The existence of subsurface aqueous oceans in Jupiter and Saturn's large icy moons, theorized in the 1970s, has been confirmed by data collected by the Voyager, Galileo and Cassini-Huygens missions. The composition of chondritic and cometary materials and the data from these missions suggest that magnesium sulfate and carbon dioxide may be major components of these extra-terrestrial oceans. In order to understand the implications of the presence of these two constituents, new experiments were carried out in the H2O-CO2 and H2O-MgSO4 systems at the high pressures (0 - 2 GPa), low temperatures (250 - 350 K) and compositions (water-rich systems) expected in the hydrospheres of Ganymede, Callisto and Titan. The results from these experiments led to the first global description of the H2O-CO2 system at these conditions. The domain of stability of the two CO2 hydrates and the solubility of CO2 in water at high pressure bring new constraints on the trapping and transfer of this volatile in large icy moons. These data now make possible the high-pressure thermodynamic modeling of the CO2-CH4 sI clathrate hydrate, a phase likely involved in the segregation of these main volatile carbon molecules throughout the hydrospheres of icy moons. The first set of data acquired to constrain the eutectic composition of the H2O-MgSO4 system at high pressures complete available density data and provide the means to understand the evolution of dense oceans within massive hydrospheres. These data support the recent hypothesis of deep oceans at the bottom of the icy mantles of large icy satellites, giving a new perspective on the evolution and dynamics of these bodies.
