This blog post is based on a White Paper and #EPSC2020 presentation by Tristan Guillot, available on Vimeo. Uranus and Neptune are key to the understanding of planets with hydrogen atmospheres. These are the last worlds never to have been visited by an orbiter, and are probably the building blocks for formation of giant planets. Their interiors and evolution, and hence their composition, are poorly constrained. They are unique laboratories for understanding heat transfer, compositional variations and temporal variations. These planets are active, with methane clouds, seasonal variations, and activity most probably linked to convective phenomena.
What have we learned from the other giants, like Juno at Jupiter? Equilibrium cloud structures have methane clouds near the top of the observable atmosphere, where the optical depth is relatively low. This is opposite for the water clouds in Jupiter and Saturn. Juno has shown that the atmosphere is not as simple as we expected. Ammonia is varying in altitude and latitude down to great depths, tens of bars or more. The presence of water storms lofting ice crystals, that dissolve ammonia, and then bring down ammonia and water gas down to great depth. This precipitation forms intense cold downdrafts that can penetrate deep. How deep to they fall with no surface? Hydrogen atmospheres always have heavy condensates, contrary to the earth case. Downdrafts in the Sun are important for the solar convective zone.
Moist convection can be inhibited by composition. The molecular weight effect inhibits convection locally, and this occurs when water is more abundant than 10x solar, and methane more abundant than 40x solar, so should occur on Saturn and the Ice Giants. Furthermore, we don't know what the temperature profiles look like below the 1-bar level - what sort of adiabats should they follow, and what are the implications for interior modelling?
We need to evolve from a standard picture of uniform clouds based on equilibrium, to something that is more variable with strong updrafts, precipitation, and downdrafts. We know that clouds are extremely important for understanding hot and warm Jupiters and their compositions, and also important for brown dwarfs. A mission is sorely needed, with an orbiter and a probe.