溶解气体在低质量低密度行星大气保持中的作用
On the Role of Dissolved Gases in the Atmosphere Retention of Low-Mass Low-Density Planets
摘要：
开普勒在超地球质量体系中发现的低质量低密度行星通常具有大半径的推断质量，这意味着H $_2$ -He大气的存在。由于母恒星的XUV通量加热，这些行星容易受到大气质量损失的影响。将大气质量损失与热演化相结合的模型预测了行星半径的双峰分布，这已经得到了观测的支持。然而，在以前的研究中被忽视的一个关键组成部分是这些气体溶解到构成其大部分质量的岩石和铁的熔融核心中。这类行星在核心-包层边界处具有高温（$>$ 2000 K）和高压（$\sim$ kbars），确保了熔融的表面和地下氢库，其氢库可能比大气大5-10倍。这项研究通过将行星的热演化和大气的质量损失与溶解的H $_2$和大气H $_2$（亨利定律）之间的热力学平衡相结合来弥合这一差距。在行星形成阶段，内部的溶解使行星能够建立一个更大的氢储存库。我们表明，溶解氢排气缓冲大气质量损失。行星的缓慢冷却也会导致脱气，因为溶解度会随着温度的降低而降低。因此，氢在内部的溶解增加了超级地球的大气保持能力。这项研究强调了包括岩浆海洋中气体的温度和压力依赖性溶解度以及行星演化模型耦合放气的重要性。
英文摘要：
Low-mass low-density planets discovered by Kepler in the super-Earth mass regime typically have large radii for their inferred masses, implying the presence of H$_2$-He atmospheres. These planets are vulnerable to atmospheric mass loss due to heating by the parent star's XUV flux. Models coupling atmospheric mass loss with thermal evolution predicted a bimodal distribution of planetary radii, which has gained observational support. However, a key component that has been ignored in previous studies is the dissolution of these gases into the molten core of rock and iron that constitute most of their mass. Such planets have high temperatures ($>$2000 K) and pressures ($\sim$kbars) at the core-envelope boundary, ensuring a molten surface and a subsurface reservoir of hydrogen that can be 5-10 times larger than the atmosphere. This study bridges this gap by coupling the thermal evolution of the planet and the mass loss of the atmosphere with the thermodynamic equilibrium between the dissolved H$_2$ and the atmospheric H$_2$ (Henry's law). Dissolution in the interior allows a planet to build a larger hydrogen repository during the planet formation stage. We show that the dissolved hydrogen outgasses to buffer atmospheric mass loss. The slow cooling of the planet also leads to outgassing because solubility decreases with decreasing temperature. Dissolution of hydrogen in the interior therefore increases the atmosphere retention ability of super-Earths. The study highlights the importance of including the temperature- and pressure-dependent solubility of gases in magma oceans and coupling outgassing to planetary evolution models.
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