The martian meteorite ALH 84001, and its possible evidence of ancient martian life, were among the featured topics at the 60th Meeting of The Meteoritical Society. The meeting, hosted by the University of Hawaii, was held July 21-25, 1997, on the island of Maui.
Talks related to ALH 84001 occupied a full session, titled "Environment and Nature of Bacterial Life on Earth (and Mars?). One additional talk was in the later session "History of Mars from Martian Meteorites."
Abstracts of talks for the Meeting of the Meteoritical Society are online below; I've summarized the abstracts about ALH 84001 and the possible traces of life in it. To read a full abstract from the conference, click on the highlighted title, which will connect you to the online abstracts in pdf format. To view the abstracts, you need version 3.0 of the pdf reader, which can be obtained free of charge from Adobe.
Papers cited are listed after the summaries, which are by Allan Treiman, Lunar and Planetary Institute.
Mahji P., Devouard B., Posfai M., Hua X. Bazylinski D.A., Frankel R.B., and Buseck P.R. Structural features of magnetite from magnetotactic bacteria.
McKay et al. (1996) claimed that the small magnetite grains in the ALH84001 are biogenic because (among other things) they are rounded and structurally perfect; Bradley et al. (1996, 1997) showed that many of these magnetites were elongate and contained structural imperfections. The authors here show that magnetites produced by Earth bacteria can also be elongate and structurally imperfect, depending on the bacteria that produced them. The authors looked at magnetites from five strains of bacteria -- magnetites from two strains contained twin boundaries (spinel-law twins). Elongate magnetite grains made by bacteria have been reported in the literature.
Bradley J.P., McSween H.Y.Jr., and Harvey R.P. Epitaxial growth of single-domain magnetite in martian meteorite ALH84001.
McKay et al. (1996) found fields of aligned bacteria-shaped objects, and inferred that they were fossil bacteria. However, Bradley et al. (1997) suggested that the bacteria-shaped objects were magnetite crystals that had grown in alignment on the host carbonate mineral. Here, the authors show that at least one grain of magnetite is in good crystallographic alignment with its host carbonate mineral; this relationship is called epitaxy. This alignment seems unlikely if the magnetites were formed by bacteria. But the alignment is expected if, as the authors suggest, the magnetite grew at high temperature from a vapor. "The widely published images of aligned nanofossils in ALH 84001 are probably magnetite whiskers with parallel orientations resulting from epitaxial growth on carbonate substrates."
Thomas-Keprta K., Wentworth S.J., McKay D.S., Taunton A.E., Allen C.C., Romanek C.S., and Gibson E.K.Jr. Subsurface terrestrial microfossils from Columbia River basalt samples: Analogs of features in martian meteorite ALH 84001?
The supposed fossil bacteria in ALH 84001 are smaller than fossil bacteria known on Earth. The authors here investigated bacteria from an igneous rock (Columbia River basalt), looking for biological objects the same sizes as those in ALH 84001. They found many "normal" (micrometer-sized) bacteria, about 10-100 times longer than the ALH 84001 objects, and also found many filaments of approximately the same size (10-30 nanometers) as the ALH 84001 objects. The filaments are composed of iron oxide minerals, and the authors think that the filaments are fossilized bacteria (like the shapes in ALH 84001) or fossilized appendages of normal bacteria.
Flynn G.J., Keller L.P., Jacobsen C., Wirick S., Bajt S., and Chapman H.N. The spatial distribution and bonding states of carbon associated with ALH 84001 carbonates.
One of the indicators of martian life in ALH 84001 is the presence of organic molecules in its carbonate globules (McKay et al., 1996). The spatial distribution of the organics is not known well, as the analysis method used by McKay et al. (1996) had a spot size of ~50 - 100 micrometers, almost as big as the carbonate globules themselves. Here, the authors mapped a sample of ALH 84001 carbonate with a spot size of 0.01 micrometer, using an X-ray technique that can distinguish carbon in organic material from carbon in carbonate minerals. Preliminary results show that organic carbon is dispersed in and near the carbonate globules, possibly as distinct grains, or as films between grains of carbonate minerals.
Becker L., McDonald G.D., Glavin D.P., Bada J.L., and Bunch T.E. Sublimation: A mechanism for the enrichment of organics in Antarctic ice.
This talk continues a controversy about organic contamination of ALH 84001. McKay et al. (1996) suggested that PAH organic molecules in ALH 84001 were relics of ancient martian life. Becker et al. (1997) replied that the PAHs were probably organic contamination from the Antarctic ice. Wright et al. (1997) countered that the ice contains so little PAH that 3000-8000 liters (quarts, more or less) of melted ice would have to flow through ALH 84001 to deposit enough PAHs. Now, Becker and co-workers respond with a mechanism that could concentrate organics in the Antarctic ice before it melted and permeated ALH 84001. Much of the ice in Antarctica evaporates away (like ice in a self-defrosting refrigerator), and its PAHs may be left behind and concentrated on the surface of the remaining ice. So, when ice melts around a meteorite, the water is rich in organic material and more able to deposit much more PAHs in meteorites like ALH 84001.
