Recombination rates of O and CO on solid CO 2: Implications for the composition of the martian atmosphere

Abstract

An atomic oxygen flow system and a C 14 radiochemical technique have been used to show that the reactions O + CO → CO 2 and O + O → O 2, are heterogeneously catalysed by solid CO 2 at 77 K. The O-CO recombination is first order in CO and inhibited by O, whereas the O-O recombination is first order in O and weakly inhibited by CO. Assuming simple first order kinetics, recombination coefficients γ co = 1.3(±0.9) × 10 −5 and γ O = 0.05± 0.02 are determined. A recombination mechanism involving an intermediate adsorbed CO 3 is proposed. If the kinetic results are assumed to apply under Martian surface conditions, then conversion of CO to CO 2 by reaction on the solid CO 2 at the polar caps occurs at ~10 times the total column recombination rates for homogeneous reactions previously proposed; night-side CO 2 ice clouds would also constitute an important recombination surface.