can be at most 4 × 10–5 times that of Earth’s (Weiss et al., 2000). These results suggest that distribution of hypothetical martian biota may not be limited by the availability of energy, but rather by the availability of liquid water.
The metabolic activity of a potential martian biosphere is one of several possible explanations for recent claims of the detection of roughly 10 ppb of methane in the martian atmosphere (Mumma et al., 2004; Krasnapolsky et al., 2004; Formisano et al., 2004). If this detection is correct, the approximately 300-year photochemical lifetime of methane in the martian atmosphere implies the presence of a significant and recent methane source. Candidate abiogenic methane sources include recent volcanism, hydrothermal sources, cometary and meteorite impacts, and the serpentization of basalt (Wallendahl and Treiman, 1999; Krasnopolsky et al., 2004; Formisano et al., 2004). Candidate biogenic sources include fossil biogenic methane diffusing out of the martian crust and ongoing methane production from a hypothetical martian biosphere (Farmer, 1996; Fisk and Giovannoni, 1999; Max and Clifford, 2000; Krasnopolsky et al., 2004; Formisano et al., 2004). The global atmospheric source strength of methane on Mars is less than one-millionth that of methane on Earth. This suggests that, if the amount of methane that appears to be present in the atmosphere is in equilibrium with its production rate (i.e., there is no net accumulation of methane in the subsurface), then the metabolic rate of any hypothetical martian biosophere is quite small. Neither the validity of this assumption nor the accuracy of the identification of atmospheric methane can currently be confirmed. If subsequent investigations were to verify the presence of methane and also confirm that it is of biological origin, the motivation and requirements for preventing the forward contamination of Mars would have to be thoroughly reassessed.
On Mars today, the limiting requirement for habitability appears to be the presence of liquid water. The potential for forward contamination of Mars by Earth microorganisms is therefore closely tied to the existence, state, and distribution of water reservoirs across the planet. Thus, water is the key indicator for special regions (see Box 1.1) on Mars. Knowledge of the distribution and behavior of water in all its forms on Mars is at present incomplete, particularly with respect to liquid water.
The subsections below summarize current understanding of the total martian reservoir and discuss the prospects for liquid water in the deep and near subsurface.
Mars exhibits widespread evidence of extensive modification by the effects of impacts, volcanism, liquid water, ice and wind—processes that appear to have been more active in the planet’s past. Evidence for a water-rich Mars is provided by the geomorphic interpretation of a long list of landforms (e.g., Carr and Schaber, 1977; Rossbacher and Judson, 1981; Carr, 1986, 1996; Squyres et al., 1992; Malin and Edgett, 2000, 2003) and by geochemical and sedimentary evidence, recently acquired by the Mars Exploration Rover (MER), of episodic inundation by shallow surface water at Meridiani Planum (Squyres et al., 2004a,b).4
The most persuasive geomorphic evidence for large amounts of water on Mars are the outflow channels—broad scoured depressions hundreds of kilometers long that exhibit braided and streamlined forms within their beds. The channels generally emerge abruptly from large areas of collapsed and disrupted terrain, the apparent result of a massive release of groundwater. The distribution, size, and range of ages of these features suggest that a significant body of groundwater was present on Mars throughout much of its geologic history and may still persist today (Baker, 1982; Tanaka, 1986; Tanaka and Scott, 1987; Carr, 1986, 1996; Baker et al., 1991).
Using a conservative estimate of the volume of water required to erode the outflow channels, and the likely extent of their subsurface source regions, Carr (1986) estimates that Mars may possess a planetary inventory of