . cannot be simple equilibrium . . ., and must include changing conditions and kinetic effects. Whether such models are more plausible than biogenic models is a matter of judgment."
As an aside, the fifth point of Anders' comments seems to refer to the shapes and structures of the carbonate globules, while McKay et al. here responded about the sausage-shaped things that might be fossil bacteria. Some critical sentence or idea may have been lost.
Gibson E. K. Jr., McKay D. S., Thomas-Keprta K. L., and Romanek C. S. (1996) Science, 274, 2125.
The authors respond directly to Shearer and Papike's (1996) claim that sulfur isotope ratios on pyrites near the carbonate globules probably mean that they formed without help from bacteria. The authors note that the pyrite grains may not be relevant to McKay et al.'s hypothesis because pyrite is not in the carbonate globules, it did not grow with the structually flawless submicroscopic magnetites, and it is not associated with the PAHs. The submicroscopic sulfur-bearing minerals, pyrrhotite and greigite, that are part of McKay et al.'s hypothesis will be very difficult to analyze for sulfur isotope ratios. These sulfur-bearing grains are so small that the carbonate and magnetite grains around them would also end up being analyzed for sulfur. The carbonate and magnetite grains don't contain sulfur, but they do contain lots of oxygen, and oxygen molecules can masquerade as sulfur atoms in these isotope analyses. Sulfur atoms with mass 32, 32S, can be mimicked by the oxygen molecule 16O16O; and sulfur atoms with mass 34, 34S, can be mimicked by the oxygen molecule 16O18O.
Greenwood et al. (1997) report that the isotope ratios for sulfur from the carbonate globules, presumably from pyrrhotite, is nearly the same as for the pyrite grains. Their sulfur isotope of the pyrrhotite analyses are quite imprecise (d34S somewhere between +12 and _1), and it is not clear if they considered the possible interferences from molecular oxygen.
II. A number of complete scientific papers on ALH 84001 are related to McKay et al.'s hypothesis that ALH 84001 contains traces of ancient martian life. These summaries and commentaries are arranged chronologically.
Shearer C. K., Layne G. D., Papike J. J., and Spilde M. N. (1996) Sulfur isotope systematics in alteration assemblages in martian meteorite ALH 84001. Geochim. Cosmochim. Acta, 60, 2921-2926.
The element sulfur has two stable (not radioactive) isotopes, 32S and 34S. The relative abundances of these sulfur isotopes, called the sulfur isotope ratio, can be affected by chemical processes, including metabolism by bacteria. Many Earth bacteria can "eat" sulfur compounds and use them as fuel for growth. Sulfur processed this way by bacteria is typically very depleted in 34S compared to the starting sulfur. Nonbiological processes can enrich or deplete sulfur in 34S, but usually not so much as biological processes.
The authors here analyzed the isotopic composition of three pyrite grains associated with the carbonate globules of ALH 84001. The pyrite grains all were enriched in 34S compared to the solar system average; in the jargon, they had d34S (pronounced "delta thirty-four S") between 5 and 8 "per mil," (or parts per thousand). These enrichments in 34S suggest that the pyrite (and also the carbonate globules) formed at 'low' temperatures, and that the sulfur in the pyrite was probably never processed by bacteria like those on Earth.
However, these results are ambiguous because the isotope ratio in Mars' starting sulfur is not well known . If Mars' starting sulfur has d34S near zero (the solar system average), the high d34S of the pyrites could not come from biological processing, at least by bacteria like those on Earth. Nor could the high d34S develop during high-temperature chemical processes. More likely, the pyrites grew from alkaline, oxygen-poor water at less than 150°C. Lunar soils also have a high d34S, which develops as meteorite impacts vaporize some of the soils.
On the other hand, if Mars' starting sulfur was rich in 34S (had a high value of d34S), the isotopic composition of the sulfur could be consistent with either a high temperature or a biogenic origin. At high temperatures, inorganic processes do not separate the sulfur isotopes well, so a fluid rich in 34S would deposit pyrite rich in 34S. Acidic waters at low temperature also would not separate sulfur isotopes well, so a fluid rich in 34S would deposit pyrite rich in 34S. If Earth-type sulfur-eating bacteria were fed sulfur that was very rich in 34S, they would accumulate in them sulfur that was not so rich in 34S, perhaps similar to the sulfur in the pyrites. Of course, if martian bacteria process sulfur differently from Earth bacteria, all bets are off.
