Atmospheric circulation and mysteries of the Weather Layers of Uranus and Neptune

The Icy Giants Uranus and Neptune are the least explored of Solar System planets. Their apparent size in ground-based observations are 3.7 and 2.3 arcsec difficulting their observation. Their distance also implies that they have been visited only once during the Voyager 2 flybys. Both planets have fast rotation periods, highly tilted rotation axis and cold atmospheres where methane condenses to form most of the visible clouds, although some modern evidences point to some visibility of the deeper H2S clouds. Voyager 2 and ground-based observations showed that the atmospheres of these planets are home to intense zonal winds at the visible cloud level. Unlike the gas giants Jupiter and Saturn, Uranus and Neptune have broad retrograde equatorial jets and broad mid-latitude prograde jets. Both planets have complex deeply extended weather layers where inhomogeneous abundances of condensables will largely determine the atmospheric stability. Existing data show signatures of vertical wind shear and variable methane abundance at different latitudes implying secondary circulations that should extend deeply into their weather layers. A small number of bright cloud systems in both planets are good candidates for moist convective storms, possibly powered by methane condensation. However the stabilizing effects of the vertical gradient of molecular weight should inhibit moist convection at most levels through the weather layer. The combination of extended weather layers, deep massive clouds, weak solar and internal heat forcing, extremely long seasons and inhibited convection place these planets in a dynamic regime unlike any other in the Solar System. While most of the focus of the talk will cover Uranus, I will also show key differences and similarities with Neptune.