Experimental study on the chemical strucutre of the Jovian planets and their satellistes

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We have studied the phase relation of H_2O-NH_3 system up to 10 GPa by using the diamond anvil cell, in order to understand the chemical structure of icy satellites of the Jovian Planets. The starting materials were the aqueous ammonia solutions containing ammonia up to 26.8 wt%. Determination of the phase relations was conducted by microscopic observation of the sample at high pressure, while identification of the phases was made by in situ X-ray diffraction at high pressure.The pressure of the liquidus in the H_2O-14wt%NH_3 composition, which is the composition of the cosmic ratio of H_2O and NH_3, was 1.2 Gpa at 295 K, where Ice VI begins to crystallize as the liquidus phase. The solidus locates at around 2 GPa at 295 GPa, where Ice VI and an ammonia hydrate are stable. The ammonia hydrate seems to transform into unknown phase at around 4 GPa at 295 K. The structures of the unknown ammonia hydrates are not identified due to lack of quality of the X-ray diffraction data.On the basis of the present results, the estimated temperature distributions in the icy satellites, and the density relationship between polymorphs of ice and the aqueous ammonia solution, it is suggested that the ammonia enriched liquid formed by partial melting in the interior of the satellites tends to move upwards and to disperse into the space. Thus highly volatile components such as NH_3 could be depleted in the present icy satellites of the Jovian Planets.