Life on Mars? The Viking experience The observational evidence that the surface environment of Mars has been more amenable for life in the past inspired experiments on the Viking landers. Both craft had instrument payloads that included a gas chromatograph-mass spectrometer (GC-MS) and three separate biological experiments. The GC-MS was designed to test surface soils for the presence of organic compounds, and to identify them if present. The biological experiments were designed to test for metabolic action. Unfortunately, the results obtained were, on balance, negative. Although one of the experiments did give a positive signal that might have implied the presence of a metabolising agent, the overall conclusion from the mission was that there was no detectable trace of organic matter in the surface soils at either of the two landing sites, and thus no likelihood of the presence of extant life. The exploration continues Despite these conclusions, Mars has remained a prime target in the search for extraterrestrial life. The recognition of extremophile micro-organisms that can survive (and thrive) in conditions previously thought unfriendly to life, but that might exist at or below the surface of Mars, has given impetus to remote martian exploration. Since the Viking mission, there have been two very successful missions to Mars (NASA's Pathfinder and Global Surveyor). Although neither of these missions included experiments specifically intended to test for traces of past or present life at the martian surface, they are part of NASA's programme of missions leading up to the return of samples from Mars to Earth. However, before the sample return mission takes place, the European Space Agency's (ESA) Mars Express mission will be launched in June 2003, arriving at Mars in late December 2003. Extreme lifeforms on Mars? Mars has had liquid water and ice flowing across its surface in the past. When water was present on the surface, it was warm and briny, and restricted in flow. In other words, it might have been locked in enclosed basins that overflowed in episodes of flash flooding. Although the surface of Mars is now apparently dry, and, as a result of oxidation by solar ultraviolet (UV) radiation, presumably devoid of organic compounds, we have no knowledge of what is present in the subsurface soil layers. They might be reservoirs of permafrost, or even liquid water trapped within pore spaces, giving rise to organisms like the cryptoendolithic bacteria of the Dry Valleys of Antarctica. Residues from evaporating brines might host desiccated halophilic or sulphur-loving microorganisms. But in the absence of suitable meteoritic material, the answer to the question of the existence (either now or in the past) of life on Mars must await results from future space missions to the red planet.

Outline of what are believed to be possible microscopic fossils of bacteria-like organisms found in the meteorite ALH84001. The Viking 1 Lander sampling arm created a number of deep trenches as part of the surface composition and biology experiments on Mars. The digging tool on the sampling arm (at lower centre) could scoop up samples of material and deposit them into the appropriate experiment (NASA).