The evidence for life
The indication of life hinges on three important pieces of evidence, all discovered within mineralized fractures in the meteorite in close proximity to each other. One is the discovery of abundant polycyclic aromatic hydrocarbons (PAHs) on the fracture surfaces. These are a family of complex organic molecules which are commonly found on dust grains and certain types of meteorites in outer space, presumably formed by non-biological chemical reactions. However, when micro-organisms die they break down into PAHs as well. The mixture of PAHs found on ALH84001 is very different from that found on dust grains and other meteorites, suggesting the possibility of a biological origin. Thousands of different types of PAHs are found all over the Earth, but those in ALH84001 do not appear to be contaminants which have leaked into the meteorite.
Another line of evidence involves unusual mineral phases found beside the PAHs. These carbonate minerals form "globules" about 50 micrometers across, some of which have cores containing manganese and rings of iron carbonate and iron sulfides, and also contain magnetite and pyrrhotite. These minerals bear strong resemblance to mineral alterations caused by primitive bacteria on Earth. This diversity of minerals in such a small area, formed under the presumed conditions, seem to make a non-biological origin unlikely.
Finally, high-resolution scanning electron microscopy has revealed the presence of tiny "ovoids" which may actually be fossil remnants of tiny (20 to 100 nanometer) bacteria. If so, they are 100 times smaller than any bacteria microfossils found on Earth, except for some supposed "nanofossils" recently discovered in very young terrestrial rocks, a finding currently not generally accepted as fossil organisms. Taken together, the findings are thought to be strong evidence pointing to primitive bacterial life on Mars. The PAHs, unusual mineral phases, and "microfossils" were all located within a few micrometers of one another, indicating a relationship which may require a biological explanation. However, much work will be done on this in the future, including searching for amino acids, other fossil structures such as cell walls, other types of fossils, and fossils of bacteria frozen in the act of reproducing. Mars is almost certain to have been warmer and wetter in its distant past, so the existence of primitive life has been a tantalizing possibility for some time, but the real search may be just beginning.