Constraining the effect of convective inhibition on the thermal evolution of Uranus and Neptune

TL;DR

This study investigates how condensation and convective inhibition affect the thermal evolution of Uranus and Neptune, finding they can only partially explain the planets' observed heat flows and internal temperatures.

Contribution

It provides the first quantitative constraints on how methane and water condensation influence the thermal evolution of Uranus and Neptune, including effects on cooling times.

Findings

Condensation effects can speed up or slow down cooling by up to 15%.

Methane condensation can accelerate cooling, water condensation can decelerate it.

Convective inhibition impacts planetary interior modeling and requires finite activation energy to disrupt.

Abstract

The internal heat flows of both Uranus and Neptune remain major outstanding problems in planetary science. Uranus' surprisingly cold effective temperature is inconsistent with adiabatic thermal evolution models, while Neptune's substantial internal heat flow is twice its received insolation. In this work we constrain the magnitude of influence condensation, including latent heat and inhibition of convection, can have on the thermal evolution of these bodies. We find that while the effect can be significant, it is insufficient to solve the Uranus faintness problem on its own. Self-consistently considering the effects of both latent heat release and stable stratification, methane condensation can speed up the cool down time of Uranus and Neptune by no more than 15%, assuming 5% molar methane abundance. Water condensation works in the opposite direction; water condensation can slow down the cool down timescale of Uranus and Neptune by no more than 15% assuming 12% molar water abundance. We also constrain the meteorological implications of convective inhibition. We demonstrate that sufficiently abundant condensates will relax to a state of radiative-convective equilibrium requiring finite activation energy to disrupt. We also comment on the importance of considering convective inhibition when modeling planetary interiors.