• Lefèvre, Franck
• Vinci, Tommaso
• Redmer, R.
• Hernandez, J. A.
• Guarguaglini, M.
• Benuzzi-Mounaix, A.
• French, M.
• Bethkenhagen, M.
• Datchi, F.
• Ravasio, A.
• Ninet, S.

Abstract

Super-Earths, mini-Neptunes and Neptune-sized (exo-)planets have mantles potentially composed of large amounts of H, He, C, N and O. Modelling of their structure and dynamics requires knowledge of the equations of state and transport properties of the relevant mixtures up to several Mbar and thousands of Kelvin. At these conditions, highly hydrogenated molecular compounds (e.g H<SUB>2</SUB>O, NH<SUB>3</SUB>, CH<SUB>4</SUB>, hydrocarbons) present complex phase diagrams with multiple solid phases, superionic regimes, dissociations, metal-insulator transitions. In the last two decades, much effort has been dedicated to H<SUB>2</SUB>O. The recent discovery of superionic water ices in laser-shock experiments [Millot et al., Nature 569, 2019] illustrates this success, validating a two-decade-old prediction [Cavazzoni et al., Science 283, 1999]. <P />In this study, we focus on NH<SUB>3</SUB>, another end-member showing strong hydrogen disordering in the solid phase, and whose behavior in the warm dense regime remains unexplored. We investigate the equation of state, the optical properties and the electrical conductivity of warm dense ammonia by combining both laser-driven shock experiments coupled to static compression devices (pre-shock pressures ranging between 14 bar and 3.1 GPa) and state-of-the-art first-principles atomistic simulations. <P />Temperature measurements along the Hugoniot of liquid NH<SUB>3 </SUB>(initial state at 14 bar and 295 K)shows a subtle slope change at 7000 K and 90 GPa, which coincides with the gradual transition from a liquid dominated by molecules to a plasma state in our new ab initio simulations. Measurements in shocked solid ammonia III provide strong constraints on the melting curve of ammonia around 100 GPa. Our reflectivity data furnish the first experimental evidence of electronic conduction in high pressure ammonia and are in excellent agreement with the reflectivity computed from atomistic simulations. Corresponding conductivity values are found up to one order of magnitude higher than in water in the 100 GPa regime, with possible implications on the generation of magnetic dynamos in large icy planets' interiors....

Topics

• mineral
• surface
• compound
• phase
• experiment
• simulation
• Hydrogen
• phase diagram
• electrical conductivity
• ion chromatography