Abstract DGP2026-83

We investigate hydrogen-water phase separation in Uranus and Neptune using a combination of experimental and computational data to predict the atmospheric water abundances of these planets and the pressure at which a transition from a water-poor to a water-rich envelope occurs as a result of hydrogen-water demixing [1]. Our results indicate a strong water depletion of the atmospheres due to to rain-out. These findings are consistent with a classical, few-layer interior structure for the ice giants. Hydrogen-water demixing also affects the thermal evolution of both planets, with phase separation causing a rapid reduction of luminosity while simultaneously increasing the planet radius. This process may account for Uranus' low intrinsic brightness and provides insights into the thermal evolution of Neptune. In addition, we present our revision of the AQUA water equation of state (EoS) [2]. A component of the original AQUA describing the liquid, supercritical fluid and superionic states [3] contained an error in the computation of the entropy. We recomputed the entropies in this high-pressure, high-temperature regime and updated the AQUA EoS accordingly. Moreover, we replaced entropies in the superionic regime [4] to further improve the revised AQUA. We find that the entropy of superionic water is offset by approximately 10% relative to the fluid phase, suggesting a first-order phase transition accompanied by a latent heat release that may contribute to power the luminosity of the ice giants [5].

 

[1] Cano Amoros, M., Nettelmann, N., Tosi, N., Baumeister, P., & Rauer, H. 2024, A&A, 692, A152 

[2] Haldemann, J., Alibert, Y., Mordasini, C., & Benz, W. 2020, A&A, 643, A105 

[3] Mazevet, S., Licari, A., Chabrier, G., & Potekhin, A. Y. 2019, A&A, 621, A128 

[4] French, M., Desjarlais, M. P., & Redmer, R. 2016, Phys. Rev. E, 93, 022140

[5] Cano Amoros, M., Nettelmann, N., Tosi, N., Mazevet, S. 2026, submitted to A&A