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The atmosphere of Uranus, like those of the larger gas giants, Jupiter and Saturn, 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. The Uranian atmosphere can be divided into three main layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between and the hot thermosphere (and exosphere) extending from an altitude of 4,000 km to several Uranian radii from the nominal surface at 1 bar pressure. 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, has studied it in detail. No other missions to Uranus are currently scheduled. == Observation and exploration == Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope (the region accessible to remote sensing) is called its atmosphere. Remote sensing capability extends down to roughly 300 km below the 1 bar level, with a corresponding pressure around 100 bar and temperature of 320 K. The observational history of the Uranian atmosphere is long and full of errors and frustrations. Uranus is a relatively faint object, and its visible angular diameter is smaller than 4″. The first spectra of Uranus were observed through a prism in 1869 and 1871 by Angelo Secchi and William Huggins, who found a number of broad dark bands, which they were unable to identify. They also failed to detect any solar Fraunhofer lines—the fact later interpreted by Norman Lockyer as indicating that Uranus emitted its own light as opposed to reflecting light from the Sun. In 1889 however, astronomers observed solar Fraunhofer lines in photographic ultraviolet spectra of the planet, proving once and for all that Uranus was shining by reflected light. The nature of the broad dark bands in its visible spectrum remained unknown until the fourth decade of the twentieth century. The key to deciphering Uranus's spectrum was found in the 1930s by Rupert Wildt and Vesto Slipher, who found that the dark bands at 543, 619, 925, 865 and 890 nm belonged to gaseous methane. They had never been observed before because they were very weak and required a long path length to be detected. This meant that the atmosphere of Uranus was transparent to a much greater depth compared to those of other giant planets. In 1950, Gerard Kuiper noticed another diffuse dark band in the spectrum of Uranus at 827 nm, which he failed to identify. In 1952 Gerhard Herzberg, a future Nobel Prize winner, showed that this band was caused by the weak quadrupole absorption of molecular hydrogen, which thus became the second compound detected on Uranus. Until 1986 only two gases, methane and hydrogen, were known in the Uranian atmosphere. The far-infrared spectroscopic observation beginning from 1967 consistently showed the atmosphere of Uranus was in approximate thermal balance with incoming solar radiation (in other words, it radiated as much heat as it received from the Sun), and no internal heat source was required to explain observed temperatures. No discrete features had been observed on Uranus prior to the Voyager 2 visit in 1986. In January 1986, the Voyager 2 spacecraft flew by Uranus at a minimal distance of 107,100 km providing the first close-up images and spectra of its atmosphere. They generally confirmed that the atmosphere was made of mainly hydrogen and helium with around 2% methane. The atmosphere appeared highly transparent and lacking thick stratospheric and tropospheric hazes. Only a limited number of discrete clouds were observed. In the 1990s and 2000s, observations by the Hubble Space Telescope and by ground based telescopes equipped with adaptive optics systems (the Keck telescope and NASA Infrared Telescope Facility, for instance) made it possible for the first time to observe discrete cloud features from Earth. Tracking them has allowed astronomers to re-measure wind speeds on Uranus, known before only from the Voyager 2 observations, and to study the dynamics of the Uranian atmosphere. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atmosphere of Uranus」の詳細全文を読む スポンサード リンク
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