ALH84001: The claim of life in a Martian meteorite 10 years later

We had an interesting in-house seminar yesterday at the LPI (Lunar and Planetary Institute) given by Dr. Allan Treiman. The topic was the Martian meteorite ALH84001, which is the rock that gained much fame and notoriety in 1996 when a team of scientists from the Johnson Space Center announced that they had found evidence for possible relic biogenic activity (read: traces of small dead bugs from Mars) within cracks in the meteorite. Unless you've been living in a cave for the past ten years, you've probably heard about this rock. In fact, this Martian rock is without question the single most studied rock on the planet. A quick search of google scholar and the NASA Astrophysical Data System reveals several hundred subsequent studies that cite the original publication in Science. The impact of this paper was huge and cannot be understated. By how do things stand today? Has this paper been re-affirmed or disproved? McKay et al. (science-speak for McKay and co-authors) gave four lines of evidence in their initial paper of non-terrestrial biologic activity in the meteorite:

1. Small bacteria-shaped objects.
   
2. Abundant PAHs or polycyclic aromatic hydrocarbons, which are often formed from the breakdown of organic molecules.
3. Carbonate globules.
4. Tiny magnetite grains associated with the carbonates that are similar to those produced by known terrestrial bacteria.
   

Although each of these lines of evidence, taken individually, is not conclusive, the authors argues that the collective weight of the evidence supported a Martian biologic origin. In their words,

None of these observations is in itself conclusive for the existence of past life. 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.

Personally, I have always been bothered by this statement. Essentially, this statement is akin to Occam's Razor, a philosophic tool which states that "one should not increase, beyond what is necessary, the number of entities required to explain anything." Occam's Razor is often paraphrased as "the simplest explanation is often the best." McKay et al. are saying that although individual components may be explained by non-biologic processes, it is difficult to conceive of a single non-biologic explanation that explains all of these observations. Therefore, biology is the most likely culprit.

While this idea is not wholly without merit, I believe this is an utterly insufficient criterion in this case. Carl Sagan if often quotes as having said "extraordinary claims require extraordinary evidence." I can think few claims more extraordinary than the existence of life on another planet. The idea of accepting such a fantastic claim as true on the basis of faith in antithetical to the very nature of science. Therefore, the burden of proof lies upon those making the claim.

Treiman presented an alternative, non-biologic scenario. Partly based on his own calculations and partly based upon the work of other scientists, Treiman proposed that the carbonates and associated magnetites were formed from the relatively high temperature (~400° C) decomposition of siderate (an iron-bearing carbonate). The features which resemble bacteria turn out to be much smaller than most terrestrial bacteria and may thus have insuffient interior volume to contain a cell wall, nucleus, and other components necessary for self-sustained operation. They may instead be parasitic, or more troubling may be artifacts of the sample preparation process. The PAHs found in ALH84001 are not terrestrial in origin, but may also be from the breakdown of organic compoundss that are not associated with once-living organisms.

So where do we stand? I think the consensus is that biologic activity does not seem to be required to have produced thefeaturess observed in ALH84001. Most, if not all, of the observed features can be plausibly explained without invoking a biologic explanation. But the same argument I raised earlier applies here: while biology is not required, neither is it entirely ruled out. No one can say for certain that biology played no role.

Was it all for naught? After all that work, the best answer we have is "we don't know for sure?" That's what NASA's been spending my tax dollars on? Before you get on the phone to your congressperson, consider the following. The study of this rock has advanced our knowledge considerably. Scientists in many different sub-disciplines to make considerable advances in analytical techniques. Our understanding of contamination procedures have been improved. I think we are better prepared as a result of this effort to study samples that we will bring from Mars and other bodies. We now have a better idea of the kinds of biomarkers and traces of life we should be looking for. In short, it was be no means a wasted effort. Sometimes science raises more questions than it answers. And believe me, that is a good thing.