The isotopic signature of atmospheric carbon offers a unique tracer for the history of the Martian atmosphere and the origin of organic matter on Mars. The photolysis of CO2 is known to induce strong isotopic fractionation of the carbon between CO2 and CO. However, its effects on the carbon isotopic compositions in the Martian atmosphere remain uncertain. Here, we develop a 1D photochemical model to consider the isotopic fractionation via photolysis of CO2, to estimate the vertical profiles of the carbon isotopic compositions of CO and CO2 in the Martian atmosphere. We find that CO is depleted in 13C compared with CO2 at each altitude, due to the fractionation via CO2 photolysis: the minimum value of the δ 13C in CO is about −170‰ under the standard eddy diffusion setting. This result supports the hypothesis that fractionated atmospheric CO is responsible for the production of the 13C-depleted organic carbon in the Martian sediments detected by the Curiosity Rover, through the conversion of CO into organic materials and their deposition on the surface. The photolysis and transport-induced fractionation of CO that we report here leads to a ∼15% decrease in the amount of inferred atmospheric loss when combined with the present-day fractionation of the atmosphere and previous studies of carbon escape to space. The fractionated isotopic composition of CO in the Martian atmosphere may be observed by ExoMars Trace Gas Orbiter and ground-based telescopes, and the escaping ion species produced by the fractionated carbon-bearing species may be detected by the Martian Moons eXploration mission in the future.

The atmosphere of Mars is mainly composed by carbon dioxide (CO2). It has been predicted that photodissociation of CO2 depletes 13C in carbon monoxide (CO). We present the carbon 13C/12C 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 12C16O, 13C16O, and 12C18O 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 12C16O 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 δ 13C value obtained from the 13C16O/12C16O ratios is −263‰, and the standard deviation and standard error of the mean are 132‰ and 4‰, respectively. The relatively large standard deviation is due to the strong temperature dependences in the 12C16O lines. We also examine the 13C16O/12C18O ratio, whose lines are less sensitive to temperature. The mean δ value obtained with 12C18O instead of 12C16O is −82‰ with smaller standard deviation, 60‰. These results suggest that CO is depleted in 13C when compared to CO2 in the Martian atmosphere as measured by the Curiosity rover. This depletion of 13C in CO is consistent with the CO2 photolysis-induced fractionation, which might support a CO-based photochemical origin of organics in Martian sediments.