It is easy to think that a low formation temperature for the carbonates in ALH 84001 means that they formed from martian life. But temperature and biology are separate issues. Here, Shearer et al. infer that the pyrite and carbonates in ALH 84001 formed at low temperature without life!
Since this work was published, Greenwood et al. (1997) have also analyzed the isotopic composition of sulfur ALH 84001, and in martian meteorites that have no known or suspected signs of life in them. For ALH 84001, Greenwood et al. got essentially the same sulfur isotope values as this paper; for the other martian meteorites, Greenwood et al. got values between about d34S = +3 and _3. These low numbers, so close to the average for the solar system, suggest that Mars' original sulfur was not very different from the solar system average, and so support Shearer's inference of a nonbiological origin for the pyrite. The temperature of pyrite formation is not clear yet: Shearer et al. suggest low temperature, while Greenwood et al. suggest high temperature. This work is continued in Shearer and Papike (1997) and Shearer (1997).
Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1996) Magnetite whiskers and platelets in ALH 84001 martian meteorite: Evidence of vapor phase growth. Geochim. Cosmochim. Acta, 60, 5149-5155.
McKay et al. (1996) found that submicroscopic magnetite grains in the ALH 84001 carbonate globules are ". . . cuboid, teardrop, and irregular in shape" and have ". . . no structural defects." These magnetite crystals are similar to crystals produced by bacteria on Earth, and so McKay et al. suggested that the magnetites in ALH 84001 could have been made by martian bacteria.
The authors here show that the submicroscopic magnetite grains also occur in other shapes and with structural defects. Using transmission electron microscopy, the authors discovered whisker-shaped magnetite crystals, five times as long as they are wide (10 millionths of a millimeter by 50 millionths of a millimeter). Many of these magnetite whiskers contain a common kind of structural defect, a screw dislocation. The authors also discovered blade- and plate-shaped crystals of magnetite, and many of them contain a structural defect called twinning.
On searching through other technical papers, the authors found that magnetite (and similar substances) grow in whisker shapes only from hot gases, hotter than 500°C. Hot gas like this occurs in nature near volcanos, in structures called fumaroles, where the hot gases from a volcano or lava flow escape into the air. In fact, whisker-shaped magnetite crystals were reported from a fumarole deposit in Indonesia by Symonds (1993). Also, Bradley et al. could find no descriptions of bacterial magnetites that were blade-shaped, plate-shaped, whisker-shaped, or contained structural defects.
The authors conclude that the magnetites in the ALH 84001 carbonate globules formed at high temperatures, and not from biological processes. In addition, they note that the magnetite whiskers are approximately the same sizes and shapes as some of the possible fossilized bacteria shown in the McKay et al. (1996) paper.
This work can be viewed in two ways: as a refutation of McKay et al.'s claims that the magnetites were made by microorganisms, or as an ambiguous result that merely shows that McKay et al. were a bit exuberant in claiming that all the magnetite crystals were structurally perfect.
In the first view, it is clear that some of the magnetite crystals in the ALH 84001 carbonates do not have the shapes and structures of common biogenic magnetites. This fact alone can be seen as a refutation of part of the McKay et al. hypothesis. Because magnetite has a cubic crystal structure, it almost always grows as cubes, octahedra, or other compact shapes. Elongated magnetite crystals are known to grow only from high-temperature gases, whether in nature or in the laboratory. And this inference of high temperature, while not conclusive, is certainly inconsistent with life.
In the second view, most of the arguments in Bradley et al. (1996) are ambiguous. While they all are interesting observations, none of them invalidates the hypothesis of McKay et al.
1. From the description in their paper, it is not clear that Bradley's magnetites are from the same layers and veins as the magnetites studied by McKay et al.
2. Although Bradley et al. did find structurally imperfect whisker-shaped magnetites, it would still appear that most of the magnetite crystals in the ALH 84001 carbonates are structurally perfect cuboids (and similar shapes). So far, there is no proof that the whisker and cuboid magnetites formed at the same temperature.
