In August 1996, McKay et al. (1996) proposed that the Antarctica meteorite ALH84001, believed to come from Mars, contained evidence of ancient life. Two questions naturally occur to the interested outsider. First, how is it known that the meteorite is from Mars? And secondly, how strong is the evidence of life? The answers to both these questions are revealing not only for the fascinating science involved but also for the insight they provide to how scientific truth is established.

 The evidence for the Martian origin of this sample (and 11 other rare meteorites) is now considered well established by the meteoritics community. It rests on a pyramid of many specific details about the chemistry of these samples, all of which are consistent withan origin from Mars, and one specific finding which is extremely difficult to explain in any other way.

Meteorites are distinct from terrestrial rocks in many ways. Not the least of these differences is that in many cases these samples are actually observed to fall from the sky, in a path that can be traced backwards to an orbit from the asteroid belt. Their origin in space is unquestioned.

The vast majority of stony meteorites, chondrites, are made from the conglomeration of tiny (1mm or less in diameter) spheres of once-molton rock called chondrules. Though these pieces of the sample were once molten, the sample as a whole was assembled cold; some meteorites are remarkably inhomogeneous in their composition and structure. In addition, these meteorites have several other important traits which distinguish them from terrestrial rocks.

First, the vast majority were formed 4.6 billion years ago. Though various techniques give a similar age for the Earth as a whole, individual rocks from Earth's surface are generally no older than 3 billion years, and most of the Earth's crust has been molten and re-formed (with consequent re-setting of these radioactive clocks) within the last one hundred million years.

Second, the oxidation state of the iron in these meteorites is much lower than on Earth. Unlike rocks on Earth's surface, meteorites were not formed in equilibrium with an oxygen-rich atmosphere. Indeed, the weathering of meteorites - the rusting of the bits of metal found in most meteorites - proceeds rapidly once they hit Earth. (The iron in meteorites is also rich in nickel and other trace metals not normally seen in terrestrial iron.)

The third characteristic difference is the meteorite's isotope abundances. In particular, various classes of meteorites have distinctive abundances of oxygen-16 (measured relative to oxygen-17 and -18 abundances) which can only be explained by each type of meteorite having been formed in a region of space that had a mixture of oxygen isotopes distinctively different from other types - or Earth's.

A small number of meteorites, about 5% of all our samples, are not chondrites but rather the frozen remains of basaltic lavas. Nonetheless, even these basaltic meteorites are still roughly 4.6 billion years old, have a low oxidation state, and have distinctive oxygen isotopes. And of the 15,000 meteorites in our collection, about a dozen appear to be pieces of the Moon. They share the relatively younger ages of Moon rocks and also the lunar (and terrestrial) oxygen isotope abundances.

Another twelve meteorites, however, are quite distinct. Most of them are even younger than Moon rocks, some dating at a mere 1 billion years, others less than 100 million years in age. The iron is as oxidised as terrestrial iron, suggesting they were formed in the presence of an oxidising atmosphere. But their oxygen isotopes, the same for all twelve samples, are distinctly different from Earth's. (All of these samples were either seen to fall or discovered in Antarctic meteorite fields; there is no question that they are meteorites.)

Though all these factors are consistent with a Martian origin, none of them are decisive. One could envision an asteroid with perhaps a large ice content producing local spots rich in oxygen. For a body to have been molten as recently as one billion years ago would suggest, by all our understanding of how planets are heated and cool off, that these samples must have come from a Mars-sized body; but our theories could be incomplete. Indeed, other theorists had a hard time envisioning a way in which the sample could be ejected from a planet as big as Mars.

However, in the early 1980's, it was discovered that some of these samples (most notably EET79001) had gas bubbles trapped in veins of once-molten rock. When analysed, the composition of this gas was an excellent fit for the Martian atmosphere. Since the Martian atmosphere is known to have unique abundances and isotope patterns for several gases, this match was compelling evidence.

Evidence for life in these meteorites is not yet so convincing. The meteorite in question, ALH84001, is unique in many ways. Its mineralogy is different from the other Martian meteorites (or any other meteorite). It has a very old age, close to 4.6 billion years. And it contains many minerals which apparently grew quickly, too fast for chemical equilibrium to be maintained; thus attempting to use its chemistry to determine the physical conditions under which it formed is problematical. Nonetheless, it does share two important features that allow it to be classified with the other Martian meteorites. It has the same unusually high oxidation state; and its oxygen isotope values place it squarely among all the other Mars meteorites (and away from any other known class).

What is the evidence for relict life? In unusual carbonate minerals within this meteorite the McKay et al. group discovered iron sulfide crystals associated with magnetite in a manner normally seen in terrestrial rocks only when both are the product of bacterial action. Some magnetite crystals themselves had a morphology usually taken as evidence of biogenic activity. Polycyclic aromatic hydrocarbons (PAH's) were found in these carbonates, but not in other parts of the rock. And, most dramatically, within these carbonates themselves TEM images have revealed tubular, segmented shapes reminiscent of purported terrestrial nanobacteria or parts of larger known terrestrial fossil bacterial forms.

This evidence has been strongly attacked by several scientific groups, and alternate hypotheses have been put forward to explain each line of evidence. These alternate explanations have themselves in turn been scrutinised and strongly challenged. (For more information on this debate, the web site maintained by Dr. Alan Trieman at http://cass.jsc.nasa.gov/lpi/meteorites/mars_meteorite.html is highly recommended.)

No one disputes the data presented by McKay et al. but the interpretation of those data are in great dispute. Every line of evidence brought forward so far could be explained by mechanisms other than Martian life (though no one mechanism can explain all the evidence together except life.)

To be able to identify these samples as having come from Mars was possible only because the Martian atmosphere was unique, and well known independently; and because the meteorites could be fit into a much larger, well understood picture of both Mars and meteorite geochemistry. By contrast, our understanding of the conditions under which ALH84001 was formed is not well understood; nor do we have any idea of what type of "life" if any should be expected to be found on Mars.

If we already knew of life on Mars, from actually having observed evidence of its existence there, the evidence within the meteorites would be much more easily accepted as a sign of ancient life. Without that external knowledge, the very same evidence is considered less than compelling.

Non-scientists often speak of a "scientific proof" as if one key experiment can provide irrefutable understanding. In fact, nothing in science is so "proved"; nor is such an idea of proof the goal or intent of a scientist. One piece of evidence may be a keystone to a proof, but you need more than a keystone to build a building. How evidence and proof are used to determine the origin of these meteorites and the presence or not of life forms is a clear illustration of that fact.

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 ALH84001. Science 273, 924-930.