Keywords:icy bodies, gas hydrate, crystal growth
Gas hydrates are clathrate compounds where hydrogen-bonded water cages include a guest gas molecule. Methane hydrate is one of famous gas hydrates because it is naturally found in deep-sea and permafrost sediments on the Earth. Gas hydrates would exist in icy bodies as well. One of the candidates is Pluto where the presence of subsurface ocean is suggested. Gas hydrates may exist between the ice shell and subsurface ocean and play an important role as insulator to keep warm inside. The presence of subsurface ocean is also suggested in some icy satellites like Enceladus. INMS (Ion and Neutral Mass Spectrometer) aboard the Cassini spacecraft has investigated composition of the plumes there that includes H2O (< 90 %), CH4, CO2, NH3, and other various organic materials. These results could reflect the composition of the subsurface ocean and imply the presence of clathrate hydrates in the ocean. Density of gas hydrates depends on guest gas molecules. For example, methane hydrate is lighter than the water, whereas CO2 hydrate is heavier. If CO2 hydrate forms at the bottom of the icy shell, the average density may become large and the ice (water ice + CO2 hydrate) may start to sink locally. If the size of each gas hydrate become small and granular gas hydrates exist in subsurface ocean, heat can be transfer by not only thermal conduction but also convection. This means that icy bodies may be cooled faster. To elucidate how gas hydrates form in the subsurface ocean in icy bodies, we have developed the observation system of gas hydrate formation and dissociation below the ice sheet. In the presentation, we report the progress of the observation system.