[U-12] Biogeochemistry of CO world

[U12-04] Evolution of the atmospheric composition of the Martian CO₂-driven dry climate system

*Yasuto Watanabe¹, Eiichi Tajika¹, Arihiro Kamada², Yoshiaki Endo³, Kazumi Ozaki³ (1.The University of Tokyo, 2.Tohoku University, 3.Institute of Science Tokyo)

Keywords:Mars

The climate of the present Mars is characterized by a cold and dry condition with a thin CO₂ atmosphere. In such a CO₂-driven dry climate system, the atmospheric pCO₂ is determined by physical processes that determine the distribution of CO₂ between surface reservoirs (atmosphere, polar ice caps, and adsorption to surface regolith) (e.g., Nakamura and Tajika, 2003). This provides a unique response of the Martian dry climate system to the variations of orbital parameters, long-term evolutions of solar luminosity, and total mass of exchangeable CO₂. Another important characteristic of the Martian dry climate system is the runaway of atmospheric CO originating from photodissociation of atmospheric CO₂ (Zahnle et al., 2008; Koyama et al., 2021). The large negative δ¹³C signal recorded in Martian sediment (House et al., 2022), which would indicate the development of CO-rich atmospheres on early Mars (Ueno et al., 2024; Yoshida et al., 2023), would further strengthen this view. However, the evolution of the distribution of CO₂ in the Martian climate system and associated changes in climate and atmospheric composition of the Mars remains uncertain. For this reason, the possibility and durations of such a CO-rich atmosphere on early Mars remains ambiguous. Here we employed a one-dimensional energy balance climate model for the Martian dry climate system. We estimated the evolutions of the distribution of CO₂ between the atmosphere, ice caps, and surface regolith since 3.8 Ga under different orbital parameters by driving the model with the evolution of the solar luminosity (Gough, 1981) and total mass of exchangeable CO₂ (Hu et al., 2015). We further estimate the evolution of the atmospheric composition of Mars with specific focus on atmospheric CO using the one-dimensional atmospheric photochemical model Atmos (Arney et al., 2016).

We show that under the present Martian obliquity (25.19 deg.), the atmospheric pCO₂ at 3.8 Ga was ~200-300 mbar, which was the largest exchangeable reservoir of CO₂. This atmospheric pCO₂ may cause a sudden increase in the atmospheric pCO under sufficient supply of reducing gases, which is consistent with the previous studies (Zahnle et al., 2008; Koyama et al., 2021). We further show that the atmospheric pCO₂ is dependent on obliquity, and it decreases sharply if the obliquity is lower than ~20 deg. under early Mars conditions. This phenomenon is called the atmospheric collapse, which would have occurred repeatedly following the obliquity cycles. For the case of low obliquity (10 deg.), the atmospheric pCO₂ was estimated to be ~1 mbar at 3.8 Ga, which would not be sufficient for keeping atmospheric pCO high. These results infer that the atmospheric pCO may have changed dramatically following the obliquity cycle of Mars, supporting the previous discussion (Koyama et al., 2021). The atmospheric pCO₂ decreases following the decline in total exchangeable CO₂ caused by ion sputtering and photochemical escape and deposition of carbonates. Under the present Martian obliquity, the atmospheric pCO₂ gradually decreases on early Mars and drops down to ~4 mbar at ~3.0 Ga. Since then, it slightly increased following the increase of the solar luminosity. In summary, our results indicate that the values of obliquity are especially important for the Martian atmospheric pCO₂ and pCO, both of which would be critical for understanding the evolution of the Martian surface environment.