Thermodynamic data and modeling of the water and ammonia-water phase diagrams up to 2.2 GPa for planetary geophysics

摘要

We present new experimental data on the liquidus of ice polymorphs in the H_2 O- NH_3 system under pressure, and use all available data to develop a new thermodynamic model predicting the phase behavior in this system in the ranges (0-2.2 GPa; 175-360 K; 0-33 wt % NH_3). Liquidus data have been obtained with a cryogenic optical sapphire-anvil cell coupled to a Raman spectrometer. We improve upon pre-existing thermodynamic formulations for the specific volumes and heat capacities of the solid and liquid phase in the pure H_2 O phase diagram to ensure applicability of the model in the low-temperature metastable domain down to 175 K. We compute the phase equilibria in the pure H_2 O system with this new model. Then we develop a pressure-temperature dependent activity model to describe the effect of ammonia on phase transitions. We show that aqueous ammonia solutions behave as regular solutions at low pressures, and as close-to-ideal solutions at pressure above 600 MPa. The computation of phase equilibria in the H_2 O- NH_3 system shows that ice III cannot exist at concentrations above 5-10 wt % NH_3 (depending on pressure), and ice V is not expected to form above 25%-27% NH_3. We eventually address the applications of this new model for thermal and evolution models of icy satellites.