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

The strikingly low luminosity of Uranus ( T eff ≃ T eq ) 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 × solar. Finally, we allow for an equilibrium evolution ( T eff ≃ T eq ) 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 T eff ≃ T eq 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. [ABSTRACT FROM AUTHOR]