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

Tatsuya Yoshida (1) , Shohei Aoki (2) , Yuichiro Ueno (3, 4, 5) , Naoki Terada (1) , Yuki Nakamura (1, 6) , Kimie Shiobara (1) , Nao Yoshida (1) , Hiromu Nakagawa (1) , Shotaro Sakai (1) , Shungo Koyama (1)

1 Department of Geophysics [Sendai]
2 Department of Complexity Science and Engineering [Tokyo]
3 Department of Earth and Planetary Sciences [TITECH Tokyo]
4 ELSI - Earth-Life Science Institute [Tokyo]
5 JAMSTEC - Japan Agency for Marine-Earth Science and Technology
6 HELIOS - LATMOS

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 CO 2 is known to induce strong isotopic fractionation of the carbon between CO 2 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 CO 2 , to estimate the vertical profiles of the carbon isotopic compositions of CO and CO 2 in the Martian atmosphere. We find that CO is depleted in 13 C compared with CO 2 at each altitude, due to the fractionation via CO 2 photolysis: the minimum value of the δ 13 C 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 13 C-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.

Domaines: Planète et Univers [physics]

Tatsuya Yoshida, Shohei Aoki, Yuichiro Ueno, Naoki Terada, Yuki Nakamura, et al.. Strong Depletion of ¹³C in CO Induced by Photolysis of CO₂ in the Martian Atmosphere, Calculated by a Photochemical Model. The Planetary Science Journal, 2023, 4, 53 (8pp). ⟨10.3847/psj/acc030⟩. ⟨insu-04045592⟩