Strong Depletion of ¹³C in CO Induced by Photolysis of CO₂ in the Martian Atmosphere, Calculated by a Photochemical Model 

Abstract

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.

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

“…This is consistent with the measured negative δ value by NOMAD, indicating the depletion of not only 18 O but also 13 C in CO, as found from the analysis of 13 C 16 O/ 12 C 16 O. The calculation by one-dimensional photochemical model by Yoshida et al (2023) predicted that the δ 13 C can be between −165‰ and −117‰ with vertical transport and between −225‰ and −178‰ without vertical transport at 30-50 km due to the fractionation induced by the photolysis of CO 2 . Despite its relatively large uncertainty, our analysis with the TGO/NOMAD measurements suggests that such a depletion of 13 C in CO may actually be present.…”

“…This large uncertainty in the obtained 13 Despite the fact that the uncertainties are relatively large, our results suggest a depletion of 13 C in CO. This is consistent with the results predicted by a one-dimensional photochemical model (Yoshida et al 2023), which shows that a fractionation in 13 C/ 12 C occurs in the photolysis of CO 2 . This result might support the hypothesis that the atmosphere of early Mars was CO-rich and the recently discovered depleted 13 C in the organic carbon at Gale crater (House et al 2022) As described in the main body of the text, there are some challenges to work on the NOMAD spectra taken in diffraction orders 183-186.…”

“…Atmospheric CO 2 can be photodissociated with ultraviolet light, but the CO 2 -rich atmosphere is maintained by the reaction between CO and OH to reproduce CO 2 (CO + OH → CO 2 + H). Schmidt et al (2013) showed with quantum mechanical calculations that the UV absorption cross-section of 13 CO 2 is several tenths of a percent less than that of 12 CO 2 , which suggests that faster photolysis of 12 CO 2 with respect to 13 CO 2 could induce an isotopic depletion of 13 C relative to 12 C in CO, δ 13 C (see definition below) by about −200‰ relative to initial CO. With calculation by a one-dimensional photochemical model considering the isotopic fractionation, Yoshida et al (2023) have predicted that CO can be depleted in 13 C compared with CO 2 by about −170‰ in the lower atmosphere (below 50 km) via photolysis of CO 2 . A similar depletion of 13 C in CO that is probably due to photolysisinduced fractionation has in fact been observed in Earth's mesosphere (Beale et al 2016).…”

Radio Absorption in the Nightside Ionosphere of Mars During Solar Energetic Particle Events

“…For this study, the initial particle velocities are assumed to have an isotropic distribution over the hemisphere pointing down toward the planet, and we solved the three‐dimensional equation of motion but only tracked the vertical location of the particles. PROTEUS is a one‐dimensional photochemical model, which has been successfully applied to the Martian atmosphere (Nakamura, Leblanc, et al., 2023; Yoshida et al., 2023). PTRIP calculates the ion production rates, from which PROTEUS computes the ion and electron density profiles.…”

Numerical Prediction of Changes in Atmospheric Chemical Compositions During a Solar Energetic Particle Event on Mars

“…(2022) and Nakamura et al. (2023), which was designed for adaptability to many planetary atmospheres, for example, Jovian ionosphere (Nakamura, Terada, Tao, et al., 2022) and Martian atmosphere (T. Yoshida et al., 2023).…”