Isotopic Composition of CO<sub>2</sub> in the Atmosphere of Mars: Fractionation by Diffusive Separation Observed by the ExoMars Trace Gas Orbiter
Juan Alday, Colin Wilson, P. G. J. Irwin, Аlexander Trokhimovskiy, Franck Montmessin, Anna Fedorova, Denis Belyaev, Kevin Olsen, Oleg Korablev, Franck Lefèvre, Ashwin Braude, Lucio Baggio, Andrey Patrakeev, Alexey Shakun
Abstract
Abstract Isotopic ratios in atmospheric CO 2 are shaped by various processes throughout Mars' history, and can help understand what the atmosphere of early Mars was like to sustain liquid water on its surface. In this study, we monitor the O and C isotopic composition of CO 2 between 70 and 130 km for more than half a Martian year using solar occultation observations by the Atmospheric Chemistry Suite onboard the ExoMars Trace Gas Orbiter. We find the vertical trends of the isotopic ratios to be consistent with the expectations from diffusive separation above the homopause, with average values below this altitude being consistent with Earth‐like fractionation ( δ 13 C = −3 ± 37‰; δ 18 O = −29 ± 38‰; and δ 17 O = −11 ± 41‰). Using these measurements, we estimate that at least 20%–40% of primordial C on Mars has escaped to space throughout history. The total amount of C lost from the atmosphere is likely to be well in excess of this lower limit, due to carbonate formation and further sink processes. In addition, we propose a photochemical transfer of light O from H 2 O to CO 2 to explain the larger enrichment in the ratio in H 2 O than in CO 2 .