Depletion of 13C in CO in the Atmosphere of Mars Suggested by ExoMars-TGO/NOMAD Observations

The atmosphere of Mars is mainly composed by carbon dioxide (CO2). It has been predicted that photodissociation of CO2 depletes C-13 in carbon monoxide (CO). We present the carbon C-13/C-12 isotopic ratio in CO at 30-50 km altitude from the analysis of the solar occultation measurements taken by the instrument Nadir and Occultation for Mars Discovery on board the ExoMars Trace Gas Orbiter (ExoMars-TGO). We retrieve (CO)-C-12-O-16, (CO)-C-13-O-16, and (CO)-C-12-O-18 volume mixing ratios from the spectra taken at 4112-4213 cm(-1), where multiple CO isotope lines with similar intensities are available. The intensities of the (CO)-C-12-O-16 lines in this spectral range are particularly sensitive to temperature, thus we derive the atmospheric temperature by retrieving CO2 density with simultaneously measured spectra at 2966-2990 cm(-1). The mean delta C-13 value obtained from the (CO)-C-13-O-16/(CO)-C-12-O-16 ratios is -263 parts per thousand, and the standard deviation and standard error of the mean are 132 parts per thousand and 4 parts per thousand, respectively. The relatively large standard deviation is due to the strong temperature dependences in the (CO)-C-12-O-16 lines. We also examine the (CO)-C-13-O-16/(CO)-C-12-O-18 ratio, whose lines are less sensitive to temperature. The mean delta value obtained with (CO)-C-12-O-18 instead of (CO)-C-12-O-16 is -82 parts per thousand with smaller standard deviation, 60 parts per thousand. These results suggest that CO is depleted in C-13 when compared to CO2 in the Martian atmosphere as measured by the Curiosity rover. This depletion of C-13 in CO is consistent with the CO2 photolysis-induced fractionation, which might support a CO-based photochemical origin of organics in Martian sediments.