Abstract
Abstract The internal heat flows of both Uranus and Neptune remain major outstanding problems in planetary science. Uranus’s 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 cooldown 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 cooldown 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.
Highlights
Giant planet atmospheres are primarily heated by a combination of sunlight and internal heat leftover from formation
Luminosity is a crude indicator of thermal evolution, since planets can store heat internally and may have internal heat sources
We find instead that the profile relaxes into a state of global radiative–convective equilibrium; eventually, the upper layer stops cooling as the heat it loses to space balances with the heat radiated across the stable layer
Summary
Giant planet atmospheres are primarily heated by a combination of sunlight and internal heat leftover from formation. Measurements of the ice giants’ electromagnetic emission to space began in the 1960s (Kellermann & Pauliny-Toth 1966), with high-quality far-infrared measurements constraining the effective temperatures beginning in the 1970s (Fazio et al 1976; Loewenstein et al 1977; Stier et al 1978) These early observations concluded that Uranus appeared approximately in equilibrium with its received sunlight, while Neptune emitted more than twice the radiation it received. We present a mechanism that inhibits convection near the methane cloud level, thereby trapping internal heat beneath the clouds This mechanism has already been theorized and discussed (e.g., Guillot 2005; Friedson & Gonzales 2017; Leconte et al 2017), but the effect of methane on the ice giants has not yet been explicitly quantified and worked into a thermal.
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