Despite the popular image of little green Martians in early science fiction films, you won't find any Martians in Star Trek. In the Star Trek: Enterprise series, it is clear that Mars was not inhabited by any humanoid species when Earth established a human colony there. However, in the Star Trek: The Next Generation episodes "The Last Outpost" and "Datalore", it is revealed that scientists from Earth had discovered microscopic evidence of ancient life on Mars.

Strikingly, such a discovery had indeed been made decades later in 1996, when fossilized bacteria was found in a meteorite thought to have come from Mars, the Allan Hills 84001 (commonly abbreviated ALH 84001) meteorite found in Allan Hills, Antarctica. Under scanning electron microscopes, structures were revealed that may be the remains—in the form of fossils— of bacteria-like lifeforms 20-100 nanometres in diameter - similar in size to the theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are really fossilized lifeforms, they would be the first solid evidence of the existence of extraterrestrial life, aside from the chance of their origin being terrestrial contamination.

The search for life in Mars has always been a particular focus of current astrobiology research. Water is considered to be an essential precursor to the development of life Mars missions have already sent back views of the Martian surface that seem to show evidence of dry riverbeds, flood plains and gullies. Sedimentary deposits also suggest the large presence of water at some point in the history of Mars. More recently, strange globs seen on the landing strut of the Phoenix Mars lander could be the first proof that the Mars surface hosts liquid water. Large plumes of methane - a gas usually produced by lifeforms or decaying organic matter - have also been detected in Mars.

Missions specifically designed to search for life include the Viking program and Beagle 2 probes, both directed to Mars. However, the Viking results were inconclusive and Beagle 2 failed to transmit from the surface and is assumed to have crashed.

Chain structures in meteorite fragment ALH84001

Gullies and river beds on the Mars surface