3. To support a high-temperature origin for the ALH 84001 magnetites, Bradley et al. refer to Symonds (1993), who found that whisker-shaped magnetite crystals grew from the hot gases given off by a volcano. But Symonds suggested that temperature alone did not control whether the magnetite crystals grew as cubes or whiskers. In fact, the highest-temperature magnetites grew as cubes, while the whisker-shaped crystals formed at lower temperatures where they grew very quickly (i.e., the gas was very supersaturated). Whisker-shaped magnetites apparently have not been reported in low-temperature carbonate deposits, but it is quite possible that no one has looked carefully.
Becker L., Glavin D. P., and Bada J. L. (1997) Polycyclic aromatic hydrocarbons (PAHs) in Antarctic martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta, 61, 475-481.
McKay et al. (1996) discovered that ALH 84001 contains polycyclic aromatic hydrocarbon molecules (PAHs) in moderate abundance, found that these PAHs were distinct from meteoritic and terrestrial PAHs, and found that the PAHs in ALH 84001 were intimately associated with the carbonate minerals that host other possible indications of fossil life. Here, the authors evaluate whether the PAHs in ALH 84001 might be contaminants.
To see if the association of PAHs and carbonate minerals in ALH 84001 really suggests that they formed together, the authors put carbonate mineral grains in water samples that contained PAHsa standard, and a sample of Antarctic ice from the Allan Hills. In both cases, the PAHs in the water attached themselves to the carbonate mineral grains. From this result, the authors infer that the PAHs in ALH 84001 might have become associated with the carbonate minerals without any biologic action.
To see if the PAHs in ALH 84001 were actually different from those in other sources, the authors analyzed PAHs in the martian meteorite EETA 79001 (both carbonate minerals and bulk rock), in two carbonaceous chondrite meteorites, and in Antarctic ice from the Allan Hills. The PAHs from these other samples are all similar to those in ALH 84001, especially in having strong signals from the few simplest PAHs (called parent or nonalkylated molecules). The ALH 84001 PAHs are most similar to PAHs in carbonate minerals in the EETA 79001; both meteorites have similar simple PAHs and in similar small amounts of big complex PAHs. The carbonates in EETA 79001 are known to be contaminated with carbon and organic molecules from Earth (Jull et al., 1995; McDonald and Bada, 1995), and therefore probably contaminated with Earth PAHs. So, Becker et al. conclude that the PAHs in ALH 84001 are probably a mixture of PAHs from Antarctic ice and PAHs from carbonaceous meteorites or interplanetary dust, which could have entered ALH 84001 either on Earth or on Mars. They see no clear evidence in the PAHs for a biological origin on Mars, and suggest that amino acids would be better biomarkers than PAHs.
This article is important for characterizing the PAHs from Earth that are likely to collect on meteorites as they sit in Antarctica, and would seem to weaken McKay et al.'s case for traces of martian fossils in ALH 84001. But many questions are not yet answered.
1. The PAHs in ALH 84001 are not merely a mixture of PAHs from CM chondrites and from the Allan Hills ice. The ice contains strong signals from the PAHs naphthalene (mass 128) and coronene (mass 300), while carbonates in ALH 84001 contain neither (their Table 1); other differences are apparent in the relative strengths of some PAH signals, and in the presence or absence of signals from some less-abundant PAHs. Are these differences artificial, for instance, because Becker et al. and McKay et al. used slightly different analytical techniques? Or could the differences be real and significant for the origin of the PAHs?
2. The authors here showed that PAHs in water stick strongly to a calcium carbonate mineral, but is this relevant to ALH 84001? Calcium-rich carbonate minerals are rare in ALH 8400_most of its carbonate is rich in magnesium and iron. Further, the calcium carbonate in the experiments was not characterized, and may not have the same crystal structure as the carbonates in ALH 84001 (calcite vs. aragonite vs. vaterite structure types); PAHs may bond differently to different carbonate mineral structures.
3. Becker et al. suggest that the PAHs in ALH 84001 are associated with the carbonate minerals because their experiment showed that PAHs in water stick strongly to a carbonate mineral. But do PAHs prefer to stick to carbonates compared to the other minerals in ALH 84001? The experiments of Becker et al. shed no light on this question.
