Uranus

By solar and stellar occultations and visual, infrared, and radio observations from Voyager 2, the structure and circulation of Uranus' atmosphere has been mapped from just below the cloud layers to its exosphere. The main components of the atmosphere are hydrogen and helium, the most abundant elements in the universe. Methane comprises 1–2% of the observable troposphere. Water vapor and ammonia are inferred to be important components of the atmosphere below the clouds, but they have not been detected in the observable part of the troposphere because it is too cold and freezes them out.

Uranus' upper atmosphere is dominated by hydrogen, mainly molecular. Methane and other hydrocarbons are nearly all frozen out by the underlying cold atmosphere, especially the lower stratosphere (at 428°F [220°C] temperature). Molecular hydrogen is broken down into atomic hydrogen mainly by the absorption of ultraviolet radiation from the sun, and some atomic hydrogen is ionized into free protons and electrons, forming an ionosphere. The temperature of the upper atmosphere increases to 482°F (250°C) at 497 mi (800 km) above the cloud layers, and continues to increase to over 932°F (500°C) at 3,105 mi (5,000 km) above the clouds. Atomic hydrogen becomes the main component of Uranus' upper atmosphere above 4,658 mi (7,500 km) above the clouds, forming a hydrogen thermal corona in its exosphere that extends at least 15,525 mi (25,000 km) above the clouds (extending through the zone of the rings from 9,936 mi [16,000 km] to 16,146 mi [26,000 km] above the clouds). The source of most of the heating of Uranus' upper atmosphere is still unknown as is also true for the heating of the upper atmospheres of Earth, Jupiter, and of Saturn.