還元的な惑星大気における有機物エアロゾルの成長過程に関する実験的研究

抄録

Organic aerosol layers have been observed and suggested to form in CH₄-containing reducing planetary atmospheres. In Titan’s atmosphere, composed mainly of N₂ and CH₄, organic aerosols are produced through photochemical reactions driven by solar ultraviolet light. Such organic aerosols may have existed in the CH₄-CO₂ atmosphere of early Earth and exoplanets. Since the aerosol layers in CH₄-containing atmospheres control the atmospheric structure and surface temperature, their formation mechanisms are important for the evolution of planetary atmospheres. Although the Cassini spacecraft has revealed that aerosol particles are initially formed in Titan’s upper atmosphere by irradiation of extreme ultraviolet light, the growth mechanisms of aerosol particles during precipitation in the middle atmosphere remain poorly constrained. Here, we conduct laboratory experiments on photochemical and heterogeneous reactions to constrain the aerosol growth mechanisms in CH₄-containing atmospheres. Our results of the experiments and photochemical modeling suggest that the addition reaction of CH₃ radicals formed by CH₄ photolysis is the major process for the growth of aerosol particles in the middle atmosphere of Titan. This reaction would affect the carbon budget and cycles on Titan, through increasing the amounts of organic aerosols and reducing the amounts of C₂H₆.