Volatile delivery to planets in habitable zones during planet formation

井田 茂 (Shigeru Ida)
東京工業大学 地球生命研究所 (ELSI, Tokyo Institute of Technology)

Degassed atmospheres of terrestrial planets are composed of volatile elements that condense only in outer, cold disk regions, such as N2, CO2, and H2O. Actually, the Earth is severely depleted in N and C compared to the Solar compositions, and H2O ocean mass is only 0.02 wt.% of the bulk Earth. Atmospheric molecules would have been delivered from the outer regions by some mechanisms such as scattering of volatile- bearing planetesimals by a giant planet(s), radial diffusion of the planetesimals due to mutual scattering, migrations of icy planets due to disk-planet interactions, or migrations of pebbles/planetesimals induced by gas drag during the disk evolution phase where the ice lines of the volatiles move inside of the terrestrial planet orbits. If H2O ices are not delivered to the terrestrial planets, water vapor atmosphere cannot exist. If organic materials are not delivered to the terrestrial planets, N2 or CO2 atmosphere cannot exist. In that case, the terrestrial planets cannot be "habitable" even if their orbits are well within the classical habitable zones. On the other hand, some fraction of H-He gas can also be captured by a terrestrial planet from the protoplanetary disk, if the planet becomes massive enough before the disk gas depletion. This capture also provides volatiles to the planet. Reaction of H in the gas can react with FeO in the magma ocean on the surface of the planet. Because H-He molecules are light, H-He atmosphere tends to escape from the planet on long timescales and heavier elements such as C and N can be dragged with the H-He flow. However, the atmosphere can also retain as main components of the atmosphere, depending on planetary mass and environments. The delivery and capture are determined by planet accretion process. Furthermore, reactions with magma ocean on early planetary surface can change the atmosphere amount and compositions a lot. In the case of pebble accretion, impact- generated magma oceans would not exist because accreting pebbles are decelerated or ablated by atmospheric gas drag. I will explain the current understanding of these issues and I want to discuss open problems.

Place: 2F Conf. room / 研究管理棟２階会議場（1236号室）