3:45 PM - 4:00 PM
[PPS06-02] Co-evolution of Earth's proto-atmosphere and magma ocean
Keywords:Earth, porto-atmosphere, magma ocean, evolution
During accretion, Earth is expected to have acquired a hydrogen-rich, reduced proto-atmosphere through impact degassing and the capture of nebular gas (e.g., Kuramoto and Matsui, 1996; Schaefer and Fegley, 2010; Zahnle et al., 2020). If a deep magma ocean was formed by a giant impact during this period, it has been suggested that ferrous iron (Fe2+) would undergo disproportionation under lower mantle pressures, yielding metallic iron and ferric iron (Fe3+), and the subsequent segregation of metallic iron into the core and the homogenization of ferric iron through mantle convection would lead to the oxidation of the entire magma ocean (e.g., Armstrong et al., 2019; Deng et al., 2019; Kuwahara et al., 2023; Zhang et al., 2024). Since the oxidation of the magma ocean would also result in the oxidation of the proto-atmosphere, understanding the chemical interaction efficiency between the atmosphere and magma ocean, as well as their thermochemical co-evolution, is crucial. However, these processes have not been thoroughly investigated.
In this study, we developed a coupled atmosphere-interior evolution model that accounts for planetary radiation and solar radiation transfer and balance in the atmosphere, chemical interactions between the magma ocean and atmosphere, and the partitioning of oxides between the melt and solidified mantle. Using this model, we estimated the solidification process of the magma ocean and the chemical co-evolution of the proto-atmosphere. For the radiative transfer calculations, we considered H2O line and continuum absorption, H2-H2 collision-induced absorption as sources of radiative absorption, and Rayleigh scattering by H2O and H2 as sources of scattering. The cooling of the magma ocean was assumed to occur in response to the balance between planetary radiation flux and incoming solar radiation flux. To estimate chemical interactions between the magma ocean and atmosphere, we incorporated the diffusion transport and redox reactions of H2 and Fe3+ within the magma. Additionally, equilibrium partitioning of FeO, FeO1.5, MgO, SiO2, CaO, and Al2O3 between the magma ocean and mantle was considered, and the redox state of the magma ocean was inferred from the relative ratio of FeO to FeO1.5.
For a high-pressure H2-H2O atmosphere with a total mass comparable to or greater than that of Earth's oceans, planetary radiation flux is significantly suppressed due to Rayleigh scattering, in addition to H2O absorption and H2-H2 collision-induced absorption. The Rayleigh scattering affects planetary radiation below ~1 micron, and when the surface temperature exceeds 2000 K, planetary radiation flux decreases by more than an order of magnitude compared to cases without scattering. Furthermore, the reaction rate between H2 and the magma ocean is limited by the diffusion rate of H2 in the boundary layer at the atmosphere-magma ocean interface. If the viscosity of the magma ocean is low and boundary layer renewal occurs frequently, the maximum H2 oxidation rate can reach ~103 bar/Myr. These findings suggest that if a deep magma ocean with low viscosity formed immediately after a giant impact, chemical reactions would proceed until equilibrium is reached between the primordial atmosphere and the magma ocean during the cooling process, potentially producing an H2O inventory comparable to or greater than Earth's present ocean mass.
In this study, we developed a coupled atmosphere-interior evolution model that accounts for planetary radiation and solar radiation transfer and balance in the atmosphere, chemical interactions between the magma ocean and atmosphere, and the partitioning of oxides between the melt and solidified mantle. Using this model, we estimated the solidification process of the magma ocean and the chemical co-evolution of the proto-atmosphere. For the radiative transfer calculations, we considered H2O line and continuum absorption, H2-H2 collision-induced absorption as sources of radiative absorption, and Rayleigh scattering by H2O and H2 as sources of scattering. The cooling of the magma ocean was assumed to occur in response to the balance between planetary radiation flux and incoming solar radiation flux. To estimate chemical interactions between the magma ocean and atmosphere, we incorporated the diffusion transport and redox reactions of H2 and Fe3+ within the magma. Additionally, equilibrium partitioning of FeO, FeO1.5, MgO, SiO2, CaO, and Al2O3 between the magma ocean and mantle was considered, and the redox state of the magma ocean was inferred from the relative ratio of FeO to FeO1.5.
For a high-pressure H2-H2O atmosphere with a total mass comparable to or greater than that of Earth's oceans, planetary radiation flux is significantly suppressed due to Rayleigh scattering, in addition to H2O absorption and H2-H2 collision-induced absorption. The Rayleigh scattering affects planetary radiation below ~1 micron, and when the surface temperature exceeds 2000 K, planetary radiation flux decreases by more than an order of magnitude compared to cases without scattering. Furthermore, the reaction rate between H2 and the magma ocean is limited by the diffusion rate of H2 in the boundary layer at the atmosphere-magma ocean interface. If the viscosity of the magma ocean is low and boundary layer renewal occurs frequently, the maximum H2 oxidation rate can reach ~103 bar/Myr. These findings suggest that if a deep magma ocean with low viscosity formed immediately after a giant impact, chemical reactions would proceed until equilibrium is reached between the primordial atmosphere and the magma ocean during the cooling process, potentially producing an H2O inventory comparable to or greater than Earth's present ocean mass.