The Meteoritical Society will hold its 63rd annual meeting this fall in Chicago, IL (USA), from August 28 through September 1, 2000. Special sessions at the meeting will focus on Pre-Solar Grains in meteorites and on Future Missions and Sample Returns. Martian meteorites are represented well with two sessions of talks and a session of posters. Among them, studies of the many new finds of the last year (Los Angeles, Dhofar 019, Sayh al Uhaymir 005/008, and Dar al Gani 670/476/489) have eclipsed work on ALH84001. To the cognoscenti, most of the ALH84001 players and topics will be familiar.
Abstracts of talks for the Meeting of the Meteoritical Society are available on the internet. 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, double-click on the highlighted title, which will connect you to the on-line abstract in .pdf format. To view the abstracts, you need a pdf reader, which can be obtained free of charge from Adobe.
Abstracts are summarized in presentation order. Summaries are by Allan Treiman, Lunar and Planetary Institute.
Stephan T. and Jessberger E.K. Polycyclic Aromatic Hydrocarbons in Allan Hills 84001: A Result of Terrestrial Contamination?.
In earlier works, the authors found that polycyclic aromatic hydrocarbons (PAHs) were less common on carbonate globules in ALH84001 than on silicate minerals (contrary to McKay et al., 1996). A new experiment shows that contamination in the laboratory did not produce this difference in PAH abundances. Also, the authors found that the amount of PAHs on mineral surfaces correlate well with the amount of lead present. That lead is probably from Earth, as its isotope ratios are similar to those of Earth lead in Antarctic meteorites. Thus, they infer that the lead and associated PAHs in ALH84001 are Earth contaminants.
Thomas-Keprta K.L., Clemett S.J., Bazylinski D.A., Kirschvink J.L., McKay D.S., Wentworth S.J., Vali H., and Gibson E.K.Jr. Biogenic Martian Magnetite Crystals? A Comparison of Prismatic Magnetite Crystals in the Allan Hills 84001 Carbonate Globules with Those from Magnetotactic Bacteria Strain MV-1
Earlier, these authors showed that a population of the submicometer-sized magnetite crystals in the ALH84001 carbonate globules is similar to those made by one type of Earth bacteria. Here, the authors examined magnetites from that bacteria in detail, and find that they are identical to the population of ALH84001 magnetites in: chemical composition, morphology (including crystal faces present), and size distribution (including width/length). A mathematical model of the size distributions is in press. If such magnetite crystals were found on Earth, they would be considered ("...defined as...") biogenic.
Treiman A.H. and Keller L.P. Magnetite-bearing Layers in Allan Hills 84001 Carbonate Globules: Bulk and Mineral Compositions.
Chemical compositions and mineral grains in the magnetite-bearing layers of ALH84001 carbonate globules were examined with electron microprobe and transmission electron microscopy. The magnetite-bearing layers also contain magnesite, Fe-Mg clay, brucite (magnesium hydroxide), amorphous iron oxide/hydroxide, and iron sulfide. The chemical compositions of magnetite-bearing layers contain significant silicon and sulfur, but little calcium, and so are distinct from compositions of pure carbonate minerals in the globules.
Holland G., Lyon I.C., Saxton J.M., and Turner G. Very Low Oxygen-Isotopic Ratios in Allan Hills 84001 Carbonates: A Possible Meteoric Component?.
Oxygen in the carbonate globules in ALH84001 shows a large range in its isotopic composition - the ratio of "heavy" 18O to "light" 16O. Here, the authors confirm the range found by earlier workers and extend it to even "lighter" oxygen. This range and its relation to elemental compositions (Mg, Fe, and Ca) are not consistent with the carbonates forming in a high-temperature event. The water that formed the "light-oxygen" carbonate was itself quite "light," comparable to Earth groundwater.
Holland G., Lyon I.C., Cliff B., Lockyer N.P., and Vickerman J.C. Halogen Concentration and Distribution in Allan Hills 84001 Measured by Time-of-Flight Secondary Ion Mass Spectrometry.
The authors analyzed for chlorine, bromine, and iodine in ALH84001 as potential tracers of biologic activity. The carbonate globules contain ~ 1000 ppm Cl (0.1%), which accounts for all of the chlorine in the rock. This is significantly more than in common terrestrial carbonates, and could be consistent with an evaporitic origin for the globules. Bromine and iodine were only detected in chromite grains, which is difficult to explain.
Gibson E.K.Jr., McKay D.S., Thomas-Keprta K.L., Westall F., and Clemett S.J. What is the Status of the Hypothesis of Evidence of Biogenic Activity Within Martian Meteorites: Alive or Dead?.
The four lines of evidence for ancient martian life in ALH84001 are still valid and have not been refuted. The carbonate globules (hosts to the evidence) formed at temperatures consistent with life. Some magnetites from the globules are characteristic of bacterial products on Earth. Organic compounds occur in the globules, and structures resembling fossil bacteria are present. Similar structures and materials in other martian meteorites reinforce the validity of the original evidence.