We investigate the thermodynamic stability of clathrate hydrates at cryogenic temperatures from the 0 K limit to 200 K in a wide range of pressures, covering the thermodynamic conditions of interstellar space and the surface of the hydrosphere in satellites. Our evaluation of the phase behaviors is performed by setting up quantum partition functions with variable pressures on the basis of a rigorous statistical mechanics theory that requires only the intermolecular interactions as input. Noble gases, hydrocarbons, nitrogen, and oxygen are chosen as the guest species, which are key components of the volatiles in such satellites. We explore the hydrate/water two-phase boundary of those clathrate hydrates in water-rich conditions and the hydrate/guest two-phase boundary in guest-rich conditions, either of which occurs on the surface or subsurface of icy satellites. The obtained phase diagrams indicate that clathrate hydrates can be in equilibrium with either water or the guest species over a wide range far distant from the three-phase coexistence condition and that the stable pressure zone of each clathrate hydrate expands significantly on intense cooling. The implication of our findings for the stable form of water in Titan is that water on the surface exists only as clathrate hydrate with the atmosphere down to a shallow region of the crust, but clathrate hydrate in the remaining part of the crust can coexist with water ice. This is in sharp contrast to the surfaces of Europa and Ganymede, where the thin oxygen air coexists exclusively with pure ice.

我们研究了笼形水合物在从 0 K 极限到 200 K 的低温温度下在很宽的压力范围内的热力学稳定性，涵盖了星际空间和卫星水圈表面的热力学条件。我们对相行为的评估是通过在严格的统计力学理论的基础上设置具有可变压力的量子分配函数来执行的，该理论只需要分子间相互作用作为输入。惰性气体、碳氢化合物、氮气和氧气被选为客体物种，它们是此类卫星中挥发物的关键成分。我们探索了富水条件下这些包合物水合物的水合物/水两相边界和富客条件下的水合物/客体两相边界，其中任何一种都发生在冰卫星的表面或地下。得到的相图表明包合物水合物可以与水或客体在远离三相共存条件的广泛范围内保持平衡，并且每个包合物水合物的稳定压力区在强烈冷却时显着扩大。我们对泰坦中水的稳定形式的发现意味着，地表的水仅以笼形水合物的形式存在于大气中，直到地壳的浅层区域，但地壳其余部分的笼形水合物可以与水共存冰。这与木卫二和木卫三的表面形成鲜明对比，那里稀薄的氧气空气只与纯冰共存。得到的相图表明包合物水合物可以与水或客体在远离三相共存条件的广泛范围内保持平衡，并且每个包合物水合物的稳定压力区在强烈冷却时显着扩大。我们对泰坦中水的稳定形式的发现意味着，地表的水仅以笼形水合物的形式存在于大气中，直到地壳的浅层区域，但地壳其余部分的笼形水合物可以与水共存冰。这与木卫二和木卫三的表面形成鲜明对比，那里稀薄的氧气空气只与纯冰共存。得到的相图表明包合物水合物可以与水或客体在远离三相共存条件的广泛范围内保持平衡，并且每个包合物水合物的稳定压力区在强烈冷却时显着扩大。我们对泰坦中水的稳定形式的发现意味着，地表的水仅以笼形水合物的形式存在于大气中，直到地壳的浅层区域，但地壳其余部分的笼形水合物可以与水共存冰。这与木卫二和木卫三的表面形成鲜明对比，那里稀薄的氧气空气只与纯冰共存。