Uranus evolution models with simple thermal boundary layers

The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2--3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of $1\times$ solar. Finally, we allow for an equilibrium evolution (Teff ~ Teq) that begun prior to the present day, which would therefore no longer require the current era to be a "special time" in Uranus' evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When Teff ~ Teq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones.Comment: 13 pages, Accepted to Icaru

X-ray free electron laser heating of water and gold at high static pressure

The study of water at high pressure and temperature is essential for understanding planetary interiors but is hampered by the high reactivity of water at extreme conditions. Here, indirect X-ray laser heating of water in a diamond anvil cell is realized via a gold absorber, showing no evidence of reactivity

Virial expansion of the electrical conductivity of hydrogen plasmas

International audienceThe low-density limit of the electrical conductivity σ (n ,T ) of hydrogen as the simplest ionic plasma is presented as a function of the temperature T and mass density n in the form of a virial expansion of the resistivity. Quantum statistical methods yield exact values for the lowest virial coefficients which serve as a benchmark for analytical approaches to the electrical conductivity as well as for numerical results obtained from density functional theory-based molecular dynamics simulations (DFT-MD) or path-integral Monte Carlo simulations. While these simulations are well suited to calculate σ (n ,T ) in a wide range of density and temperature, in particular, for the warm dense matter region, they become computationally expensive in the low-density limit, and virial expansions can be utilized to balance this drawback. We present new results of DFT-MD simulations in that regime and discuss the account of electron-electron collisions by comparison with the virial expansion

Uranus evolution models with simple thermal boundary layers

The strikingly low luminosity of Uranus (Teff ~ Teq) constitutes a long-standing challenge to our understanding of Ice Giant planets. Here we present the first Uranus structure and evolution models that are constructed to agree with both the observed low luminosity and the gravity field data. Our models make use of modern ab initio equations of state at high pressures for the icy components water, methane, and ammonia. Proceeding step by step, we confirm that adiabatic models yield cooling times that are too long, even when uncertainties in the ice:rock ratio (I:R) are taken into account. We then argue that the transition between the ice/rock-rich interior and the H/He-rich outer envelope should be stably stratified. Therefore, we introduce a simple thermal boundary and adjust it to reproduce the low luminosity. Due to this thermal boundary, the deep interior of the Uranus models are up to 2--3 warmer than adiabatic models, necessitating the presence of rocks in the deep interior with a possible I:R of $1\times$ solar. Finally, we allow for an equilibrium evolution (Teff ~ Teq) that begun prior to the present day, which would therefore no longer require the current era to be a "special time" in Uranus' evolution. In this scenario, the thermal boundary leads to more rapid cooling of the outer envelope. When Teff ~ Teq is reached, a shallow, subadiabatic zone in the atmosphere begins to develop. Its depth is adjusted to meet the luminosity constraint. This work provides a simple foundation for future Ice Giant structure and evolution models, that can be improved by properly treating the heat and particle fluxes in the diffusive zones

Equation of state and electrical conductivity of warm dense ammonia at the conditions of large icy planets' interiors.

International audienceSuper-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 H2O, NH3, CH4, 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 H2O. 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]. In this study, we focus on NH3, 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. Temperature measurements along the Hugoniot of liquid NH3 (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

Equation of state and electrical conductivity of warm dense ammonia at the conditions of large icy planets' interiors.

International audienceSuper-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 H2O, NH3, CH4, 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 H2O. 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]. In this study, we focus on NH3, 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. Temperature measurements along the Hugoniot of liquid NH3 (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

Metallization of Shock-Compressed Liquid Ammonia

International audienceAmmonia is predicted to be one of 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 equation of state and transport properties. Despite its great significance, the experimentally accessed region of the ammonia phase diagram today is still very limited in pressure and temperature. Here we push the probed regime to unprecedented conditions, up to ∼350 GPa and ∼40000K. Along the Hugoniot, the temperature measured as a function of pressure shows a subtle change in slope at ∼7000K and ∼90 GPa, in agreement with ab initio simulations we have performed. This feature coincides with the gradual transition from a molecular liquid to a plasma state. Additionally, we performed reflectivity measurements, providing the first experimental evidence of electronic conduction in high-pressure ammonia. Shock reflectance continuously rises with pressure above 50 GPa and reaches saturation values above 120 GPa. Corresponding electrical conductivity values are up to 1 order of magnitude higher than in water in the 100 GPa regime, with possible significant contributions of the predicted ammonia-rich layers to the generation of magnetic dynamos in ice giant interiors

Equation of state and electrical conductivity of warm dense ammonia at the conditions of large icy planets' interiors.

International audienceSuper-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 H2O, NH3, CH4, 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 H2O. 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]. In this study, we focus on NH3, 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. Temperature measurements along the Hugoniot of liquid NH3 (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