09:30 〜 09:45
[U08-03] UV shielding by hydrocarbons in an early reduced Earth's atmosphere
キーワード：地球、大気光化学、大気進化、有機物、初期大気
Recent cosmochemical studies of isotopic compositions of Earth’s materials and primitive meteorites indicate that the materials accreted by Earth comprised a large fraction of enstatite-type impactors throughout the accretion phase (Dauphas, 2017). This implies that volatiles simultaneously accreted on Earth were reduced by metallic iron in building blocks to form impact-generated vapor enriched in reduced species like H2 and CH4 (Kuramoto and Matsui, 1996; Zahnle et al., 2020). Such a reduced atmosphere is a promising site for the photochemical production of organic matter including prebiotically important molecules such as HCN and H2CO (e.g., Schlesinger and Miller, 1983).
A reduced atmosphere enriched in H2 and CH4 would have evolved through photochemistry (Zahnle et al., 2020) as well as the hydrodynamic escape of hydrogen (Yoshida and Kuramoto, 2021). CH4 is estimated to be oxidized efficiently through the reactions with OH and O, which are mainly produced by the photolysis of H2O (Zahnle et al., 2020). On the other hand, hydrocarbons such as C2H6 and C2H2 are also expected to be produced from CH4 as demonstrated by the atmospheres of Titan and gas-giant planets (e.g., Yung and DeMore, 1998). In that case, their UV shielding may affect the oxidation rate of CH4 through the suppression of the photolysis of H2O. Furthermore, it may also affect the production rates of prebiotically important molecules by the suppression of their photolysis. However, the effect of UV shielding by hydrocarbons in the early reduced Earth’s atmosphere has not been fully investigated.
In this study, we develop a 1-D photochemical model for a reduced Earth’s atmosphere mainly composed of H2 and CH4 based on PROTEUS (Nakamura et al., 2023) to estimate the production rates of organic matter and oxidized components with clarifying the UV shielding effect by hydrocarbons. Based on the calculation results, we estimate the evolution of the reduced atmosphere through photochemistry and atmospheric escape. As for the chemical processes, we consider 322 chemical reactions for 61 chemical species (Tian et al., 2011). To calculate the profiles of the photolysis rates, we adopt the UV spectrum from 100 to 1000 nm estimated for the young Sun at the age of 100 Myr (Claire et al., 2012). The model also considers the vertical transport of each chemical species due to molecular diffusion and eddy diffusion. The mixing ratio of water vapor in the stratosphere is fixed at 1 ppm. At the lower boundary corresponding to the surface, the number densities of H2, CH4, and N2 are fixed. The basal CH4/H2 ratio is taken to be a free parameter.
We find that most of the products from CH4 are hydrocarbons in CH4-rich conditions with a basal CH4/H2 ratio>~0.5. This is due to the UV shielding effect by hydrocarbons such as C2H2 and C2H6. As the mixing ratio of CH4 in the stratosphere increases, the production rates of hydrocarbons increase. As a result, the photolysis of H2O is suppressed since the UV wavelength range absorbed by hydrocarbons and H2O overlaps each other, leading to a decrease in the production rate of oxides. The production and deposition rate of HCN increases with the CH4 mixing ratio since the reaction between CH3 and N proceeds efficiently and the photolysis of HCN is suppressed under the hydrocarbon-rich condition. Here most of N produced through the photolysis of N2 in the thermosphere/ionosphere is converted to HCN. H2CO is also produced efficiently in the reduced atmosphere, whose production and deposition rate mainly correlates with the abundance of CO and CO2. Our results suggest that the organic matter including prebiotically important molecules such as HCN and H2CO was produced efficiently in the early atmosphere, leading to the formation of an organic soup in the ocean potentially linked to the emergence of living organisms.
A reduced atmosphere enriched in H2 and CH4 would have evolved through photochemistry (Zahnle et al., 2020) as well as the hydrodynamic escape of hydrogen (Yoshida and Kuramoto, 2021). CH4 is estimated to be oxidized efficiently through the reactions with OH and O, which are mainly produced by the photolysis of H2O (Zahnle et al., 2020). On the other hand, hydrocarbons such as C2H6 and C2H2 are also expected to be produced from CH4 as demonstrated by the atmospheres of Titan and gas-giant planets (e.g., Yung and DeMore, 1998). In that case, their UV shielding may affect the oxidation rate of CH4 through the suppression of the photolysis of H2O. Furthermore, it may also affect the production rates of prebiotically important molecules by the suppression of their photolysis. However, the effect of UV shielding by hydrocarbons in the early reduced Earth’s atmosphere has not been fully investigated.
In this study, we develop a 1-D photochemical model for a reduced Earth’s atmosphere mainly composed of H2 and CH4 based on PROTEUS (Nakamura et al., 2023) to estimate the production rates of organic matter and oxidized components with clarifying the UV shielding effect by hydrocarbons. Based on the calculation results, we estimate the evolution of the reduced atmosphere through photochemistry and atmospheric escape. As for the chemical processes, we consider 322 chemical reactions for 61 chemical species (Tian et al., 2011). To calculate the profiles of the photolysis rates, we adopt the UV spectrum from 100 to 1000 nm estimated for the young Sun at the age of 100 Myr (Claire et al., 2012). The model also considers the vertical transport of each chemical species due to molecular diffusion and eddy diffusion. The mixing ratio of water vapor in the stratosphere is fixed at 1 ppm. At the lower boundary corresponding to the surface, the number densities of H2, CH4, and N2 are fixed. The basal CH4/H2 ratio is taken to be a free parameter.
We find that most of the products from CH4 are hydrocarbons in CH4-rich conditions with a basal CH4/H2 ratio>~0.5. This is due to the UV shielding effect by hydrocarbons such as C2H2 and C2H6. As the mixing ratio of CH4 in the stratosphere increases, the production rates of hydrocarbons increase. As a result, the photolysis of H2O is suppressed since the UV wavelength range absorbed by hydrocarbons and H2O overlaps each other, leading to a decrease in the production rate of oxides. The production and deposition rate of HCN increases with the CH4 mixing ratio since the reaction between CH3 and N proceeds efficiently and the photolysis of HCN is suppressed under the hydrocarbon-rich condition. Here most of N produced through the photolysis of N2 in the thermosphere/ionosphere is converted to HCN. H2CO is also produced efficiently in the reduced atmosphere, whose production and deposition rate mainly correlates with the abundance of CO and CO2. Our results suggest that the organic matter including prebiotically important molecules such as HCN and H2CO was produced efficiently in the early atmosphere, leading to the formation of an organic soup in the ocean potentially linked to the emergence of living organisms.