H${}_2$-H${}_2$O demixing in Uranus and Neptune: Adiabatic structure models

Abstract

Demixing properties of planetary major constituents influence the interior structure and evolution of planets. Comparing experimental and computational data on the miscibility of hydrogen and water to adiabatic profiles suggests phase separation between these components occurs in the ice giants Uranus and Neptune. We aim to predict the atmospheric water abundance and transition pressure between the water-poor outer envelope and the water-rich deep interior in Uranus and Neptune. We construct seven H2-H2O phase diagrams from the available experimental and computational data. We compute interior adiabatic structure models and compare these to the phase diagrams to infer whether demixing is occurring. We obtain a strong water depletion in the top layer due to rain-out of water and find upper limits on the atmospheric water mass fraction Z_atm of 0.21 for Uranus and 0.16 for Neptune. The transition from the water-poor to the water-rich layer is sharp and occurs at pressures P_Z between 4 and 11 GPa. Using these constraints on Z_atm and P_Z, we find that the observed gravitational harmonics J2 and J4 can be reproduced if P_Z > 10 GPa in Uranus and > 5 GPa in Neptune, and if the deep interior has a high primordial water mass fraction of 0.8, unless rocks are also present. The agreement with J4 is improved if rocks are confined deeper than P_Z, for instance below a rock cloud level at 2000 K (20-30 GPa). These findings confirm classical few-layer models and suggest that a layered structure may result from a combination of primordial mass accretion and subsequent phase separation. Reduced observational uncertainty in J4 and its dynamic contribution, atmospheric water abundance measurements from an Orbiter with a Probe mission to Uranus (UOP) or Neptune, and better understanding of the mixing behaviour of constituents are needed to constrain the interiors of ice giants.

Accepted for publication in Astronomy and Astrophysics

References in corpus (17)

• Separating gas-giant and ice-giant planets by halting pebble accretion
• A new equation of state for dense hydrogen-helium mixtures
• Ab initio equations of state for hydrogen (H-REOS.3) and helium (He-REOS.3) and their implications for the interior of Brown Dwarfs
• Layered convection as the origin of Saturn's luminosity anomaly
• Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune
• A new equation of state for dense hydrogen-helium mixtures II: taking into account hydrogen-helium interactions
• Understanding Jupiter's Interior
• An Exploration of Double Diffusive Convection in Jupiter as a Result of Hydrogen-Helium Phase Separation
• The formation of mini-Neptunes
• The Formation of Uranus & Neptune: Challenges and Implications For Intermediate-Mass Exoplanets
• Thermal evolution of Uranus and Neptune I: adiabatic models
• Machine learning inference of the interior structure of low-mass exoplanets
• The measured compositions of Uranus and Neptune from their formation on the CO iceline
• Bayesian Evolution Models for Jupiter with Helium Rain and Double-diffusive Convection
• Evolution of Jupiter and Saturn with helium rain
• Empirical Structure Models of Uranus and Neptune
• Random Models for Exploring Planet Compositions I: Uranus as an Example