Séminaire - Superionic phases of water and ammonia in the deep interior of ice giant planets - Mandy Bethkenhagen

Lundi 22 novembre 2021 à 10 h 30

IMPMC - Sorbonne Université - 4 place Jussieu, 75005 Paris, tour 23, 4e étage, couloir 22-23, salle 401

Mandy Bethkenhagen, postdoctorante à l'ENS Lyon

Abstract

The interiors of the ice giant planets Uranus and Neptune are dominated by a mixture of water, ammonia, and methane at temperatures between 1000 K and 20 000 K. Many observable properties of these planets, such as luminosity, gravitational moments and magnetic fields, are thought to be determined by the physical and chemical properties of matter within this ice layer. In particular, the superionic phases of water and ammonia, characterized by highly mobile hydrogen ions diffusing through a lattice of oxygen and nitrogen ions, respectively, have gained much attention, because they are suggested to be related to the complex magnetic field structures of Uranus and Neptune. Determining the stability domain of such phases, and especially their melting curves, is therefore crucial to constrain the location and extent of the dynamo region in the planets’ mantles.

This presentation provides an overview of the recent advances made in the study of superionic water and ammonia by applying density functional theory molecular dynamics (DFT-MD) incombination with experiments and machine learning methods. First, the high-pressure phase diagram of ammonia is explored by combining laser-driven shock compression, diamond anvil cells, and DFT-MD simulations. The equation of state is probed along several pre-compressed Hugoniots and the melting curve of superionic ammonia is measured between 70 and 125 GPa. The reflectivity data furnish the first experimental evid ence of electronic conduction in warm dense ammonia and the calculated electrical conductivity values are found up to one order of magnitude higher than in water in the 100 GPa regime. Second, the phase transitions of superionic water are studied by combining DFT-MD with machine learning and free-energy methods. Close-packed superionic phases, which have a fraction of mixed stacking for finite systems, are predicted to be stable over a wide temperature and pressure range, whereas a body-centred cubic superionic phase is only thermodynamically stable in a small window but is kinetically favored. The resulting phase boundaries are consistent with existing, albeit scarce, experimental observations, and provide a step forward to resolve the fractions of insulating ice, different superionic phases and liquid water inside ice giants.