APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022;
Chicago
Session N15: Frontiers of High-Energy Density Physics
11:30 AM–2:30 PM,
Wednesday, March 16, 2022
Room: McCormick Place W-183C
Sponsoring
Unit:
GSCCM
Chair: Peter Celliers, Lawrence Livermore Natl Lab; Ivan Oleynik, University of South Florida
Abstract: N15.00005 : Exploring metallic and superionic ammonia in ice giant interiors
1:54 PM–2:30 PM
Abstract
Presenter:
Alessandra Ravasio
(Laboratoire d'Utilisation de Lasers Intenses, Ecole Polytechnique)
Author:
Alessandra Ravasio
(Laboratoire d'Utilisation de Lasers Intenses, Ecole Polytechnique)
Mixtures of water, ammonia and methane are predicted to be the major components in the depths of the ice giant planets Uranus and Neptune. Their dynamics, evolution, and interior structure are insufficiently understood and models rely imperatively on data for equations of state and transport properties. The physical and chemical behavior of such systems at the extreme pressures and temperatures of planetary interiors is also extremely interesting on its own, since those conditions are characterized by a coexistence of dissociated atoms, atomic clusters, chains and superionic lattices. In spite of its great appeal, exploring these phenomena is a real challenge for both ab initio calculations and experiments so that dedicated studies remain very limited. Here we present our recent work obtained combining laser-driven shock experiments and state-of-the-art density functional theory molecular dynamics (DFT-MD) simulations, focusing on ammonia. We investigate the equation of state, the melting line, the optical properties and the electrical conductivity over a wide range of pressure and temperature conditions. We find experimental evidence of metallic ammonia, through a gradual transition from a liquid dominated by molecules to a plasma state at ~7000 K and 90 GPa. Shock compression of ammonia in solid phase III allows us to observe the melting of superionic ammonia between 70 and 125 GPa. The melting line is subsequently further constrained with DFT-MD up to 275 GPa and 4500K. The reflectivity data provide the first experimental evidence of electronic conduction in high pressure ammonia and are in excellent agreement with the reflectivity computed from atomistic simulations. Corresponding electrical conductivity values are found up to one order of magnitude higher than in water at conditions relevant to pertinent interior models, with possible implications on the generation of magnetic dynamos in large icy planets' interiors.