The atmosphere of Uranus, like that of Neptune, is different from those of the larger gas giants, Jupiter and Saturn. While still composed primarily of hydrogen and helium, it possesses a higher proportion of volatiles (dubbed "ices") such as water, ammonia and methane. Unlike Jupiter and Saturn, Uranus is not believed to possess a metallic hydrogen mantle or envelope below its upper atmosphere. Instead, its inner regions are believed to consist of an "ocean" composed of ammonia, water and methane, which then makes a gradual transition without a clear boundary into a gaseous atmosphere dominated by hydrogen and helium. Due to these differences, many astronomers group Uranus and Neptune into their own separate category, the ice giants, to distinguish them from Jupiter and Saturn. It is very similar to neptune's colour but instead of a sapphire blue it has a aquamarine appearance.
Although there is no well-defined solid surface within Uranus' interior, the outermost part of Uranus' gaseous envelope that is 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 tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure. The Uranian atmosphere can be divided into three layers: of 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 0.1 and 10–10 bar; and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere.
The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of molecular hydrogen and helium. The helium molar fraction, i.e. the number of helium atoms per molecule of hydrogen/helium, was determined from the analysis of Voyager 2 far infrared and radio occultation observations. The currently accepted value is 0.15 ± 0.03 in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05. This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01, indicating that helium has not settled towards the centre of the planet as it has in the gas giants. The deuterium abundance ratio relative to light hydrogen 5.5+3.5−1.5 × 10−5 was measured in the 1990s by the Infrared Space Observatory (ISO), and appears to be higher than the protosolar value of 2.25 ± 0.35 × 10−5 measured in Jupiter. This deuterium is found almost exclusively in hydrogen deuteride molecules which it forms with normal hydrogen atoms.
The fourth most abundant constituent of the Uranian atmosphere is methane (CH4), the presence of which has been known for some time as a result of the ground-based spectroscopic observations. Methane possesses prominent absorption bands in the visible and near-infrared making Uranus aquamarine or cyan in color. Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at 1.3 bar; about 20 to 30 times that found in the Sun. The mixing ratio is much lower in the upper atmosphere due to its extremely low temperature, which lowers the saturation level and causes excess methane to freeze out. The abundances of less volatile compounds such as ammonia, water and hydrogen sulfide in the deep atmosphere are poorly known. However they are probably also higher than solar values.
Infrared spectroscopy, including measurements with Spitzer Space Telescope (SST), and UV occultation observations, found trace amounts of various hydrocarbons in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar UV radiation. They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H). Infrared spectroscopy also uncovered traces of water vapour, carbon monoxide and carbon dioxide in the stratosphere, which can only originate from an external source such as infalling dust and comets.
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude. The temperature falls from about 320 K at the base of troposphere at −300 km to 53 K at 50 km. The temperatures in the cold upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude, with the lowest temperature reached near 25° southern latitude. The troposphere holds almost all of the mass of the atmosphere, and the tropopause is also responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.
The troposphere is believed to possess a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar, ammonium hydrosulfide clouds in the range of 20 and 40 bar, ammonia or hydrogen sulfide clouds at between 3 and 10 bar and finally thin methane clouds at 1 to 2 bar. Although Voyager 2 directly detected methane clouds via a radio occultation experiment, all other cloud layers remain speculative. The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, convection, bright clouds and seasonal changes.
The middle layer of the Uranian atmosphere is the stratosphere, where temperature generally increases with altitude from 53 K in the tropopause to between 800 and 850 K at the base of the thermosphere. The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons, that form in this part of the atmosphere as a result of methane photolysis. Heating from the hot thermosphere may also be significant. The hydrocarbons occupy a relatively narrow layer at altitudes of between 100 and 280 km corresponding to a pressure range of 10 to 0.1 mbar and temperatures of between 75 and 170 K. The most abundant hydrocarbons are acetylene and ethane with mixing ratios of around × 10−7 relative to hydrogen, which is similar to the mixing ratios of methane and carbon monoxide at these altitudes. Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower. The abundance ratio of water is around 7 × 10−9. The temperature and hydrocarbon mixing ratios vary strongly in time and with latitude; the stratosphere at the poles is both poorer in hydrocarbons and cooler than elsewhere.
Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause forming haze layers, which may be partly responsible for the bland appearance of Uranus. The concentration of hydrocarbons in the Uranian stratosphere is significantly lower than in the stratospheres of the other giant planets. This, in addition to weak vertical mixing makes it less opaque (above the haze layer) and, as a result, colder than on the other giant planets.
Thermosphere and corona
The outmost layer of the Uranian atmosphere is thermosphere/corona, which has a uniform temperature around 800 to 850 K. This is much higher than the 420 K in the thermosphere of Saturn. The heat sources necessary to sustain such a high value are not understood, since neither solar FUV/EUV radiation nor auroral activity can provide the necessary energy, although weak cooling efficiency due to the lack of hydrocarbons in the upper part of the stratosphere may also contribute. Helium is thought to be absent here, because it is diffusivelly separated at lower altitudes. In addition to molecular hydrogen, the thermosphere-corona contains a large proportion of free hydrogen atoms. Their small molecular mass together with the high temperatures may help to explain why the corona extends as far as 50,000 km or two Uranian radii from the planet. This extended corona is a unique feature of Uranus. Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings. The hot thermosphere of Uranus produces intense hydrogen quadrupole emissions in the near-infrared.
The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus. The main sources of information about the ions are Voyager 2 measurements and infrared emissions of the H3+ ion detected from Earth-based telescopes. The observations show that the ionosphere occupies altitudes from 2,000 to 10,000 km. The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere. The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity. Auroral activity is not as significant as at Jupiter and Saturn. The upper ionosphere (thermosphere region) is the source of the UV emission from Uranus that is known as 'dayglow' or 'electroglow', which like H3+ IR radiation emanates exclusively from the sunlit part of the planet. This phenomenon, which occurs in 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 that is excited by solar radiation, with a possible contribution from photoelectrons.
* Climate of Uranus
1. ^ a b c d e f g h i j k l m Lunine, Jonathan. I. (1993). "The Atmospheres of Uranus and Neptune". Annual Review of Astronomy and Astrophysics 31: 217–263. doi:10.1146/annurev.aa.31.090193.001245. http://adsabs.harvard.edu/abs/1993ARA%26A..31..217L.
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