11:30 〜 11:45
[PPS07-09] 水素—水蒸気大気をもつ地球型惑星の固化タイムスケール
キーワード：酸化還元状態、マグマオーシャン、保温効果
The mass and composition of planetary atmosphere affect a crystallization rate of a magma ocean via its thermal blanketing effect. The overlying atmosphere controls heat escaping from the planet and determine time to reach a warm habitable state from the extremely hot molten state. Furthermore, a physical state of planetary mantle, i.e. in a molten state or in a solid state, could affect origins of disk-forming materials ejected by a successive giant impact. Thus, the strength of the thermal blanketing effect by planetary atmosphere overlying a magma ocean thus may be one of crucial factors for the composition of the Moon.
The most investigated to date is a thermal blanketing effect by oxidized atmospheres consisting of H2O and/or CO2, whereas early atmospheres could have a reducing composition, depending on a redox state of the underlying magma ocean. We present thermal evolution of a magma ocean under a H2-H2O atmosphere for Earth-like planets with a varying initial oxygen fugacity of the surface magma. We find that the magma ocean sustains longer as initial oxygen fugacity decreases, for a given initial H2O inventory. Instead, for a given initial total H inventory, the magma ocean can solidify on a comparable timescale, despite the 8 orders of magnitude difference in initial oxygen fugacity. The crystallization timescale is the order of 1 Myr or less, when the total H inventory is less than 5 ocean mass in water equivalent. In more volatile-rich and reducing cases, crystallization of the magma ocean proceeds more slowly at a rate controlled by H2 escape, resulting in an extended lifetime by several orders of magnitude. Our results suggest that early Earth would have crystallized on a timescale of the order of 0.1-1 Myr, leaving early atmosphere whose composition would have strongly reflected the magma redox state. The crystallization timescale is shorter than a typical interval of giant impacts at a late stage of planet formation. This timescale comparison suggests that the Moon-forming impact would have occurred onto an already solidified proto-Earth, unless it was more reducing and volatile-rich, or originally located closer to the Sun, compared with Earth.
The most investigated to date is a thermal blanketing effect by oxidized atmospheres consisting of H2O and/or CO2, whereas early atmospheres could have a reducing composition, depending on a redox state of the underlying magma ocean. We present thermal evolution of a magma ocean under a H2-H2O atmosphere for Earth-like planets with a varying initial oxygen fugacity of the surface magma. We find that the magma ocean sustains longer as initial oxygen fugacity decreases, for a given initial H2O inventory. Instead, for a given initial total H inventory, the magma ocean can solidify on a comparable timescale, despite the 8 orders of magnitude difference in initial oxygen fugacity. The crystallization timescale is the order of 1 Myr or less, when the total H inventory is less than 5 ocean mass in water equivalent. In more volatile-rich and reducing cases, crystallization of the magma ocean proceeds more slowly at a rate controlled by H2 escape, resulting in an extended lifetime by several orders of magnitude. Our results suggest that early Earth would have crystallized on a timescale of the order of 0.1-1 Myr, leaving early atmosphere whose composition would have strongly reflected the magma redox state. The crystallization timescale is shorter than a typical interval of giant impacts at a late stage of planet formation. This timescale comparison suggests that the Moon-forming impact would have occurred onto an already solidified proto-Earth, unless it was more reducing and volatile-rich, or originally located closer to the Sun, compared with Earth.