Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Although fictional Martians have been a recurring feature of popular entertainment, it remains an open question whether life currently exists on Mars, or has existed there in the past.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 onEarth. This diversity of minerals in such a small area,formed under the presumed conditions, seem to make a non-biological origin unlikely.
