The Conference on Early Mars, held April 24–27 in Houston, Texas, is the first major conference on Mars science since the 28th Lunar and Planetary Science Conference in March. On the morning of April 27, there was a formal debate on the hypothesis of McKay et al. (1996) that martian meteorite ALH 84001 contains traces of ancient martian life. But the ideas raised by McKay et al. will pervade the conference as we discuss and debate early Earth and the origin of life; the physical environment of early Mars, whether it was warm or cold; the chemical and mineralogical environments of early Mars; environmental challenges for life on early Mars; life in extreme environments; and the future of Mars exploration.
Abstracts of posters and talks presented at the Conference on Early Mars are on line. Below are summaries of the abstracts that relate most closely to ALH 84001 and the possible traces of life within it. The list is short, so no indexes are provided. Most of the talks and abstracts at the Early Mars conference are relevant to the existence and detection of life on Mars, and many are indirectly relevant to ALH 84001. I've not summarized them all, and encourage you to look at the program and to read these abstracts that sound interesting.
To read a full abstract from the conference, double-click on the highlighted title, which will connect you to the online abstract in .pdf format. To view the abstracts, you need a pdf reader, which can be obtained free of charge.
Papers cited are listed after the abstracts.
Summaries by Allan Treiman, Lunar and Planetary Institute
Becker L., McDonald G. D., and Bada J. L. Biomarkers for analysis of martian samples.
The authors suggest that the search for biological activity in martian samples should emphasize amino acids. Amino acids in Earth life are all of the same chirality (handedness), are relatively resistant to decomposition, and can be detected at subpicomole quantities. Polycyclic aromatic hydrocarbon (PAH) molecules, such as have been reported as evidence of ancient martian life in ALH 84001, are poor markers of biological activity. PAHs are widespread in the cosmos, and form readily under physical and chemical conditions where life as we know it is impossible. PAHs also form readily from the decomposition of organic matter (as suggested by McKay et al.), including terrestrial organic matter. So, contamination from these abundant PAH sources is a significant problem.
Bogard D. D. and Garrison D. H. 39Ar-40Ar age of ALH 84001.
The authors investigated the age of ALH 84001 using the potassium-argon radio-isotope chronometer, in its avatar as 39Ar-40Ar. This radio-isotope system dates when argon gas was last able to leak out of the rock; here, it is probably the age of metamorphism or the age of shock-melting of the plagioclase. The authors' new high-precision results involving heating up a sample of ALH 84001 from room temperature to 1550°C in 20 steps, collecting the gas that comes off, and analyzing their argon isotope abundances.
The results here are similar to earlier 39Ar-40Ar analyses, an age near 4.0 billion years ago, and have uncovered some complexities. Some of the 40Ar in ALH 84001 is associated with 36Ar, and so cannot be from decay of potassium; it must come from another gas source, like the martian atmosphere, the martian mantle, or the Earth's atmosphere. If all the 36Ar were from the martian atmosphere, some of the gas releases would give negative ages (which is impossible); most of the gas releases would imply an age of 3.81 billion years, which is the absolute minimum 39Ar-40Ar age of ALH 84001. More likely, the 36Ar is from the Earth's atmosphere or the martian mantle, and ages up to 4.29 billion years are possible. The authors prefer a 39Ar-40Ar age of 4.2 billion years.
Fisler D. K., Cygan R. T., and Westrich H. R. Cation diffusion in carbonate minerals: Determining closure temperatures and the thermal history for the ALH 84001 meteorite.
The carbonate globules in ALH 84001 contain most of the proposed evidence of ancient martian life, so it is important to know how hot the globules were when they formed. The carbonate minerals in the globules are zoned, from magnesium-rich at the rims to calcium- and iron-rich in the cores, and this zoning could not survive a long time at a high temperature. This paper is a start at quantifying "how hot for how long". The authors studied the movement (diffusion) of calcium and magnesium in the mineral calcite, which is calcium carbonate. They found that magnesium moves faster than calcium, and that both move rapidly, at least in geological terms. If the zoned carbonate globules formed at ~650°C (Harvey and McSween, 1996), they must have cooled faster than a degree C per thousand years to retain their zoning.
Hofmann B. A. and Farmer J. D. Microbial fossils from terrestrial subsurface hydrothermal environments: Examples and implications for Mars.
