Giant Planet Atmospheres

Summary

The giant planets, Jupiter, Saturn, Uranus, and Neptune, all have vibrant and dynamic atmospheres. The iconic belt-zone structure of Jupiter, together with the Great Red Spot, is instantly recognizable. Saturn, with its dramatic ring system and more muted atmosphere, is a formidable jewel in the Solar System. In the outer reaches, the pale blue Uranus and Neptune are found, worlds about which ultimately very little is known. The atmospheres of these planets are dominated by hydrogen and helium, and unlike the Earth, they do not have a solid surface. These differences generate inherently different types of atmospheres, but there are also similarities. For example, the condensation of water, which forms the familiar clouds on Earth, also occurs on the giant planets. Broadly speaking, the atmosphere can be divided into different regimes defined by their temperature gradients. In the troposphere, where weather occurs, the temperatures decrease as a function of increasing altitude as convection moves internal heat upward; the rising material expands and cools. Above this region lies the stratosphere, defined by a positive temperature gradient, where hydrocarbons are heated by ultraviolet radiation from the Sun (analogous to ozone heating in the terrestrial stratosphere), which also drives substantial photochemistry. This is followed by a mesosphere that cools as a function of altitude, a region that is ill-defined at the giant planets. Finally, the upper atmosphere connects to the space environment and is heated by both solar extreme ultraviolet light and auroral processes. The giant planets are energized both by internal heat and by solar heating. These energy inputs, along with the fast rotation rates of these planets, drive dynamics by establishing global circulation patterns and generating both waves and instabilities. They also drive chemistry within the atmosphere, which alters composition and can influence the temperature structure, particularly in the stratosphere. The atmospheres of these planets can be observed using telescopes on the ground and in space, which can reveal composition, dynamics, and long-term changes. Much of the detailed knowledge of these planets comes from spacecraft such as the iconic Grand Tour of Voyager 1 and 2 in the 1970s and 1980s, as well as the Galileo and Juno missions to Jupiter and the Cassini mission to Saturn. In the 2030s, both the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission and the NASA Europa Clipper mission will arrive at Jupiter, and momentum within the planetary science community is building toward a mission to Uranus that would arrive at some point in the 2040s.