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 Planetary Sci. 32, 91-96.
As ALH 84001 traveled between Mars and the Earth, it was bombarded by cosmic rays, high-energy particles from the Sun and the galaxy. Interactions of cosmic-ray particles with meteorites leave characteristic signatures like the nuclear tracks produced by cosmic-ray heavy nuclei and trace abundances of the noble elements (e.g., neon and argon) resulting from nuclear interactions of cosmic-ray protons with meteoritic matter. Here, the authors investigated the evidence for cosmic-ray bombardment in ALH 84001 to understand what happened to this meteoroid after it left Mars and before it landed in Antarctica. They found that ALH 84001 formed approximately 4 billion years ago, and spent approximately 17 million years exposed to cosmic rays; these numbers are consistent with results from many other groups. In addition, the authors here deduce that ALH 84001 was approximately 20 centimeters in diameter before it encountered the Earth, and that ~85% of it burnt up as it passed through the Earth's atmosphere. They also suggest that ALH 84001 did not break up into multiple fragments as it fell through the Earth's atmosphere, and so it is also unlikely that additional fragments of this meteorite exist.
There may be calls for the Antarctic Search for Meteorites Program, ANSMET, to return to the Allan Hills area of Antarctica to search for more fragments of ALH 84001 rock. The results in this paper suggest that returning to the Allan Hills for martian meteorites would be no more fruitful than collecting elsewhere in Antarctica. In fact, ANSMET field parties have gathered meteorites from the Allan Hills area many times since their first visit in 1976. In that time, only two martian meteorites have been found in the Allan Hills: ALHA 77005 and ALH 84001. These two meteorites are quite different, and could not be separate fragments from a single meteorite fall.
Jull A. J. T., Eastoe C. J., and Cloudt S. (1997) Isotopic composition of carbonates in the SNC meteorites Allan Hills 84001 and Zagami. J. Geophys. Res., 102, 1663-1669.
The authors investigated the sources of the carbon in ALH 84001 (and other martian meteorites), especially using radioactive carbon-14 (14C) as a marker for carbonates that formed on Earth. Radioactive 14C forms continuously in the Earth's atmosphere (and from nuclear bomb tests) and forms only sparingly in space, so the abundance of 14C in the carbonates is a clue to how much they have reacted with carbon from Earth. The authors find that most of the carbonate in ALH 84001 contains 14C, so much 14C that it must have either formed on Earth or traded some of its martian carbon for Earth carbon. The carbon in ALH 84001 with the least 14C is also the richest in the stable carbon isotope 13C, and its 13C abundance is the same as measured for martian carbonates in ALH 84001 and other martian meteorites.
This work and Jull et al. (1995) are important for understanding terrestrial contamination in ALH 84001. The authors argue that a great proportion of the carbon and oxygen in the ALH 84001 carbonates originated on Earth, and then diffused into the carbonate mineral grains in the meteorite. This argument, if true, lends plausibility to the idea that the PAHs in ALH 84001 are also terrestrial (Becker et al., 1997). However, Wright et al. (1997) suggest that the amount of 14C found here could also mean only limited contamination by Earth carbon.
Kirschvink, J. L., Maine A. T., and Vali H. (1997) Paleomagnetic evidence of a low-temperature origin of carbonate in the martian meteorite ALH 84001. Science, 275, 1629-1633.
When a rock forms or cools down, it can trap some of the local magnetic field; magnetic minerals in the rock become little bar magnets, aligned with the planet's magnetic field. This trapped magnetic field, called natural remnant magnetism or NRM, can stay in the rock indefinitely, and can be used to unravel the history of the magnetic minerals and the rock. The strength of the trapped magnetic field can tell how strong the planet's field was. If the rock is broken or bent, the magnetic field trapped in it will point in a different direction from the original field. If the rock gets heated above a critical temperature, the old trapped magnetic field is lost, and a new one is trapped when it cools down again.