Trying to find Earth equivalents for the possible microbial fossils in ALH 84001, the authors briefly describe a number of microfossil localities from the Earth. Fossils of subsurface microbial life (like the purported fossil forms in ALH 84001) had previously been known only from salt deposits and ore-bearing veins. The authors report that fossilized bacterial filaments, 50-200 µm in diameter, are common in cavities and open fractures inside buried volcanic rocks on Earth. In all reported cases, the filaments were replaced by iron oxides and silica, and then encased in more silica. These mineralized filaments can be as long as 10 cm. So, volcanic rocks with abundant cracks or cavities (vesicles) could be important targets in the search for fossil microbial life on Mars.
Kring D. A., Swindle T. D., Gleason J. D., and Grier J. D. Relative ages of maskelynite and carbonate in ALH 84001 and implications for early hydrothermal activity on Mars.
The authors reexamined thin sections of ALH 84001 to find further clues to the origin and age of the carbonate globules (hosts to the possible microfossils), and conclude that the carbonate was deposited after the rock's original plagioclase had been converted to glass, maskelynite. The authors noted that where the carbonate and maskelynite abut, the carbonate forms ellipsoidal globules of needle-shaped crystals, all radiating from a point. However, carbonate tends to replace crystalline plagioclase along its fractures, cleavage planes, and twin boundaries. Thus, the authors conclude that the "plagioclase" in ALH 84001 was already a glass when the carbonates were deposited. This inference, and laboratory studies on glass dissolution, limit the duration of carbonate formation to <10,000 to 1,000,000 years at 22°C or only years at 300°C. If the carbonate was indeed produced by solution/precipitation involving plagioclase glass, it probably did not involve biological activity.
Gibson E. K. Jr., McKay D. S., Thomas-Keprta K., Romanek C. S., Clemett S. J., and Zare R. N. Biogenic activity in martian meteorite ALH 84001 - Status of the studies.
This is a review (somewhat partisan) of the evidence to date bearing on the hypothesis of McKay et al. (1996) that martian meteorite ALH 84001 contains traces of ancient martian life. None of the claims made by McKay et al. have been disproved. (1) The carbonate globules, hosts to the possible traces of martian life, were suggested to be 3.6 billion years old, but they may be much younger. (2) Evidence accumulates that the carbonate globules formed at low temperature. New oxygen isotope data (Valley et al., 1997) reenforces the arguments of Romanek et al. (1994) for a low temperature. Elongate magnetite grains, cited by Bradley et al. (1996) as evidence of high temperature, could also be formed biogenically at low temperature. (3) The bacteria-shaped forms found by McKay et al. have been criticized as too small, but Earth bacteria that small are common, if unfamiliar. It is also possible that the bacteria shapes are fossilized filaments from bacteria, if not the bacteria themselves. (4) The organic compounds, PAHs, in ALH 84001 are unlike any combination of other known meteorite sources or terrestrial contaminants. Assertions by Becker et al. (1997) of extensive terrestrial contamination cannot be confirmed.
Two new lines of evidence have come forward since McKay et al. (1996). There are some hints that ALH 84001 contains some organic carbon that is very rich in the stable isotope 12C (as opposed to 13C). If confirmed, these could most easily be explained by biologic activity. Recent work has uncovered structured, possibly organic, films on grain surfaces in ALH 84001; the authors assert that these are biofilms produced by bacteria.
Gilmour J. D., Wogelius R. A., Grime G. W., and Turner G. Major- and trace-element distributions in ALH 84001 carbonate: Indications of a high formation temperature.
Using proton-induced X-ray emission (PIXE) and Rutherford backscatter (RBS) methods, the authors analyzed carbonate minerals, maskelynite (feldspar glass), and chromite in ALH 84001 for their major- and trace-element contents. The major element analyses are comparable to those from more familiar techniques. Most interesting were that the analyzed carbonate globule had detectable and significant abundances of potassium and chromium, elements not normally present in Ca-Mg-Fe carbonate minerals. The authors suggest that the potassium and chromium abundances are unlikely to have arisen if the carbonate globules formed at low temperature, but are much more likely if the carbonate globules formed rapidly at high temperature.
Saxton J. M., Lyon I. C., and Turner G. Oxygen isotope ratio zoning in ALH 84001 carbonates.
The authors measured the abundances of oxygen isotopes in the carbonate minerals, following earlier work by Romanek et al. (1994), Valley et al. (1997), and Leshin et al. (1997). The results here are consistent with those of Valley and Leshin -- the carbonate globules have "heavy" oxygen (d18O = 5 - 25 "per mil", or parts per thousand), and oxygen isotope ratios are strongly zoned in the globules, with heavier oxygen (higher d18O) toward the rims. The author's data do not resolve the difference in interpretations of the oxygen isotope data: carbonate formation at low temperature from a fluid streaming through the rock (Valley et al.); or carbonate formation at high temperature from a fluid trapped in the rock (Leshin et al.).
