Low–temperature specific heat capacity of water--ammonia mixtures down to the eutectic: applications in geodynamic modelling of icy moons

Abstract

Robust thermodynamic data are essential for the development of geodynamic and geochemical models of ocean worlds. The water–ammonia system is of interest due to its purported abundance in the outer solar system, geological implications and potential importance for origins of life. In support of developing new equations of state, we conducted 1bar specific heat capacity measurements (Cp) using a differential scanning calorimeter at low temperatures (184−314K) and low mass fractions of ammonia (5.2−26.9wt%) to provide novel data in the parameter space relevant for planetary studies. This is the first known set 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 above the liquidus increases with temperature. Deviations of our data from the currently adopted equation of state by Tillner-Roth and Friend are negative (ranging from +1% to −10%) and larger at lower temperatures. This result suggests that suppression of the critical behavior of super-cooled water (rapid increase in specific heat with decreasing temperature) by ammonia starts at a smaller concentration than that set by Tillner-Roth and Friend. Cp measurements of the liquid were also obtained in the partial melting domain between the eutectic and liquidus. This novel dataset will be useful in future investigations of conditions where such partial melt may exist, such as the ice shell– ocean boundary or the interiors of ocean worlds that may contain relatively large proportions of dissolved ammonia.