Phase diagram and density of fluids in the water-methanol system: experiments and implications for the crystallization and dynamics of subsurface oceans in icy moons

Methanol, CH3OH, has been recently observed in several comets and at the surface of Saturn's icy moon Enceladus, [Hodyss et al., 2009]. Its plausible presence in the subsurface ocean could significantly affect the thermal and structural evolution of the satellite [Deschamps et al., 2010]. Methanol lowers the melting temperature of water ice [Vuillard & Sanchez, 1961; Miller & Carpenter, 1964], hence decreasing the efficiency of convective heat transfer through the outer ice Ih shell, and affects the subsurface ocean density and thermo-chemical evolution. However, the phase diagram and the fluid density of the H2O - CH3OH system remains largely unknown at the high pressures and low temperature conditions relevant for the icy moon interiors. In this study, we determined experimentally the liquidus temperature of Ice Ih and Ice VI and the fluid density in the binary water-methanol system (5, 10 and 20 w% CH3OH) from sound velocity measurments by Brillouin scattering spectroscopy over the P-T range 230 - 300 K and 10-4 - 1.2 GPa. The experiments were conducted using a membrane-type diamond anvil cell (mDAC) and an in-house designed Peltier cooling system to achieve the low temperatures of interest. Melting and crystallization in the system was visually monitored and confirmed from changes in the Brillouin spectra and in the pressure dependence of the measured sound velocities. The density of fluids ρ(P, T,x) in the binary system weas determined from the inversion of sound velocities measured in the fluids as a function of pressure along isotherms from 230 to 300 K. The results are used to propose a thermodynamic model for the CH3OH-H2O system over the investigated P-T range and further used to examine the effect of the methanol on the crystallization and thermo-chemical evolution of the subsurface ocean. The implications of these results for the thermal and structural evolution of icy moons, with particular applications to Titan, will be further discussed. References : Deschamps, F., Mousis, O., Sanchez-Valle, C., and Lunine, J.I., Astrophys. J., 2010. Hodyss, R., Parkinson, C.D. Johnson, V.D., Stern, J.V., Goguen, J.D, Yung, Y.L., and Kanik, I., Geophys. Res. Lett., 1992. Miller, G.A., and Carpenter, D.A., J. Chem. Eng. Data, 1964. Vuillard, G., and Sanchez, M., Bull. Soc. Chim. France, 1961.