Swindle T. D. and Kring D. A. Studies of weathering products in the Lafayette meteorite: Implications for the distribution of water on both early and recent Mars.
The martian meteorite Lafayette contains clays and water-bearing iron oxide minerals that definitely formed on Mars. Lafayette itself crystallized from magma at about 1.3 billion years ago, and the clay-rich alteration products give ages (by the K-Ar radio-isotope method) of 100-600 million years. The noble gas contents of the clays suggest that they were formed from water that had lain long in the martian crust, not fresh water from the martian mantle. The young clay-rich alteration products in Lafayette are unlike the alteration products in ALH 84001, which may be old and contain very little (if any) clay. The lack of extensive aqueous alteration in ALH 84001 is difficult to reconcile with an early warm, wet epoch on Mars; if there were such an epoch, one would expect ALH 84001 to contain abundant traces of water-bearing silicate minerals (clays) and iron oxide minerals (like rust).
Taunton A. SEM studies of Antarctic lunar and SNC meteorites with implications for martian nanofossils.
The author is attempting to find bacteria-shaped objects, comparable to those in ALH 84001, in lunar meteorite samples and in other martian meteorites. Because there is little chance of sub-surface bacteria on the moon, a find of comparable forms in lunar meteorites would strongly suggest that the forms are not related to life. If comparable bacteria-shaped forms are found in the lunar samples, the author will ask a group of microbiologists and electron microscopy experts to examine images of the forms and comment on the likelihood of their being related to living organisms.
Wadhwa M. and Lugmair G. W. The controversy of young vs. old age of formation of carbonates in ALH 84001.
Last year, the authors briefly reported evidence that the carbonate globules in ALH 84001 had formed about 1.4 billion years ago (Wadhwa and Lugmair, 1996), not 3.6 billion years ago as McKay et al. (1996) suggested. Here, the authors present the same data in more detail. They measured isotope abundances of the elements rubidium (Rb) and strontium (Sr) for use in the 87Rb-87Sr radio-isotope dating method. Pyroxene and bulk rock analyses give an age of 3.84 billion years, comparable to the ages from potassium-argon dating (see abstract by Bogard and Garrison listed here). This is probably the last major heating (metamorphic) event that ALH 84001 experienced. However, the Rb-Sr analyses of carbonates (one globule) and feldspar glass are not consistent with 3.85 billion years. Rather, these analyses suggest that the carbonate and feldspar glass formed at 1.39 ± 0.10 billion years ago.
Wright I. P., Assanov S., Verchovsky A. B., Franchi I. A., Grady M. M., and Pillinger C. T. Further investigations of isotopically light carbon in ALH 84001.
The authors have analyzed the isotopic compositions of carbon-bearing gases released as fragments of ALH 84001 were heated in the absence of oxygen gas (i.e., pyrolysis). The carbon dioxide and carbon monoxide together are relatively rich in the stable isotope 13C, given as d13C ~ = 32.3 "per mil". This value is less rich in 13C than the carbonates which average 40 "per mil". This difference suggests the presence of a carbon-bearing substance that has relatively little 13C; this substance may have been seen in the 200°-300°C gas release, which has d13C ~ = -37 "per mil". In a separate experiment, small quantities of methane (only 2-3 times the detection limit) were released on heating. If the hydrogen in this methane were like known martian hydrogen, the carbon in the methane would have d 13C ~ < -60 "per mil". This value is extreme, and attained on Earth only through the metabolisms of bacteria.
Bradley J. P., Harvey R. P., and McSween H. Y. Jr. (1997) Magnetite whiskers and platelets in ALH 84001 Martian meteorite: Evidence of vapor phase growth. Geochim. Cosmochim. Acta, 60, 5149-5155.
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., and Harvey R. P. (1997) Oxygen isotopic constraints on the genesis of carbonates from martian meteorite ALH 84001. Lunar Planet. Sci. XXVIII, 805-806.
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.
Romanek C. S., Grady M. M., Wright I. P., Mittlefehldt D. W., Socki R. A., Pillinger C. T., and Gibson E. K. Jr. (1994) Record of fluid-rock interactions on Mars from the meteorite ALH 84001. Nature, 372, 655-657.
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.