Phase changes of methane hydrate under high-temperature and pressure and inference of interiors of giant icy planets

本研究的目的是首先阐明甲烷水合物和固体甲烷在类似于冰质行星和卫星内部的高压高温下的相变和性质，然后根据实验结果推断冰质天体的状态和内部结构。采用激光加热金刚石对顶砧（LHDAC）产生高压高温，利用X射线衍射和拉曼光谱对样品进行了表征。对于甲烷水合物，填充冰Ih结构的高压相存活至少高达90 GPa。研究了高压稳定性显著的原因。客体甲烷分子的定向有序化和主体分子氢键的对称化可能证实了填充冰Ih结构在极高压下的突出稳定性。对于固体甲烷，X-射线衍射和拉曼光谱显示的熔化条件是在10至80 GPa的宽压力范围内高于约1100 K。在1100 K以上，发生聚合以产生乙烷分子和进一步聚合的烃。在3000 K以上，产生了金刚石。这些变化取决于温度而不是压力。本研究为海王星和天王星等巨行星的中间冰层融化成富含甲烷的热海洋的理论预测提供了一些实验支持，并将有助于提高我们对巨行星内部化学过程的理解。

This study intended firstly to clarify phase changes and properties of methane hydrate and solid methane under high-pressures and high-temperatures comparable to those of interiors of icy planets and satellites, and then to infer the states and internal structures of the icy bodies based on the experimental results. High-pressures and high-temperatures were generated by using laser heated diamond anvil cells (LHDAC), and characterization of the samples were made by X-ray diffractometry and Raman spectroscopy. For methane hydrate, the high-pressure phase of filled ice Ih structure survived at least up to 90 GPa. The reasons for the remarkable high-pressure stability were examined. The orentaional ordering of guest methane molecules and symmetrization of hydrogen bond of host molecules might substantiate the outstanding stability of the filled ice Ih structure under extremely high pressure. For solid methane, X-ray diffractometry and Raman spectroscopy revealed the melting conditions to be above approximately 1100 K in a wide pressure range of 10 to 80 GPa. Above 1100 K, polymerizations occurred to produce ethane molecules and further polymerized hydrocarbons. Above 3000 K, diamond was produced. These changes proceeded depending on temperature rather than on pressure. The present study provides some experimental supports to theoretical predictions that the middle ice layer of giant planets such as Neptune and Uranus melts into a hot methane-rich ocean, and will help to improve our understanding of the chemical process of the giant planets' interiors.