Isotopic composition of CO₂ in the atmosphere of Mars: fractionation by diffusive separation observed by the ExoMars Trace Gas Orbiter

Juan Alday (1) , Colin F. Wilson (1) , Patrick G. J. Irwin (1) , Alexander Trokhimovskiy (2) , Franck Montmessin (3) , Anna A. Fedorova (2) , Denis A. Belyaev (2) , Kevin S. Olsen (1) , Oleg Korablev (2) , Franck Lefèvre (3) , Ashwin Braude (3) , Lucio Baggio (3) , Andrey Patrakeev (2) , Alexey Shakun (2)

1 AOPP - Department of Atmospheric, Oceanic and Planetary Physics [Oxford]
2 IKI - Space Research Institute of the Russian Academy of Sciences
3 PLANETO - LATMOS

Isotopic ratios in atmospheric CO₂ 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 CO₂ 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 (δ¹³C =-3 ± 37 ; δ¹⁸O =-29 ± 38 ; δ¹⁷O =-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 H₂O to CO₂ to explain the larger enrichment in the ¹⁸O/¹⁶O ratio in H₂O than in CO₂.

Domains: Sciences of the Universe [physics]

Juan Alday, Colin F. Wilson, Patrick G. J. Irwin, Alexander Trokhimovskiy, Franck Montmessin, et al.. Isotopic composition of CO₂ in the atmosphere of Mars: fractionation by diffusive separation observed by the ExoMars Trace Gas Orbiter. Journal of Geophysical Research. Planets, 2021, 127 (12), pp.e2021JE006992. ⟨10.1029/2021je006992⟩. ⟨insu-03473299v2⟩