Atmosphere of Uranus

[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.

[4][5] 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.

[6] The nature of the broad dark bands in its visible spectrum remained unknown until the fourth decade of the twentieth century.

[4] Although Uranus is presently largely blank in appearance, it has been historically shown to have occasional features, such as in March and April 1884, when astronomers Henri Joseph Perrotin, Norman Lockyer, and Charles Trépied observed a bright, elongated spot (presumably a storm) circling the equator of the planet.

[9] 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.

[12] In January 1986, the Voyager 2 spacecraft flew by Uranus at a minimal distance of 107,100 km[13] providing the first close-up images and spectra of its atmosphere.

[16] 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.

[18] Methane possesses prominent absorption bands in the visible and near-infrared, making Uranus aquamarine or cyan in color.

[29] To date the only known isotope abundance ratio is that of deuterium to light hydrogen: 5.5+3.5−1.5×10−5, which was measured by the Infrared Space Observatory (ISO) in the 1990s.

[28] The elevated S/N ratio implies depletion of ammonia in the pressure range 20–40 bar, where the ammonium hydrosulfide clouds form.

[24] The top of the deeper ammonia/hydrogen sulfide clouds were determined to be at 3 bar based on the spectroscopic data in the visible and near-infra spectral ranges (0.5–1 μm).

[47] The troposphere is very dynamic, exhibiting strong zonal winds, bright methane clouds,[48] dark spots[49] and noticeable seasonal changes.

(see below)[50] The stratosphere is the middle layer of the Uranian atmosphere, in which temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base thermosphere.

[34][52] The methane enters the stratosphere through the cold tropopause, where its mixing ratio relative to molecular hydrogen is about 3 × 10–5, three times below saturation.

[36][38] Ethane, acetylene and diacetylene condense in the colder lower part of stratosphere[34] forming haze layers with an optical depth of about 0.01 in visible light.

[55] This depletion is caused by weak vertical mixing, and makes Uranus's stratosphere less opaque and, as a result, colder than those of other giant planets.

[55][61] The hazes, like their parent hydrocarbons, are distributed unevenly across Uranus; at the solstice of 1986, when Voyager 2 passed by the planet, they were concentrated near the sunlit pole, making it dark in ultraviolet light.

[62] The outermost layer of the Uranian atmosphere, extending for thousands of kilometres, is the thermosphere/exosphere, which has a uniform temperature of around 800 to 850 K.[52][63] This is much higher than, for instance, the 420 K observed in the thermosphere of Saturn.

[64] The heat sources necessary to sustain such high temperatures are not understood, since neither solar FUV/EUV radiation nor auroral activity can provide the necessary energy.

[55] One of the sources of information about the ionosphere and thermosphere comes from ground-based measurements of the intense mid-infrared (3–4 μm) emissions of the trihydrogen cation (H3+).

[73] The upper atmosphere of Uranus is the source of the far ultraviolet (90–140 nm) emissions known as dayglow or electroglow, which, like the H3+ IR radiation, emanates exclusively from the sunlit part of the planet.

This phenomenon, which occurs in the thermospheres of all giant planets and was mysterious for a time after its discovery, is interpreted as a UV fluorescence of atomic and molecular hydrogen excited by solar radiation or by photoelectrons.

[80] The effects of this bloated exosphere include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.

[78] Uranus has a relatively bland appearance, lacking broad colorful bands and large clouds prevalent on Jupiter and Saturn.

[12][7] The most conspicuous features on Uranus observed by Voyager 2 were the dark low latitude region between −40° and −20° and bright southern polar cap.

Only a limited number of small bright clouds at middle latitudes in both hemispheres[16] and one Uranus Dark Spot have been observed since 1986.