Bishop J.L., Pieters C.M., and Hiroi T. The source of organic spectral features in ALH84001: Lab contamination, terrestrial, or extraterrestrial?
The source of organic matter in ALH 84001 is controversial. McKay et al. (1996) suggested it is martian and biogenic; Bell (1996) suggested it was from carbonaceous meteorites that fell onto Mars; and Becker et al. (1997) suggested it got into ALH 84001 on Earth. To help chose among these stories, the authors here looked at how much infrared light (3.3 - 3.7 micrometers wavelength) reflects from ALH 84001, other meteorites, and a range of possible contaminants. They found that the infrared reflection from ALH 84001 was not like any likely contaminants (oil, soap, lotion, fingerprints, ethanol). The infrared light reflection from organic materials in ALH 84001 is similar, but not identical, to reflection from some carbonaceous chondrites, especially the CM2 type (see Bell, 1996).
Noll K., Döbeli M., Tobler L., Grambole D., and Krähenbühl U. Fluorine profiles in achondrites and chondrites from Antarctica by nuclear reaction analysis (NRA).
ALH 84001 landed in Antarctica about 15,000 years ago, and certainly interacted with Earth's environment. But how badly was ALH 84001 contaminated during its stay on Earth? The authors here are calibrating a new way to measure the level of terrestrial contamination: fluorine (F) in a meteorite's fusion crust. The fusion crust, which forms on a meteorite when it passes through the Earth's atmosphere, is a thin layer of glass that has essentially no fluorine. The fusion crust absorbs fluorine from water on Earth; the longer it is wet, the more fluorine. The authors are calibrating a new, fast, nuclear method to measure fluorine. Preliminary results on ALH 84001 fusion crust show no detectable fluorine, which indicates a short exposure on the ice surface (less than 1000 years).
McKay G.A., Mikouchi T., and Lofgren G.E. Carbonates and feldspathic glass in ALH84001: Additional complications.
The claim that ALH 84001's carbonate globules contains traces of ancient martian life (McKay et al., 1996) requires that the globules formed at a temperature comfortable for life, but recent evidence suggests that some of the carbonates formed at very high temperatures (Scott et al., 1997). Here, the authors describe a newly recognized variety of carbonate minerals in ALH 84001: lacy grains dispersed in feldspathic glass. Some of the glass areas are rectangular, as if they had once been crystals. It seems clear that these lacy carbonates grew between small feldspar crystals. This carbonate-feldspar (now glass) relationship could have formed in several ways: carbonate and feldspar forming simultaneously during metamorphism (high temperature without melting); feldspar could have formed first as a porous aggregate, and carbonate later filled the holes (this seems unlikely); the carbonate could have replaced another mineral; or both carbonate and feldspar could have formed during and after shock melting (as in Scott et al., 1977). In the last case, not all of the carbonate in ALH 84001 (perhaps including the globules) need have been melted by shock (see McKay and Lofgren, 1977).
Scott E.R.D., Krot A.N., and Yamaguchi A. Formation of carbonates in martian meteorite ALH84001 from shock melts.
The claim that ALH 84001's carbonate globules contains traces of ancient martian life (McKay et al., 1996) requires that the globules formed at a temperature comfortable for life. However, Scott et al. (1977) showed that some carbonates in ALH 84001 formed at very high temperature by shock melting, and here they extend their arguments. The authors here document more evidence that small carbonate grains inside pyroxenes are remnants of shock melts, injected into the pyroxenes during the decompression phase of impact shock. They infer that the zoned carbonate disks (or pancakes; hosts to the possible traces of martian life) formed from shock melt injected into fractures. The spherical carbonate globules in feldspathic glass formed from mixtures of carbonate-rich and feldspathic shock melts. So, all the carbonates in ALH 84001 formed from melts, at temperatures that are inimical to life as we know it.
Eiler J.M., Valley J.W., and Stolper E.M. Stable isotopes in ALH 84001: An ion microprobe study.
The claim that ALH 84001s carbonate globules contains traces of ancient martian life (McKay et al., 1996) requires that the globules formed at a temperature comfortable for life. Analyses of oxygen isotope abundances in the globules have been important in estimating their formation temperatures. This work follows Valley et al. (1997), and considers how their data fit various possible models for the formation of carbonate globules. For nearly all models, oxygen isotope ratios suggest that the carbonates formed at temperatures below 300°C but "... no rigorous temperature constraint can be made using existing data...." Carbon and oxygen isotope variations do not appear to support the model of Leshin et al. (1997), that carbonate globules formed from trapped masses of carbonate-rich fluid. Earth carbonate minerals similar to those in ALH 84001 all formed at temperatures below 300°C, and typically near the Earths surface. The carbonate globules in ALH 84001 are important for Mars as a whole, because they indicate that carbon dioxide from the atmosphere can be lost to form carbonate minerals inside Mars.
