The possibility of current (extant) or past (extinct) life on Mars has been of interest for centuries. Percival Lowell's early ideas of intelligent life forms who engineered an elaborate canal system have been disproved, but the question of whether microbial life exists or has existed on Mars remains an area of intense debate. The question of martian life is one of the driving forces behind the continuing exploration of the planet.

Martian conditions relevant to biology

Surface conditions on present-day Mars are extremely inhospitable to terrestrial life forms (Clark, 1998). The thin CO2 atmosphere is inefficient at retaining daytime solar heating, resulting in a temperature range of 130–250 K with an average temperature of 240 K. No evidence of biologic replication has been observed in terrestrial organisms at temperatures < 253 K (Beaty et al., 2006). The low temperatures and low atmospheric pressure at the surface prevent liquid water from existing for extended periods of time. Terrestrial life forms require water for survival, so the lack of liquid water on Mars is a major deterrent to life on the surface.

The thin martian atmosphere and the lack of a present-day magnetic field allow harmful radiation to penetrate to the surface (except perhaps in regions where rocks retain strong remnant magnetization [Alves and Baptista, 2004]). Odyssey's MARIE instrument (Badhwar, 2004) measured the radiation environment above the martian atmosphere until October 2003 when particles from a large solar flare caused it to cease functioning.