17:15 〜 18:45
[U11-P01] Impact of volcanic outgassing speciation on CO concentrations in planetary atmospheres
キーワード：惑星大気、一酸化炭素、火山ガス、大気モデル
Atmospheric compositions are essential for the habitability of planets. The compositions of secondary atmospheres of lifeless Earth-like planets would be determined primarily by volcanic gas composition, atmospheric photochemistry, and carbonate-silicate geochemical cycle. Here, we focus on carbon monoxide (CO), which is produced mainly by CO2 photolysis in the atmosphere and has an advantage in producing organic matters in the prebiotic condition and in supporting the activity of primitive organisms owing to its high reactivity. Previous modeling studies predict that CO concentrations could be extremely high (>1%, sometimes >10%) in CO2-rich atmospheres (e.g., Kasting et al., 1983; Watanabe & Ozaki, 2024). For example, Watanabe and Ozaki (2024) systematically calculated atmospheric compositions under various CO2 partial pressures (pCO2), volcanic outgassing flux, surface temperature, and the spectral types of central stars. They found that the concentrations of CO tend to be higher at higher pCO2, higher volcanic gas fluxes, lower surface temperatures, and cooler stars. However, given the fact that physical and chemical parameters inherent in a planet (e.g., temperature, pressure, and redox state in magmas and gas bubbles) exert fundamental control on volcanic outgassing speciation, the question of how likely such CO-rich atmosphere can be achieved under probable conditions of volcanic gas compositions and fluxes for early Earth and Earth-like exoplanets remains ambiguous.
In this study, we perform Monte Carlo simulation for the simulation of the atmospheric photochemical model with varying the volcanic gas composition and flux. For this purpose, we employed a volcanic gas composition model, VolcGases, an open-source software package developed by Wogan et al. (2020), for the thermodynamic calculation of magmas. It assumes chemical equilibrium at a given temperature, pressure, redox state (or oxygen fugacity), and water and CO2 abundances in magma. We varied with pressures from 100 to 1000 bar, temperatures from 873 to 1973 K, oxygen fugacities from FMQ−4 to FMQ+5 in log units, CO2 mass fraction in magma of 10−5 to 10−2, and water mass fraction in magma of 10−5 to 10−1, following ranges assumed by Wogan et al. (2020). We also vary the magma production rate and atmospheric pCO2 from approximately 0.1 to 10 times modern Earth and from 0.001 to 10 bar, respectively. Results indicate a bimodal distribution of CO concentration, which is consistent with the results of Watanabe and Ozaki (2024). Relatively high concentrations of CO, such as >0.1%, are predicted at pCO2 above 2 bar. Additionally, among various parameters we tested, atmospheric CO levels seem highly sensitive to the assumed pCO2. Considering the operation of the carbonate-silicate geochemical cycle, our results suggest that CO concentration tends to be high at lower surface temperatures and farther from the central star inside the habitable zone.
References
Kasting et al. (1983) Precambrian Res., 20(2−4), 121−148.
Watanabe and Ozaki (2024) Astrophys. J., 961(1), 1.
Wogan et al. (2020) Planet. Sci. J., 1(3), 58.
In this study, we perform Monte Carlo simulation for the simulation of the atmospheric photochemical model with varying the volcanic gas composition and flux. For this purpose, we employed a volcanic gas composition model, VolcGases, an open-source software package developed by Wogan et al. (2020), for the thermodynamic calculation of magmas. It assumes chemical equilibrium at a given temperature, pressure, redox state (or oxygen fugacity), and water and CO2 abundances in magma. We varied with pressures from 100 to 1000 bar, temperatures from 873 to 1973 K, oxygen fugacities from FMQ−4 to FMQ+5 in log units, CO2 mass fraction in magma of 10−5 to 10−2, and water mass fraction in magma of 10−5 to 10−1, following ranges assumed by Wogan et al. (2020). We also vary the magma production rate and atmospheric pCO2 from approximately 0.1 to 10 times modern Earth and from 0.001 to 10 bar, respectively. Results indicate a bimodal distribution of CO concentration, which is consistent with the results of Watanabe and Ozaki (2024). Relatively high concentrations of CO, such as >0.1%, are predicted at pCO2 above 2 bar. Additionally, among various parameters we tested, atmospheric CO levels seem highly sensitive to the assumed pCO2. Considering the operation of the carbonate-silicate geochemical cycle, our results suggest that CO concentration tends to be high at lower surface temperatures and farther from the central star inside the habitable zone.
References
Kasting et al. (1983) Precambrian Res., 20(2−4), 121−148.
Watanabe and Ozaki (2024) Astrophys. J., 961(1), 1.
Wogan et al. (2020) Planet. Sci. J., 1(3), 58.