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 130-250K with an average temperature of240K. 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. Although it only operated for 19 months, MARIE determined that the daily radiation dose from galactic cosmic rays was between 18 and 24 millirad (1 rad=0.01 Jkg-1). Protons emitted during solar flare events were recorded to have typical doses of 1-2 rad and the largest events provided doses >7 rad (Cleghorn etal., 2004). While these are relatively small doses (the average cosmic ray exposure on Earth is 30-45 millirad yr-1), the higher dosage solar particle events require shielding to protect cells from damage.
Ultraviolet (UV) radiation is another major concern on the surface of Mars. The martian atmosphere contains very little ozone except over the winter pole, therefore allowing UV to reach the surface. Models suggest that UV fluxes are up to three orders of magnitude more damaging to DNA on the martian surface than on Earth (Cockell et al., 2000), and laboratory experiments conducted under martian UV conditions found rapid inactivation of seven species of bacillus (Schuerger et al., 2006). However, UV conditions on present-day Mars may not be drastically different from those on Archean Earth when primitive life existed (Cockell et al., 2000), so UV levels alone may not contraindicate martian life forms.
However, the high UV levels together with the highly oxidizing chemistry of the martian soils is an extremely destructive combination for life forms (Biemann et al., 1977). Very low levels of organic materials were detected in the martian regolith by the Viking landers which suggests that organic material is being destroyed as fast as or faster than it is being produced on Mars. Highly acidic conditions, such as revealed in Meridiani Planum by Opportunity, could host specialized life forms, but it is unclear if prebiotic chemical reactions could occur in such conditions to give rise initially to life forms (Squyres et al., 2004c).
The combination of low temperature, low pressure, lack of liquid water, high radiation and UV levels, and oxidizing soil conditions makes the martian surface a very inhospitable place for life forms similar to those found on Earth. However, the subsurface might provide conditions more conducive to biologic activity (Clark, 1998; Farmer and Des Marais, 1999; Cockell and Barlow, 2002; Diaz and Schulze-Makuch, 2006). Overlying surface materials provide protection from the hazardous radiation environment. Temperatures increase with depth in the martian substrate and liquid water oases could exist where a high geotherm encounters H2O-rich layers. Long-lived hydrothermal activity associated with volcanism or large impact craters could provide habitable oases for extended periods of time (Newsom et al., 2001; Varnes et al., 2003; Abramov and Kring, 2005). It is within these regions where extant martian life may occur.
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