For the McKay et al. (1996) hypothesis of fossil martian life in ALH 84001, the most important result from Kirschvink et al. is that the carbonate globules formed below 325°C, and probably below ~110°C. McKay et al. require a low formation temperature to permit bacterial growth, and many types of Earth bacteria and archaea can live and prosper at 110°C! The upper temperature limit is too high for known Earth life, but is an UPPER LIMIT, and is still better for McKay et al. than the 500°_700°C temperatures estimated by other groups.
The argument for carbonate formation below 325°C is indirect, but fairly clear. Kirschvink et al. measured the trapped magnetic fields (NRM) in two adjacent fragments of ALH 84001 from the fracture zone where McKay et al. found the most carbonate globules. The trapped fields in the two fragments were strong, equally strong, but in different orientations; the "bar-magnets" of the magnetic minerals were pointed in different directions. This meant that the two fragments had probably trapped the same original field, but had been rotated or jostled when the fracture between them formed. If ALH 84001 had ever been hotter than 325°C since the fragments were jostled, they would have lost their original magnetic fields; when they cooled, the fragments would have trapped the new magnetic field, with the same direction in both fragments! Because the fragments do have magnetic fields in different directions, ALH 84001 could not have been hotter that 325°C at any time after the fractures formed. Now, the carbonate globules are in these same fractures, and must have formed after the fractures did, and so must not have formed hotter than 325°C (otherwise the rock fragments would have their trapped magnetic fields pointing in the same direction)!
The argument for carbonate formation below ~110°C depends on the details of how the trapped magnetic field changes as the rock is heated. In ALH 84001, the trapped magnetic field is in the iron sulfide mineral pyrrhotite. When pyrrhotite is heated to temperatures below its critical temperature of 325°C, its trapped magnetic field fades away somewhat. But Kirschvink et al. found no hint of this fading in ALH 84001's trapped magnetic field. The 110°C temperature actually comes from their sample preparation, not anything inside the rock. They had to heat their samples to 110°C to allow their glue to cure. If ALH 84001 had been heated to <110°C on Mars, any magnetic effects would have been erased as the glue cured.
The results of this paper are a strong challenge to "anti-life in ALH 84001" scientists. However, the results are not (yet) proof of a low-temperature origin and certainly not proof of life on Mars. Although I am not an expert on magnetism, I see two issues in this work as it relates to McKay et al.'s hypothesis that ALH 84001 contains traces of ancient martian life.
The first issue is the timing of fracturing of ALH 84001 compared to the timing of carbonate formation. ALH 84001 was fractured at least twice, before and after the carbonate globules formed. Many carbonate globules sit in fractures, so the fractures must have been there first (McKay et al., 1996a). And the carbonate globules are themselves sliced and broken along fractures, which must have come later (Mittlefehldt, 1994; Treiman, 1995; McKay et al., 1997). So, could Kirschvink's two rock fragments be separated by a late fracture, rather than an early fracture? If this particular fracture formed after the carbonate globules were deposited, Kirschvink's results here would say nothing about formation of the carbonate globules.
The second issue is the absolute age of the carbonate globules, which should be ancient (3.6 billion years old) according to McKay et al. (1996a). The problem here is that the tiny magnetite grains in the carbonate globules have not trapped any detectable magnetic field themselves. The magnetites do contribute to other magnetic properties of the rock, just not the trapped field (the NRM). Could this mean that the magnetite grains grew when there was no field, and so are fairly young (Wadhwa and Lugmair, 1996)? Or could it mean that Kirschvink's sample had so few carbonate globules that their trapped magnetic field could not be detected?
The most important result from this paper, particularly for life on Mars, is the evidence that Mars had a strong magnetic field! Mars now has no detectable magnetic field, and had hardly any field at 1.3 billion years ago, when many of the martian meteorites formed. Kirschvink et al. have demonstrated that Mars had a strong magnetic field (possibly as strong as the Earth's is now), at about 4.0 billion years ago when ALH 84001 cooled.
First, a strong magnetic field would have protected Mars' surface from much deadly radiation from space. Its magnetic field would have deflected radiation like electrons and protons from the Sun, just as the Earth's magnetic field protects us now.
