Miscibility Calculations for Water and Hydrogen in Giant Planets

Abstract

We present results from ab initio simulations of liquid water-hydrogen mixtures in the range from 2 to 70 GPa and from 1000 to 6000 K, covering conditions in the interiors of ice giant planets and parts of the outer envelope of gas giant planets. In addition to computing the pressure and the internal energy, we derive the Gibbs free energy by performing a thermodynamic integration. For all conditions under consideration, our simulations predict hydrogen and water to mix in all proportions. The thermodynamic behavior of the mixture can be well described with an ideal mixing approximation. We suggest that a substantial fraction of water and hydrogen in giant planets may occur in homogeneously mixed form rather than in separate layers. The extentof mixing depends on the planet's interior dynamics and its conditions of formation, in particular on how much hydrogen was present when icy planetesimals were delivered. Based on our results, we do not predict water-hydrogen mixtures to phase separate during any stage of the evolution of giant planets. We also show that the hydrogen content of an exoplanet is much higher if the mixed interior is assumed.

New Models of Jupiter in the Context of Juno and Galileo

“…where ρ H,He , ρ water and ρ drysand are the densities of the H/He mixture, water and drysand, respectively, and Z water = M water /M , Z drysand = M drysand /M the mass fractions of water and drysand, respectively, with M the mass of the planet. Note that the accuracy of the AVL for the hydrogen/water mixture under the relevant conditions for Jupiter interior has been verified with QMD simulations (Soubiran and Militzer, 2015). Given the small number fraction of heavy elements compared with H and He, the P and T used to calculate the densities in the H/He/Z mixture are the ones obtained with the H/He mixture only.…”

Thermal evolution of Uranus and Neptune

“…Moreover, physical processes like sedimentation or upwelling of a major constituent in the HCNO system could also have a large influence on the thermal evolution of the ice giants. For instance, it is possible, that demixing of hydrogen and water occurs at conditions relevant for the ice giant envelope (Bali et al 2013), although ab initio simulations suggest solubility of water and hydrogen under Uranus and Neptune interior conditions (Soubiran & Militzer 2015). Deeper in the interior at pressures of about 1.5 Mbar, carbon-hydrates have been experimentally found to separate into diamond, which may sink to the core, and hydrogen, which may rise upward (Kraus et al 2017).…”

Characterization of exoplanets from their formation

“…Because of this, it is currently also not clear if, for the core-mass effect to work efficiently, a high total heavy element content today (in the envelope and/or core) is sufficient as implicitly assumed in the argument above. This is because it is currently unclear whether a centrally concentrated distribution of heavy elements (in a solid core or a strongly enriched inner region) at the beginning of detachment could shift to a more homogeneous distribution over time as heavy elements are soluble in H/He under conditions typical for giant planet interiors (e.g., Soubiran & Militzer 2015), or alternatively, that heavy elements sink over long timescale to the center. The results of Vazan et al (2016) indicate that no full mixing occurs if the initial compositional gradient is strong enough.…”