Isotopic fractionation of water and its photolytic products in the atmosphere of Mars

The current Martian atmosphere is about five times more enriched in deuterium than Earth’s, providing direct testimony that Mars hosted vastly water its early youth nowadays. Estimates of the total amount lost to space from mean D/H value depend on a rigorous appraisal relative escape between deuterated and non-deuterated water. Isotopic fractionation lower upper atmospheres has been assumed be controlled by condensation photolysis, although their respective roles influencing proportions atomic D H populations have remained speculative. Here we report HDO H2O profiles observed Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) orbit around that, once combined with expected photolysis rates, reveal prevalence perihelion season for formation at altitudes relevant escape. In addition, while condensation-induced main driver variations vapour, differential important factor determining isotopic composition dissociation products. Three years observations ExoMars TGO clarified mechanism atmospheric loss Mars, particularly fraction compared hydrogen. Analysis several isotopologues mid-atmosphere shows are produced amounts photolysis. These atoms able atmosphere.