Second, and perhaps more important, a magnetic field early in Mars' history would have protected its atmosphere. Mars' atmosphere is now quite thin, about 1/200 as thick as the Earth's. Without a thick atmosphere, Mars' surface could never have been warm enough to permit liquid water, and there is very good geologic evidence that liquid water was once abundant on the surface of Mars. What happened to Mars' atmosphere? Much of it was swept away by the solar wind, the continual stream of electron and protons that shoot off the Sun. But a strong magnetic field would have protected Mars' atmosphere, possibly letting Mars' surface be warm and wet enough for life to develop.
Valley J. W., Eiler J. M., Graham C. M., Gibson E. K. Jr., Romanek C. S., and Stolper E. M. (1997) Low-temperature carbonate concretions in the martian meteorite ALH 84001: Evidence from stable isotopes and mineralogy. Science, 275, 1633-1638.
The temperature of formation of carbonate globules in ALH 84001 is important because the globules are hosts to the possible traces of ancient martian life (McKay et al., 1996a). The first estimates of the globules formation temperature, <320°C, relied on oxygen isotope measurements (Romanek et al., 1994); here, Valley et al. revisit the oxygen isotope measurements with a new improved analytical method, and confirm the low formation temperature.
Valley et al. used an ion microprobe to determine oxygen isotope abundances in the carbonate globules and other minerals in ALH 84001. The ion microprobe can produce analyses from very small spots, about 20 micrometers (µm) in diameter, which is important because the carbonate globules are <200 µm in diameter themselves. They analyzed oxygen isotope ratios in carbonates from two separate ellipsoids, one of which was a composite of two smaller carbonate bodies. To help calibrate the ion microprobe measurements, Valley et al. also obtained chemical analyses of these and nearby spots in ALH 84001 using electron microprobe.
Valley's results are consistent with, and expand on, the earlier work of Romanek et al. (1994). They found that the carbonate minerals were variably enriched in the heavy oxygen isotope 18O, with enrichments ranging from d18O = 9.5 to 20.5 "per mil" (or parts per thousand). Carbonate near the globule rims was much richer in 18O than carbonate from the cores, and the different globules had different 18O enrichments in their cores.
Valley et al. inferred that the carbonate globules formed at low temperatures because their chemical and isotopic variations could not have been preserved, if they had formed at high temperature. Valley et al. estimate that the carbonate globules formed at <100°C. An absolute upper temperature limit from their results comes from assuming that the carbonates were in oxygen isotopic equilibrium with the surrounding pyroxene. This upper limit on temperature is ~300°C; the temperature had to have been lower because the pyroxene and carbonate were not in chemical equilibrium.
Valley et al. also made some interesting discoveries and observations during their work: (1) They also analyzed the carbonates for carbon isotope composition, and found some evidence for an organic carbon component that has relatively little of the heavy carbon isotope 13C. This finding is one of a number of hints now of very "light," possibly organic carbon in ALH 84001. (2) Valley et al. found a veinlet of silica that cut across one of the carbonate globules. This indicates that silicate minerals were mobile after the carbonate veinlets formed, and similar evidence was presented by other groups at the 28th Lunar and Planetary Science Conference. (3) Valley et al. note that the near-absence of hydrous minerals in ALH 84001, long cited as a problem for a low-temperature origin of the carbonates, is not actually a problem at all. There are many instances on Earth where low-temperature carbonate veins cut silicate rocks without formation of hydrous silicate minerals.
The oxygen isotope abundance ratios measured by Valley et al. have been confirmed and extended by two other groups using similar ion microprobe techniques: L. Leshin et al. (1997), and J. Saxton and co-workers at the University of Manchester (reported in the abstracts of the Early Mars Conference). Although there are some problems still with calibrations and interlaboratory biases, it seems indisputable that the carbonates in ALH 84001 contain relatively heavy oxygen (high d18O) and that they are strongly zoned in oxygen isotope ratios from core to rim.
However, the meaning of this zoning is quite disputable. Valley et al. have interpreted the zoning as most consistent with carbonate minerals growing, at low temperature, from a fluid that changed composition over time. Their low temperature is consistent with, but not proof of, microbial life. Leshin et al., on the other hand, interpret the oxygen isotope zoning as forming at higher temperatures in a closed system. Higher temperature here means 250°C, too high for known Earth-like bacteria, but a far cry from the 500°_700°C suggested by some other investigators.
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