Thermal evolution of Uranus and Neptune I: adiabatic models

TL;DR

This study models the thermal evolution of Uranus and Neptune using revised equations of state, revealing that neither planet's current luminosity aligns with fully adiabatic cooling, suggesting non-adiabatic interiors.

Contribution

Developed a new planetary model and evolution code incorporating updated equations of state to analyze the thermal history of Uranus and Neptune.

Findings

Cooling times are about 5.1 billion years for Uranus and 3.7 billion years for Neptune.

Neither planet's luminosity can be explained by adiabatic cooling alone.

Uncertainties in input parameters affect Uranus more than Neptune.

Abstract

The brightness of Neptune is often found to be in accordance with an adiabatic interior, while the low luminosity of Uranus challenges this assumption. Here we apply revised equation of state data of hydrogen, helium, and water and compute the thermal evolution of Uranus and Neptune assuming an adiabatic interior. For this purpose, we have developed a new planetary model and evolution code. We investigate the influence of albedo, solar energy influx, and equations of state of H and He, and water on the cooling time. Our cooling times of about $\tau_\text{U}=5.1\times 10^9\text{ years}$ for Uranus and $\tau_\text{N}=3.7\times 10^9\text{ years}$ for Neptune bracket the known age of the planets of $4.56\times 10^9\text{ years}$ implying that neither planet's present-day luminosity can be explained by adiabatic cooling. We also find that uncertainties on input parameters such as the level of irradiation matter generally more for Uranus than for Neptune. Our results suggest that in contrast to common assumptions, neither planet is fully adiabatic in the deeper interior.