Planetary Ices and the Linear Mixing Approximation

Abstract

The validity of the widely used linear mixing approximation for the equations of state (EOS) of planetary ices is investigated at pressure-temperature conditions typical for the interior of Uranus and Neptune. The basis of this study are ab initio data ranging up to 1000 GPa and 20 000 K calculated via density functional theory molecular dynamics simulations. In particular, we calculate a new EOS for methane and EOS data for the 1:1 binary mixtures of methane, ammonia, and water, as well as their 2:1:4 ternary mixture. Additionally, the self-diffusion coefficients in the ternary mixture are calculated along three different Uranus interior profiles and compared to the values of the pure compounds. We find that deviations of the linear mixing approximation from the results of the real mixture are generally small; for the thermal EOS they amount to 4% or less. The diffusion coefficients in the mixture agree with those of the pure compounds within 20% or better. Finally, a new adiabatic model of Uranus with an inner layer of almost pure ices is developed. The model is consistent with the gravity field data and results in a rather cold interior (${\mathrm{T}}_{\mathrm{core}}\sim$ 4000 K).

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• Diamond formation from hydrocarbon mixtures in planets
• The Interiors of Uranus and Neptune: Current Understanding and Open Questions
• Thermal evolution of Uranus and Neptune I: adiabatic models
• Superionic silica-water and silica-hydrogen compounds under high pressure
• New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State
• Metallization of Shock-Compressed Liquid Ammonia
• Equation of state and optical properties of shock-compressed C:H:N:O molecular mixtures
• Stochastic and Mixed Density Functional Theory within the projector augmented wave formalism for the simulation of warm dense matter
• Partially Diffusive Helium-Silica Compound in the Deep Interiors of Giant Planets
• The linear-mixing approximation in silica-water mixtures at planetary conditions