Mass-Radius Relationships for Exoplanets II: Grueneisen Equation of State for Ammonia

Abstract

We describe a mechanical equation of state for NH3, based on shock wave measurements for liquid ammonia. The shock measurements, for an initial temperature of 203 K, extended to 1.54 g/cc and 38.6 GPa. The shock and particle speeds were fitted well with a straight line, so extrapolations to higher compressions are numerically stable, but the accuracy is undetermined outside the range of the data. The isentrope through the same initial state was estimated, along with its sensitivity to the Grueneisen parameter. Mass-radius relations were calculated for self-gravitating bodies of pure ammonia, and for differentiated ammonia-rock bodies. The relations were insensitive to variations in the Grueneisen parameter, indicating that they should be accurate for studies of planetary structure.