Protoatmospheres and Surface Environment of Protoplanets

Protoatmospheres and surface environment of terrestrial protoplanets during the oligarchic accretion phase and the giant impacts phase are discussed from theoretical points of view. Mars-sized protoplanets form during the stage of the oligarchic growth. Since protoplanets are formed from more or less ‘local’ planetesimals, the surface environment of the accreting protoplanets depends on availability of volatile material in planetesimals. Even if no volatile-bearing planetesimals are available, a gravitationary captured solar composition atmosphere is formed during accretion. In such cases the surface temperature is always kept under the melting temperature of mantle silicate and only a subsurface magma ocean is formed. Core formation proceeds under dry conditions, and volatile elements are not partitioned into metallic iron. Accretion of water-bearing planetesimals results in impact degassing. A surface hydrous magma ocean forms in response to the thermal blanketing effect of the proto-atmosphere. Then, some volatile materials dissolve into the magma ocean. If we consider reaction with metallic iron, the proto-atmosphere is likely to be rich in hydrogen. In addition, a large amount of hydrogen may be partitioned into metallic iron under high pressure, and delivered to the core. In the stage of giant impacts, both dry and water-bearing protoplanets collide on the proto-Earth. Substantial amount of proto-atmosphere (including water vapor) survives giant impacts. Moreover, giant impacts on protoplanets with oceans result in relative concentration of water against other gases.