The Mechanical Greenhouse: Burial of Heat by Turbulence in Hot Jupiter Atmospheres

TL;DR

This paper proposes a turbulence-driven heat burial mechanism in hot Jupiter atmospheres that can explain their inflated radii, highlighting the role of turbulent mixing and dissipation in planetary cooling and inflation.

Contribution

It introduces a novel model where turbulent mixing and heat dissipation in the atmosphere influence planetary inflation, providing a potential explanation for hot Jupiter radii anomalies.

Findings

Turbulent mixing can slow cooling by burying heat into the interior.

Dissipation of turbulence enhances heat burial, aiding planetary inflation.

Strong mixing of heavy species like TiO may cause overinflation, challenging existing hypotheses.

Abstract

The intense irradiation received by hot Jupiters suppresses convection in the outer layers of their atmospheres and lowers their cooling rates. "Inflated" hot Jupiters, i.e., those with anomalously large transit radii, require additional sources of heat or suppressed cooling. We consider the effect of forced turbulent mixing in the radiative layer, which could be driven by atmospheric circulation or by another mechanism. Due to stable stratification in the atmosphere, forced turbulence drives a downward flux of heat. Weak turbulent mixing slows the cooling rate by this process, as if the planet was irradiated more intensely. Stronger turbulent mixing buries heat into the convective interior, provided the turbulence extends to the radiative-convective boundary. This inflates the planet until a balance is reached between the heat buried into and radiated from the interior. We also include the direct injection of heat due to the dissipation of turbulence or other effects. Such heating is already known to slow planetary cooling. We find that dissipation also enhances heat burial from mixing by lowering the threshold for turbulent mixing to drive heat into the interior. Strong turbulent mixing of heavy molecular species such as TiO may be necessary to explain stratospheric thermal inversions. We show that the amount of mixing required to loft TiO may overinflate the planet by our mechanism. This possible refutation of the TiO hypothesis deserves further study. Our inflation mechanism requires a deep stratified layer that only exists when the absorbed stellar flux greatly exceeds the intrinsic emitted flux. Thus it would be less effective for more luminous brown dwarfs and for longer period gas giants, including Jupiter and Saturn.