“Methane on Mars: A product of H2O photolysis in the presence of CO”: Response to V.A. Krasnopolsky

The detection of CH4 in the martian atmosphere, at a mixing ratio of about 10 ppb, prompted Krasnopolsky et al. [Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004. Icarus 172, 537–547] and Krasnopolsky [Krasnopolsky, V.A., 2006. Icarus 180, 359–367] to propose that the CH4 is of biogenic origin. Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320–322] proposed that CH4 can be formed in the martian atmosphere by photolysis of H2O in the presence of CO. We based our arguments on a clear demonstration that CH4 is formed in our experiments, and on thermodynamic equilibrium calculations, which show that CH4 formation is favored even in the presence of oxygen at a mixing ratio 1.3 × 10 −3 , as observed on Mars. In the present comment, Krasnopolsky [Krasnopolsky, V.A., 2007. Icarus, in press (this issue)] presents his arguments against the suggestion of Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320–322], based on the effect of O2 on CH4 formation, the absence of kinetic pathways for CH4 formation and on the inadequacy of thermodynamic equilibrium calculations to describe the martian atmosphere. In this rebuttal we demonstrate that experiments with molecular oxygen at a ratio of O 2 /CO 2 = ( 8.9 – 17 ) × 10 −3 , exceeding the martian ratio, still form CH4. Thermodynamic equilibrium calculations replicate the experimental CH4 mixing ratio to within a factor of 1.9 and demonstrate that CH4 production is favored in the martian atmosphere, which is obviously not in thermodynamic equilibrium. Consequently, we do not find the presence of methane to be a sign of biological activity on Mars.