Ground-based observations (Krasnopolsky et al., 2004) and measurements from MEx's Planetary Fourier Spectrometer (PFS) (Formisano etal., 2004) have detected methane (CH4) in the martian atmosphere with a global average of ~10 parts per billion (ppb). However, PFS detected regional variations in the concentration of CH4, ranging between 0 and 35 ppb, suggesting localized sources/sinks of the gas (Formisano et al., 2004). Methane undergoes photochemical losses in the martian
atmosphere, with a mean loss rate of 2.2X 10 cm s and a mean lifetime of ~340 years (Krasnopolsky et al., 2004; Krasnopolsky, 2006), indicating that it must constantly be replenished. The timescale of global mixing of CH4 is ~0.5yr, so constant replenishment also is indicated by the regional variations in concentration (Krasnopolsky et al., 2004).
The 1700 ppb of terrestrial atmospheric methane primarily results from biologic activity and methane-loving organisms (methanogens) are plausible candidates for martian microbes if they exist (Max and Clifford, 2000; Jakosky etal., 2003; Varnes et al., 2003). If martian atmospheric CH4 results from subsurface biological activity, this microbial population must be much smaller than on Earth because of the lower CH4 concentrations. Krasnopolsky et al. (2004) estimated that the dry biomass produced by biologic methanogenesis on Mars is between 15 X10-13 and 4 X10-14 kgm-3, well below the detection limit of 10-8kgm~3 of Viking's GCMS.
Biologic activity is not the only possible source of martian atmospheric CH4. Dust and chemical changes to the atmosphere produced during cometary impacts can produce CH4 (Kress and McKay, 2004), but the amounts are too low to explain the globally averaged concentrations of 10 ppb (Krasnopolsky, 2006). Volcanic outgassing is a possible source, but CH4 is a minor component of terrestrial volcanic emissions (Ryan et al., 2006) and no active volcanism has been detected on Mars from either visible or thermal IR analysis. Low-temperature alteration of basalt by carbon-rich hydrothermal fluids is a possible abiogenic source of CH4 (Lyons et al., 2005), as is photolysis of H2O in the presence of CO (Bar-Nun and Dimitrov, 2006). Carbon isotopic analysis is necessary to determine whether the CH4 results from biogenic or abiogenic processes.
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