Photochemical depletion of heavy CO isotopes in the Martian atmosphere

The atmosphere of Mars is enriched in heavy isotopes with respect to Earth as a result of the escape of the atmosphere to space over billions of years. Estimating this enrichment requires a rigorous understanding of all atmospheric processes that contribute to the evolution of isotopic ratios between the lower and upper atmosphere, where escape processes take place. We combine measurements of CO vertical profiles obtained by the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter with the predictions of a photochemical model and find evidence of a process of photochemistry-induced fractionation that depletes the heavy isotopes of C and O in CO (δ 13 C = −160 ± 90‰ and δ 18 O = −20 ± 110‰). In the upper atmosphere, accounting for this process reduces the escape fractionation factor by ~ 25%, suggesting that less C has escaped from the atmosphere of Mars than previously thought. In the lower atmosphere, incorporation of this 13 C-depleted CO fractionation into the surface could support the abiotic origin of recently found Martian organics.