The Liquidus Temperature for Methanol‐Water Mixtures at High Pressure and Low Temperature, With Application to Titan

Methanol is a potentially important impurity in subsurface oceans on Titan and Enceladus. We report measurements of the freezing of methanol‐water samples at pressures up to 350 MPa using a volumetric cell with sapphire windows. For low concentrations of methanol, the liquidus temperature is typically a few degrees below the corresponding ice freezing point, while at high concentrations it follows the pure methanol trend. In the Ice‐III regime, we observe several long‐lived metastable states. The results suggest that methanol is a more effective antifreeze than previously estimated and might have played an important role in the development of Titan's subsurface ocean. Plain Language Summary Saturn's largest moon, Titan, probably contains a liquid ocean beneath a frozen outer ice shell. Subsurface oceans may also exist on other icy worlds of the outer solar system, as well as on some exoplanets. Such oceans could remain liquid through a complex interplay of many factors, but the physical and mechanical properties of ice and water under high pressure and at low temperature play a central role. Those properties can be significantly affected by relatively small amounts of impurities, such as ammonia and methanol. In this work, we study the freezing temperature for methanol/water solutions as a function of pressure. We find that methanol is a more effective antifreeze than had been previously estimated, so that even relatively small amounts, such as a few percent, are worth incorporating into more complex models of icy worlds. Key Points Titan's icy shell and subsurface ocean may be affected by any impurities such as methanol We measure the freezing point of methanol/water solutions as a function of pressure Previous estimates may have underestimated the effectiveness of methanol as an antifreeze