The presence of methane in the near-surface atmosphere of Mars is a scientifically established fact (Krasnopolsky 2006; Formisano et al. 2004), and Formisano et al. (2004) have stressed the direct correlation to locations with an enhanced presence of water/ice. The global production rate of methane on Mars has been estimated to be of the order of about 3 x 105 kg methane per (terrestrial) year, or about 34 kg methane per hour. Liquid interfacial water in form of premelted ice and adsorbed water is unavoidable in case of the presence of water-ice and of atmospheric water vapor, also under martian conditions (Möhlmann, 2007). The resulting freezing point depression will cause interfacial water to remain liquid down to about 190 K. Thus, the observed direct correlation between the local or regional presence of atmospheric methane and of ice is understood as an indication for a direct coupling between methane and liquid interfacial water in the subsurface of Mars.As been shown by laboratory experiments, terrestrial methanogenic archaea isolated from Siberian permafrost soils are well able to survive martian thermophysical conditions at least over the experiment duration of three weeks (Morozova et al., 2007). These martian conditions were a diurnal temperature profile between 210 K and 280 K, a related diurnal profile of the atmospheric water content between saturation (aW = 1) and dry (aW < 0.03) conditions, all at a total pressure of 6 mbar CO2. Methanogenic archaea from terrestrial permafrost survived these conditions, while methanogens from non-permafrost environments did not show any activity after the simulation, which correspond with a drastically decrease in their cell numbers (< 5% in comparison to the amount of the beginning of the experiment; Fig. 1). Furthermore, the survival potential of methanogens from permafrost exposed to different environmental stress conditions such as low temperature (down to 78°C), high salinity (up to 6 M NaCl), starvation (up to 3 months), long-term freezing (up to two years), desiccation (up to 25 days) and radiation (F10 = 1695 J m-2; D10 = 25 kGy) makes these organisms to valuable model organisms in our effort to investigate the potential biological origin of methane in the martian atmorphere (Morozova and Wagner, 2007).The typical order of magnitude of the production rate of methane by terrestrial methanogenic archaea in soil of an average temperature of about -6° C is about 0.14 nmol h-1 g-1, where the mass (measured in g) is that of the soil (Fig, 2; Wagner et al., 2007). This corresponds to about 4 x 10-6 kg h-1 m-3, if a mean soil mass density of 2 g cm-3 is taken. Obviously, a local volume of the upper martian surface of about 1.3 x 107 m3 (corresponding to an only 235 m sized cubus) of soil with terrestrial methanogenic archaea would be necessary to produce the required mars-relevant amount of methane of about 34 kg methane per hour under terrestrial conditions. With respect to Mars, the following conditions in the upper martian subsurface are to be taken into account:a) The thickness of the biologically effective upper subsurface can assumed to be of the same order of magnitude as the diurnal thermal penetration depth L of the soil with mass density &#61554;, heat conductivity &#61548;, heat capacity c, and duration of one day P, . This is for martian conditions given by .b) The duration of the biologically active period per day can be expected to be of only a few hours. Three hours will in the following be taken as an exemplary value. Furthermore, seasonal variations may bring in another factor of about œ.c) According to the Arrhenius equation, the reaction rate of processes with an activation energy Ea decreases exponentially with temperature . The reaction rate of biological processes decreases correspondingly. Processes at T around about 240 K and 250 K are therefore slower by about one to two orders of magnitude than those around 270 K. A factor 1/30 will be taken for numerical estimates.The above given estimated values give a factor of about 2 x 10-4 (0.1 x 1/8 x œ x 1/30) with respect to the possible surface area above a hypothetical biologically active methanogenic subsurface volume. This hypothetical total surface area of seasonally and diurnally variable biologically active methanogenic subsurface parts on Mars is in the range of 1010-1011 m2, corresponding to a few 100-km sized or to possibly multiple 10-km sized surface spots above hypothetical biologically methanogenic shallow subsurface parts.In conclusion, our estimates, which are based on measured methane concentrations in the martian atmosphere and on production rates of terrestrial methanogenic archaea show that a hypothetical biologically methanogenic subsurface source, similar to methanogenic archaea in terrestrial permafrost, could be the cause of the observed quantities of methane on Mars.