Jull A.J.T., Cloudt S., Courtney C., and Eastoe C.J. Carbon-14 and stable-isotopic composition of organic material and carbonates from some SNC meteorites.
The source of the organic matter in ALH 84001 is controversial -- McKay et al (1996) suggested it is martian and biogenic, and Becker et al. (1997) suggested it got into ALH 84001 on Earth. To understand the origin of the organic matter, the authors here measured how much of the radioactive isotope carbon 14 (14C) is in the organics. Carbon 14 is produced continuously in the Earth's atmosphere. Newly formed organic material has a full complement of 14C, which decreases over time by radioactive decay; ancient organic matter (martian or not) should contain little or no 14C. The authors extracted carbon from their sample of ALH 84001 by heating it in oxygen. The organic carbon in ALH 84001 contained more than half the 14C that would be in fresh, modern-day organic material on Earth. This abundance of 14C is equivalent to a radiocarbon age of 4300 years. [This result seems to mean that the organic matter in ALH 84001 is not martian!]
Gibson E.K.Jr., Romanek C.S., McKay D.S., Thomas-Keprta K., Allen C.C., and Wentworth S.J. Nature of carbon phases in ALH84001.
This abstract reviews the many varieties of carbon in ALH 84001. Most of the carbon is as carbonate minerals: the globules and pancake structures (which formed at low temperatures), and also as some grains which were remobilized at high temperature by shock. The carbonates have isotope ratios of carbon (d 13C) and oxygen (d 18O) that suggest they formed on Mars. Organic matter in ALH 84001, PAH molecules (McKay et al., 1996), and other compounds, is unevenly distributed in the carbonate globules. This organic material is not terrestrial contamination, and has a carbon isotopic signature (d 13C) consistent with a biological origin. Finally, there are distinct traces of carbon from Mars' mantle and from its atmosphere.
Kerridge J.F. Martian exobiology in the post-ALH84001 era: Some key issues.
The furor about possible traces of life in ALH 84001 has highlighted some important issues in the search for life on Mars. First, proof of biological activity must be based on many lines of evidence ("multiple biomarkers"). Second, experimental controls are crucial, as non-biological chemicals and structures can easily be misinterpreted as biological. And third, the mineral and chemical thermometers now available cannot distinguish materials formed at biologically reasonable temperatures (< ~ 120°C) from materials formed at higher temperatures unsuitable for life. New thermometric schemes need to be developed.
"... when the claim for evidence of extinct life in ALH 84001 is finally recognized to be incorrect, the scientific case for the active exobiological exploration of Mars will remain solid." The intense public interest in possible life in ALH 84001 shows that exploration for life on Mars is an appropriate use of public funds.
Kring D.A. and Gleason J.D. Magmatic temperatures and compositions on early Mars as inferred from the orthopyroxene-silica assemblage in ALH 84001.
This abstract concerns the original igneous history of ALH 84001: what was the composition of its original magma, how hot was it, and how deep in Mars did it crystallize. In the orthopyroxene of ALH 84001, the authors have observed small grains of silica (SiO2), which they interpret as being original igneous crystals of quartz and tridymite (both silica minerals). If crystals of orthopyroxene and silica grew from the original magma, it must have been hotter than 1400°C (normal basalt magmas are closer to 1150°C). Such high temperature magmas should be expected early in the history of a planet [ALH 84001 crystallized from magma about 4.5 billion years ago]. Having quartz as an igneous silica mineral means the pressure must have been above 0.5 GPa (50,000 times Earth atmosphere pressure), which is reached at about 40 kilometers deep inside Mars. Further, the presence of silica as an igneous mineral suggests that Mars' mantle and crust contain more silicon (Si) than does the Earth's mantle and crust.
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.
Bell J.F. (1996) Evaluating the evidence for past life on Mars (letter). Science 274, 2121-2122.
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) Magnetite whiskers and platelets in the ALH84001 martian meteorite: Evidence of vapor phase growth. Lunar Planet. Sci. XXVIII, 147-148.
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.
McKay G.A. and Lofgren G.E. (1977) Carbonates in ALH 84001: Evidence for kinetically controlled growth. Lunar Planet. Sci. XXVIII, 921-922.
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.
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.
Wright I.P., Grady M.M., and Pillinger C.T. (1997) Evidence relevant to the life on Mars debate. (2) Amino acid results. (abstract). Lunar Planet. Sci. XVIII, 1587-1588.