The upshot is that there is strong, but not quite ironclad, evidence that there were primitive bacteria living on Mars about 3-1/2 billion years ago. This is the conclusion of a large research collaboration involving nine scientists (two of them students) at five institutions: David McKay and Everett Gibson at NASA Johnson Space Center, Kathie Thomas-Keprta at Lockheed-Martin, Hojatollah Vali at McGill University, Christopher Romanek at Savannah River Ecology Laboratory, and Simon Clemett, Xavier Chillier, Claude Maechling, and Richard Zare of Stanford University. Here is a summary of what they found.

The evidence for the existence of primitive life on ancient Mars is definitely not of the smoking-gun variety. However, if you try to imagine what evidence there could be that there were bacteria on Mars (or, for that matter, on Earth) 3.6 billion years ago, you will quickly realize that smoking-gun evidence is pretty well impossible -- so whatever evidence there might be would have to be pretty subtle.

The evidence comes from a meteorite which was found in the Allan Hills region of Antarctica in 1984, and which therefore acquired the prosaic name ALH84001. It is a chunk of igneous rock, about the size of a potato and with a mass of about 1.9 kg, or 4.2 lb. One of the finders, Roberta Score, recalls it having a weird, greenish appearance, but the truly remarkable features of ALH84001 only appear under microscopes and, especially, electron microscopes. In brief (for details and pictures, click here), the rock has small globules of carbonate sitting on fracture surfaces in its interior. Those globules contain tiny, elongated particles of magnetite and iron sulfides, and have strange oval and tubular structures on their surfaces. The rock also contains molecules called "polycyclic aromatic hydrocarbons," concentrated most heavily where the carbonate globules are. Among the ways such molecules are produced is the decomposition of dead microorganisms.

What do these things mean?? The research team's argument runs something like this:

• ALH84001 came from Mars. It is one of about a dozen meteorites which are known to have done so. For the evidence that this is true, click here.

• ALH84001 has led an interesting life, for a rock. It formed from volcanic magma 4.5 billion years ago (Evidence?), and was smashed, probably by asteroid strikes, 4.0 and 3.6 billion years ago (Evidence?). Between these times, it seems to have been immersed in water which was charged with carbon dioxide (Evidence?). Much later, about 16 million years ago, it was blasted off into space by another meteorite strike (Evidence?), and after floating in space for 16 million years it finally found itself in the same place as the Earth, and fell through the atmosphere to land in Antarctica a mere 13,000 years ago (Evidence?).

• The oval and tubular shapes on the surfaces of the carbonate globules look similar to fossils of some nanobacteria and filamentous bacteria which occur on Earth. The magnetite particles also resemble the tiny magnetic particles produced by some terrestrial bacteria. (Details?)

• These strange features of ALH84001 almost certainly have their origin on Mars -- they were produced long before the asteroid's relatively recent arrival on Earth (Evidence?). Finally, it is very difficult to come up with any plausible explanation for their existence without invoking the idea that they were produced by living bacteria (Details?).

The conclusion is that, radical though it may be, the most likely explanation for the appearance of these odd microscopic features of ALH84001 is that they are the fossilized remains of bacteria which lived on Mars some 3.6 billion years ago. If this conclusion holds up, it will be another in a long series of scientific findings which have chipped away at the old idea that humanity holds a privileged place at the center of the cosmos. It would mean that what we know as life has appeared on the only two planets in our solar system which were ever capable of supporting it, and so it would suggest that life may be rather common in the Universe -- making the Universe a more interesting place than it seemed to be in July.

Other sources of information

The Federation of American Scientists maintains a very comprehensive Mars page, which includes transcripts of all the news conferences, the President's remarks, and so forth. NASA has a page about the project with copies at three sites (choose the one which isn't busy!), at Johnson Space Center, at NASA Ames, and at NASA Goddard. I've found that these have a funny background color which makes them almost unreadable on some browsers, but you'll have to see whether or not yours is one of them. There is a nice Sky and Telescope article due to come out in the October '96 issue. If you want original sources, there is the NASA news bulletin released on August 7, and the prepublication text of the research article to appear in the August 16, 1996 issue of Science magazine.