Moist convection inhibition and its consequences for exoplanets

Convection governs the structure, interior temperatures and cooling of planets. When condensation occurs, the release of latent heat can lead to a more efficient heat transport. This situation is natural for us, as it occurs in the Earth atmosphere with water condensation, which can lead to powerful storms and a temperature gradient close to a moist adiabat. But when condensing species are both abundant and heavier than the surrounding gas or fluid, the opposite may be true: The release of latent heat then keeps the abundance of the condensing species higher than in the surrounding environment, leading to an inhibition of convection. Furthermore, this inhibition occurs independently of the temperature gradient itself, leading to a potentially very significant superadiabaticity, controlled by other processes (radiation, conduction) available for heat transport and by condensate precipitation. 

 

This situation occurs with methane in the atmospheres of Uranus and Neptune. It may occur with water in Jupiter and Saturn and explain some of the structures seen with Juno and Cassini. In the deep interiors of these planets, the latent heat released by helium droplets when they phase separate from hydrogen also creates a powerful barrier to convection that could explain the dichotomy that is observed on their magnetic fields. In exoplanets, moist convection inhibition should prevail in a large variety of situations: In the atmospheres of metal-rich hydrogen-dominated exoplanets, especially temperate one in which abundant condensing species such as water are present. It should also affect the interior structures of super-Earths with magma oceans. 

 

The consequences should be a slower cooling and generally (but not always) higher interior temperatures. Moist convection inhibition affects the transport of heat and chemicals, and therefore the link that we can make between the atmosphere and interior from disequilibrium species. It affects the mass-radius-age relations and therefore the inferred bulk abundances of heavy elements in fluid planets. We will review the consequences and opportunities this represents in preparation of the PLATO mission.