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

Abstract

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.

Synthesis of 13C-depleted organic matter from CO in a reducing early Martian atmosphere

“…Nonetheless, our analysis confirms that the broadband solar UV photolysis of CO 2 is associated with a large negative carbon isotope effect (1,000 ln α < −100‰). This is also supported by satellite observations showing strong 13 C depletion of CO in Earth's mesosphere 30 and in the present-day lower atmosphere of Mars [31][32][33] , where photolysis of CO 2 is the main source of CO.…”

Relative Abundances of CO2, CO, and CH4 in Atmospheres of Earth-like Lifeless Planets

“…In contrast, if a warm and wet environment were pervasive, a higher atmospheric pCO 2 level would have been required. The recent findings of anomalously low negative carbon isotopic values of the Martian atmospheric CO and organic matter buried in Gale crater suggest the importance of CO production via CO 2 photolysis in the modern and past Martian atmosphere (Zahnle et al 2008;Krasnopolsky 2015;House et al 2022;Alday et al 2023;Aoki et al 2023;Yoshida et al 2023), opening new vistas for future work intended to better understand the possibility of CO runaway on early Mars.…”

“…Notably, CO might be important in the formation of peptides (Huber & Wächtershäuser 1997, 1998. Furthermore, recent findings regarding extremely negative carbon isotopic values in the modern Martian atmospheric CO (Alday et al 2023;Aoki et al 2023) and organic matter buried in the Gale crater (House et al 2022) suggest the importance of CO 2 photodissociation-a major CO source-in the modern and past Martian atmosphere (Schmidt et al 2013;House et al 2022;Ueno et al 2022;Yoshida et al 2023).…”