In 1996, a team of scientists stunned the world by announcing possible evidence for fossil life in Martian meteorite ALH84001 (McKay, et al., 1996). In my first report on their findings, I said that the claims were very tenuous (Major, 1996), and I continue to hold that opinion. Over the intervening months, skeptics and supporters in the relevant fields have published articles and given presentations at professional meetings. At this point, the scientific community remains divided.
Alleged proof of fossil life in this meteorite came primarily from chemical remnants of past biological activity. In other words, there seemed to be evidence that living things once lived and died within this piece of rock. Most of the evidence was found in deposits of carbonate minerals. This led researchers to speculate that bacteria-laden, mineral-rich waters had entered cracks and pores before hardening into carbonate. The authors also presented pictures of round, elongated objects under extremely high magnification. In light of the other evidence, these objects were highly suggestive of bacterial cells.
The debate today centers on three main questions. First, at what temperature did the carbonate minerals form? Clearly, if the original fluids were at very high temperatures, then there is no way that they could have supported any kind of life. McKay and colleagues pinned their hopes on oxygen isotope ratios that suggested temperatures at formation of 0° to 80°C (it is thought that bacteria on Earth can survive at less than 150°C). They rejected two other results indicating a temperature of around 700°C. Subsequently, the low temperature range was revised upward to between 40° and 250°C, but there remains no conclusive evidence of carbonate formation at the lower, bacteria-friendly end of this range. To the contrary, further studies on ALH84001 suggest rapid, intense melting at the time of formation (Scott, et al., 1997).
Second, do chemical analyses reveal evidence of past life? This problem is the source of greatest ambiguity because both biological and nonbiological processes can explain certain features of the meteorite’s composition. MacKay’s team offered three main observations, and critics have challenged the significance of each one.
Polycyclic Aromatic Hydrocarbons (PAHs). Organisms can leave behind these oily, organic compounds when they decompose. However, PAHs could have come from inorganic sources or, as many critics believe, from contamination with terrestrial sources. Luann Becker and her colleagues (1997) suggested that the PAHs may have come from melted Antarctic ice. This seems unlikely, but there is no direct evidence of organic origin either.
Magnetite. This iron oxide mineral was thought to be a by-product of bacterial activity. Although some studies have shown a similarity to magnetite grains deposited by bacteria on Earth, other investigators disagree. Most of the criticism is coming from John Bradley and coworkers (1996), who believe that the shape and structure of the grains within this rock suggest an inorganic origin at very high temperatures. Other workers have not confirmed these results.
Iron Sulfides. McKay and his colleagues thought that iron sulfides were remnants of sulfur-eating bacteria. However, there are different varieties of sulfur that come in slightly different atomic weights. Bacteria tend to consume more of the lighter kind, which means they should be concentrated within the iron sulfide minerals, but this is not the case (Shearer, et al., 1996). I am not aware of any convincing counter-response, so far.
Taken together, these observations were supposed to offer compelling evidence of biological activity. However, one (based on PAHs) is ambiguous, one (based on magnetite) is questionable, and another (based on iron sulfide minerals) is contradictory.
Finally, are the rounded or elongated objects really fossil bacteria? To prove this, researchers would have to find organic molecules or unequivocal signs of cellular structures. Currently, such evidence is lacking. When the results were announced last year, critics immediately pointed out that these objects, at less than 0.2 microns in diameter, were several times smaller than the smallest known bacterial cells. If they are genuine remains of living organisms, their size would negate almost everything we know about cell walls, genetics, and the chemistry of life (Harvey, 1997).
All this speculation concerns a single potato-sized piece of rock. By now, scientists may know a lot about the composition of ALH84001 but, as we have seen, the NASA team has not succeeded in backing up its extraordinary claims.
Becker, L., D.P. Glavin, and J.L. Bada (1997), “Polycyclic Aromatic Hydrocarbons (PAHs) in Antarctic Martian Meteorites, Carbonaceous Chondrites, and Polar Ice,” Geochimica et Cosmochimica Acta, 61:475-481.
Bradley, J.P., R.P. Harvey, and H.Y. McSween, Jr. (1996), “Magnetite Whiskers and Platelets in ALH84001 Martian Meteorite: Evidence of Vapor Phase Growth,” Geochimica et Cosmochimica Acta, 60:5149-5155.
Harvey, Ralph P. (1997), “Nannobacteria: What is the Evidence?,” naturalSCIENCE, vol. 1, article 7.
Major, Trevor (1996), “Life on Mars?,” Reason & Revelation, 16:78-79, October.
McKay, David S., et al. (1996), “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001,” Science, 273:924-930, August 16.
Scott, Edward R.D., Akira Yamaguchi, and Alexander N. Krot (1997), “Petrological Evidence for Shock Melting of Carbonates in the Martian Meteorite ALH84001,” Nature, 387:377-379, May 22.
Shearer, C.K., G.D. Layne, J.J. Papike, and M.N. Spilde (1996), “Sulfur Isotope Systematics in Alteration Assemblages in Martian Meteorite ALH84001,” Geochimica et Cosmochimica Acta, 60:2921-2926.