10:45 〜 12:15
[U08-P01] Transient CO-rich atmospheres on early terrestrial planets: Implications from deep carbon cycle modeling combined with photochemistry
キーワード：地球、系外惑星、大気
Atmospheric compositions of terrestrial planets control their climates and photochemical products which could drive prebiotic chemical evolution. Though the terrestrial planets in the solar system (Venus, Earth, and Mars) today possess CO2 as the major form of atmospheric carbon, CO-rich atmospheres have been theoretically predicted for planets orbiting M-type stars and receiving lower stellar irradiation, the latter of which include early Mars. CO-rich atmospheres can form when CO production via photodissociation of CO2 exceeds CO oxidation. A CO-rich atmosphere on Archean Earth has also been suggested from sulfur isotope record. Such reducing atmospheres are more suitable to form organic compounds via atmospheric chemistry.
While previous studies on carbon speciation in terrestrial atmospheres chiefly focused on photochemical steady states, the timescale to reach a steady state for a dense atmosphere can be comparable with geologic time, implying that transient states are important for elucidating atmospheric evolution. In order to simulate time evolution of atmospheres of Earth-like worlds, we constructed a model for carbon cycling combined with atmospheric photochemistry and climate modeling and simulated atmospheric evolution from the time of magma ocean solidification and formation of oceans.
We found that, rapid CO2 drawdown from an initially-dense atmosphere via carbonate precipitation leaves CO and naturally leads to the formation of a transient CO-rich atmosphere. The transit CO-rich atmosphere forms for a wide range of initial conditions (initial carbon parititioning between the atmosphere and the mantle). Atmospheric CO is eventually lost due to the reaction with OH produced by photodissociation of H2O. However, when the CO partial pressure of the transient atmosphere is a few bars (depends on the initial condition), the CO-rich atmosphere could remain over ~1 billion years. The long lifetime is caused by the self-stabilizing effect; CO reduces water vapor mixing ratio in the atmosphere and, consequently, CO oxidation rate by photochemically-produced OH. Moreover, the lifetime of CO becomes longer if the mantle is reducing to directly release CO via volcanism.
In this presentation, we also discuss the implications for evolution of Earth and other terrestrial worlds and for observations of extrasolar systems.
While previous studies on carbon speciation in terrestrial atmospheres chiefly focused on photochemical steady states, the timescale to reach a steady state for a dense atmosphere can be comparable with geologic time, implying that transient states are important for elucidating atmospheric evolution. In order to simulate time evolution of atmospheres of Earth-like worlds, we constructed a model for carbon cycling combined with atmospheric photochemistry and climate modeling and simulated atmospheric evolution from the time of magma ocean solidification and formation of oceans.
We found that, rapid CO2 drawdown from an initially-dense atmosphere via carbonate precipitation leaves CO and naturally leads to the formation of a transient CO-rich atmosphere. The transit CO-rich atmosphere forms for a wide range of initial conditions (initial carbon parititioning between the atmosphere and the mantle). Atmospheric CO is eventually lost due to the reaction with OH produced by photodissociation of H2O. However, when the CO partial pressure of the transient atmosphere is a few bars (depends on the initial condition), the CO-rich atmosphere could remain over ~1 billion years. The long lifetime is caused by the self-stabilizing effect; CO reduces water vapor mixing ratio in the atmosphere and, consequently, CO oxidation rate by photochemically-produced OH. Moreover, the lifetime of CO becomes longer if the mantle is reducing to directly release CO via volcanism.
In this presentation, we also discuss the implications for evolution of Earth and other terrestrial worlds and for observations of extrasolar systems.