Ab-initio studies of ammonia-water mixtures at icy planet mantle conditions

The mantles of icy planets comprise large amounts of water, ammonia, and methane ices. To understand their interior structure, it is crucial to study these ices at the extreme pressure conditions they likely experience. We survey here mixtures of water and ammonia and show that high pressures stabilise ammonia-rich hydrates, through transformations from molecular crystals into fully ionic solids that involve complete deprotonation of water. We suggest that ammonia-rich hydrates can precipitate out of any ammonia-water mixture at sufficient pressures and are an important component inside icy planets.