Reaction of size-selected iron-oxide cluster cations with methane: a model study of rapid methane loss in Mars' atmosphere.

We report gas-phase reactions of free iron-oxide clusters, Fe n O m + , and their Ar adducts with methane in the context of chemical processes in Mars' atmosphere. Methane activation was observed to produce Fe n O m CH 2 + /Fe n O m CD 2 + and Fe n O m C + , where the reactivity exhibited size and composition dependence. For example, the rate coefficients of methane activation for Fe 3 O + and Fe 4 O + were estimated to be 1 × 10 -13 and 3 × 10 -13 cm 3 s -1 , respectively. Based on these reaction rate coefficients, the presence of iron-oxide clusters/particles with a density as low as 10 7 cm -3 in Mars' atmosphere would explain the rapid loss of methane observed recently by the Curiosity rover.