Melting curve and transport properties of ammonia ice up to the deep mantle conditions of Uranus and Neptune

Abstract

Motivated by the poor knowledge of the implication of planetary ice on the internal convection and anomalous magnetic field of ice giants, we perform extensive first-principles molecular dynamics simulation on ammonia ice to investigate its thermophysical behaviors at high temperature ($T$) and pressure ($P$) conditions relevant to the deep mantle of Neptune and Uranus. The melting curve, transport properties, and sound velocity of ammonia up to 350 GPa and 5500 K, spanning from fluid mixtures to a highly compressed superionic regime, are determined. A first-order phase transition of ammonia from the plastic state to the superionic state is observed explicitly, which is associated with the reduction of entropy. In contrast to previous predictions, the melting curve elucidates the existence of superionic ammonia in deep planetary interiors. Inspection of the reduction for transport properties and sound velocity along the isentrope of ice giants further evidences that superionic ammonia can sufficiently sustain the planetary dynamo mechanism and may contribute to the internal stratification which is responsible for the generation of the multipolar magnetic field. Finally, a comprehensive phase diagram of ammonia extended to a higher T-P regime is constructed, which provides a clear picture for studying the fundamental behavior and phase transition of ammonia in deep planets, and enhances our understanding of the interior structure and thermal convection of these planetary systems.