Abstract
The interiors of giant icy planets depend on the properties of hot, dense mixtures of the molecular ices water, ammonia, and methane. Here, we discuss results from first-principles molecular dynamics simulations up to 500 GPa and 7000 K for four different ammonia–water mixtures that correspond to the stable stoichiometries found in solid ammonia hydrates. We show that all mixtures support the formation of plastic and superionic phases at elevated pressures and temperatures, before eventually melting into molecular or ionic liquids. All mixtures’ melting lines are found to be close to the isentropes of Uranus and Neptune. Through local structure analyses we trace and compare the evolution of chemical composition and longevity of chemical species across the thermally activated states. Under specific conditions we find that protons can be less mobile in the fluid state than in the (colder, solid) superionic regime.
Highlights
Introduction ce pte an us criThe “ice giants” Uranus and Neptune have gaseous atmospheres and small rocky cores but they are dominated by their vast mantle regions, which comprise mixtures of the “molecular ices” water, ammonia and methane
We show here that plasticity and superionicity are general features of all ammonia water mixtures under pressure; that their melting lines are close to the isentropes of Uranus and Neptune; and provide detailed analyses of the chemical composition of the mixtures at high P − T conditions
We report here the results of a systematic computational study of all known ammonia hydrates, using ab initio molecular dynamics at pressures and temperatures that replicate the conditions in the mantle regions of giant icy planets
Summary
The “ice giants” Uranus and Neptune have gaseous atmospheres (rich in hydrogen and helium) and small rocky cores but they are dominated by their vast mantle regions, which comprise mixtures of the “molecular ices” water, ammonia and methane. Some studies are restricted to specific compounds, AMH [36] and ADH [37], and explored the high-temperature regime using ab initio molecular dynamics (AIMD) simulations These focused in particular on the appearance of superionicity – states characterized by diffusive protons in otherwise solid lattices of heavy nuclei [38]. We use AIMD simulations to explore the high-pressure/hightemperature regime of all known ammonia-water mixtures, in each case starting from the most relevant high-pressure solid phases and investigating temperature-induced phase changes Both NH3 and H2 O are reported to feature ‘plastic’ phases, where individual molecules become free rotors but remain affixed to a solid lattice [39,40,41,42].
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