Behavior of Volatile Material During Accretion of Terrestrial Planets (Invited)

Behavior of volatile material during accretion of terrestrial planets is discussed from theoretical point of view. The recent planetary formation theory suggests two stages of planetary formation; the stage of oligarchic growth (e.g., Kokubo and Ida, 1998) followed by the stage of giant impacts (e.g., Chambers and Wetherill, 1998). Mars-sized protoplanets form during the stage of the oligarchic growth. The environment of protoplanets at this stage depends on availability of volatile material. Even if no volatile-bearing planetesimals are available, an atmosphere is formed on the accreting protoplanet through capture of the surrounding solar composition gas. Since the blanket effect of the captured atmospheres on protoplanets is not strong, the surface temperature is always kept under the melting temperature of mantle silicate and only a subsurface magma ocean is formed. Reaction between the hydrogen-rich protoatmosphere and silicate rock would be limited owing to low surface temperature, and not much water would be produced. Thus, core formation proceeds under dry conditions, and volatile elements are not partitioned into metallic iron. Accretion of water-bearing planetesimals results in impact degassing, and formation of a proto-atmosphere, which sometimes called as 'a steam atmosphere.’ Since a protoplanet captures surrounding solar-composition atmosphere, a mixed proto-atmosphere of solar-type and degassed components would have formed. Most of hydrogen, carbon and nitrogen are originated from planetesimals. But some fraction of hydrogen may be nebula origin. Though the structure of the mixed atmosphere embedded in the nebula gas is not well understood yet, it would be similar to that of degassed atmosphere with extended upper atmosphere. Then, a surface hydrous magma ocean can form 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, carbon and nitrogen 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. Giant impacts heat up the proto-Earth and produce a deep magma ocean. The magma ocean cools relatively rapidly but some portions are likely kept in a partially molten state for long time. 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.