Atmosphere of Uranus

The atmosphere of Uranus is composed primarily of hydrogen and helium. At depth it is significantly enriched in volatiles (dubbed "ices") such as water, ammonia and methane. The opposite is true for the upper atmosphere, which contains very few gases heavier than hydrogen and helium due to its low temperature. Uranus's atmosphere is the coldest of all the planets, with its temperature reaching as low as 49 K.^[1]

The Uranian atmosphere can be divided into five main layers: the troposphere, between altitudes of −300^[a] and 50 km and pressures from 100 to 0.1 bar; the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10⁻¹⁰ bar; and the hot thermosphere (and exosphere) extending from an altitude of 4,056 km to several Uranian radii from the nominal surface at 1 bar pressure.^[2] Unlike Earth's, Uranus's atmosphere has no mesosphere.

The troposphere hosts four cloud layers: methane clouds at about 1.2 bar, hydrogen sulfide and ammonia clouds at 3–10 bar, ammonium hydrosulfide clouds at 20–40 bar, and finally water clouds below 50 bar. Only the upper two cloud layers have been observed directly—the deeper clouds remain speculative. Above the clouds lie several tenuous layers of photochemical haze. Discrete bright tropospheric clouds are rare on Uranus, probably due to sluggish convection in the planet's interior. Nevertheless, observations of such clouds were used to measure the planet's zonal winds, which are remarkably fast with speeds up to 240 m/s.

Little is known about the Uranian atmosphere as to date only one spacecraft, Voyager 2, which passed by the planet in 1986, obtained some valuable compositional data. The Uranus Orbiter and Probe is scheduled to launch in 2031, arriving at Uranus in 2044. Its primary science objectives include a detailed study of Uranus' atmosphere.

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