Low–Temperature Specific Heat Capacity of Water–Ammonia Mixtures Down to the Eutectic

Robust thermodynamic data are essential for the development of geodynamic andgeochemical models of ocean worlds. The water–ammonia system is of interest inthe study of ocean worlds due to its purported abundance in the outer solar system,geological implications and potential importance for origins of life. In support of thedevelopment of a new equation of state, we conducted 1 bar specific heat capacitymeasurements (Cp) using a differential scanning calorimeter (DSC) at low temperatures(184 − 314 K) and low mass fractions of ammonia (5.2 − 26.912 wt%) to provide noveldata in the parameter space most relevant for planetary studies. This is the first knownset of data with sufficient fidelity to investigate the trend of specific heat capacity with respect to temperature. The obtained Cp in the liquid phase domain abovethe liquidus generally increasing with temperature. Deviations of our data from thecurrently adopted equation of state by Tillner-Roth and Friend is generally negativeand ranges from of +1% to −10 %, where deviations are larger at lower temperatures.This suggests that suppression of the critical behavior of super-cooled water (rapidincrease in specific heat with decreasing temperature) by ammonia starts at a smallerconcentration than that arbitrarily set by Tillner-Roth and Friend. Cp measurementsof the liquid were also obtained from the partial melting domain between the eutecticand liquidus. This novel set of data will be useful in future investigations in conditionswhere such partial melt may exist, such as the ice shell–ocean boundary.