Photo-induced fractionation of water isotopomers in the Martian atmosphere

The history and size of the water reservoirs on early Mars can be constrain ed using isotopic ratios of deuterium to hydrogen. We present new laborator y measurements of the ultraviolet cross-sections of H2O and its isotopomers , and modeling calculations in support of a photo-induced fractionation eff ect (PHIFE), that reconciles a discrepancy between past theoretical modelin g and recent observations. This supports the hypothesis that Mars had an ea rly warm atmosphere and has lost at least a 50-m global layer of water. Lik ely applications of PHIFE to other planetary atmospheres are sketched.