Composition and Chemistry of the Atmospheres of Jupiter, Saturn, Uranus, and Neptune

This paper reviews the current understanding of the composition and chemistry of the Solar System’s giant planets Jupiter, Saturn, Uranus, and Neptune. The tropospheric chemistry is governed by thermochemical equilibrium, with various chemical species condensing into clouds forming at different pressure levels. In the stratospheres, photochemistry initiated by UV radiation drives the production of hydrocarbons and other molecules from methane photolysis. Photochemical models provide valuable tools to study the stratospheric composition and distribution. The predicted abundances depend on factors like solar ultraviolet flux, chemical reaction rates (and their uncertainties), and transport processes. Exogenic species like H2O, CO, and HCN are delivered to the giant planets stratospheres by interaction with their local icy environment (moons and rings), including large comet impacts and interplanetary dust particles. Their temporal evolution has been monitored to study chemistry and dynamics. Ion chemistry in the giant planet’s ionospheres is also reviewed, including the effects of precipitating auroral electrons on ion production.