11:15 〜 11:30
[MIS14-03] In-situ observation of gas hydrate formation as a simulation of the bottom of ice crust in icy bodies
キーワード：氷天体、氷地殻、内部海、ガスハイドレート
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 orbiter 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 the upper part of the subsurface ocean, heat can be transferred 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 developed the in-situ observation system of gas hydrate formation and dissociation below the ice sheet and observed gas hydrate formation.
The observation system was filled with water and set in thermostatic cooling bath. After temperature in the cell degreased enough, water ice was formed in the lower part of the cell. In this study, CF4 gas was supplied to the cell up to 7 MPa to form CF4 hydrate. The reason why we chose CF4 instead of CH4 is that CF4 hydrate is heavier than water and should sink in the water cell even if the hydrate forms in the water.
Long period observation revealed that 1) at first, tiny hydrate particles formed in the water and drifted in the water after shaking the cell, 2) then, a few mm size of hydrate crystals formed and stuck on the ice sheet, 3) hydrate particles were embedded in the ice if formation rate of ice was faster than that of the hydrate. We expect that gas hydrate layer beneath the ice sheet in icy bodies may form after the growth of the ice sheet becomes slow and ice and gas hydrate mixed layer may be formed above the gas hydrate layer.
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 the upper part of the subsurface ocean, heat can be transferred 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 developed the in-situ observation system of gas hydrate formation and dissociation below the ice sheet and observed gas hydrate formation.
The observation system was filled with water and set in thermostatic cooling bath. After temperature in the cell degreased enough, water ice was formed in the lower part of the cell. In this study, CF4 gas was supplied to the cell up to 7 MPa to form CF4 hydrate. The reason why we chose CF4 instead of CH4 is that CF4 hydrate is heavier than water and should sink in the water cell even if the hydrate forms in the water.
Long period observation revealed that 1) at first, tiny hydrate particles formed in the water and drifted in the water after shaking the cell, 2) then, a few mm size of hydrate crystals formed and stuck on the ice sheet, 3) hydrate particles were embedded in the ice if formation rate of ice was faster than that of the hydrate. We expect that gas hydrate layer beneath the ice sheet in icy bodies may form after the growth of the ice sheet becomes slow and ice and gas hydrate mixed layer may be formed above the gas hydrate layer.