The next line of evidence for martian life is organic chemical molecules. In the carbonate globules in ALH 84001, McKay’s research group found small quantities of carbon-based chemical compounds, called PAHs (for polycyclic aromatic hydrocarbons). This slide shows the shapes of some of these PAH molecules in chemists' shorthand. Each corner on these drawings represents a carbon atom, and each carbon atom (corner) can be connected to two or three other carbon atoms (corners). Where a carbon atom is connected to only two other carbon atoms, it is also connected to a hydrogen atom, which isn't drawn in.
PAH compounds form easily when plants or animals burn or decompose (they are fairly abundant in broiled steak and cigarette smoke). The only PAH that might be familiar to most people is naphthalene, the main ingredient in mothballs. McKay’s research group showed that the PAHs in the ALH 84001 meteorite are probably martian (not from Earth), and are different from most collections of PAHs in Earth materials except dead and decomposed organisms. From this, they suggested that the PAHs in the meteorite are from dead, decomposed martian life. PAH compounds can also form without the intervention of life, so the PAHs are not yet conclusive proof of life on Mars.
Allan Treiman (Lunar and Planetary Institute)