Influence of NaCl on ice VI and ice VII melting curves up to 6 GPa, implications for large icy moons

The influence of sodium chloride salt on the liquidus of ice VI and ice VII has been measured for concentrations ranging between 0.01 and 4 mol kg−1 NaCl. Dissolved NaCl significantly increases the melting pressure or decreases the melting temperature as a function of the salt concentration and induces a density crossover between ice VI and the brine. For NaCl concentrations higher than 2.5 mol kg−1 ice VI is more buoyant than the brine. The liquidus points are fitted to a second order polynomial law and a model is provided to predict the melting pressures of ice VI and ice VII (Pm,VI and Pm,VII), respectively as a function of temperature T (K) and NaCl concentration XNaCl (mol kg−1): P m , VI ( T , X NaCl ) = 9.27 + 0.996 X NaCl - ( 6.99 × 10 - 2 + 7.6 × 10 - 3 X NaCl ) · T + ( 1.414 × 10 - 4 + 1.541 × 10 - 5 X NaCl ) · T 2 P m , VII ( T , X NaCl ) = - 0.133 + 0.122 X NaCl - ( 1.28 × 10 - 4 + 4.17 × 10 - 4 X NaCl ) · T + ( 1.86 × 10 - 5 + 8.56 × 10 - 7 X NaCl ) · T 2 The temperature of the triple point between ice VI, ice VII and the brine shifts of −7 K/XNaCl along the 2.17 GPa isobar. The consequences of potentially dissolved NaCl for the internal structure of water rich planetary bodies are discussed. The density crossover between ice VI and brines suggests that migration of brines through a deep ice VI layer is enhanced for high thermal profiles and is unlikely for colder thermal conditions. It also suggests that patches of brines may actually be gravitationally stable at the interface between the bedrock and a deep ice VI basal layer allowing for a potential new class of exobiological habitat.