Atmosphère d'Uranus

Atmosphère d’Uranus
Atmosphère de la planète.
Uranus, prise par la sonde Voyager 2.
Informations générales
Épaisseur	-
Pression atmosphérique moyenne	1,20×105 Pa
Masse	-
Composition
Dihydrogène	83 %
Hélium	15 %
Méthane	1,99 %
Ammoniac	0,01 %
Éthane	0,00 025 %
Acétylène	0,00 001 %
Monoxyde de carbone	traces
Sulfure d'hydrogène	traces

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 withaut a clear boundary into a gaseous atmospherw 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.

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.

Structure

Troposphère

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.

Composition

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 $5.5{\scriptstyle {+3.5} \atop \scriptstyle {-1.5}}\times 10^{-5}$ 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^e 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.

Voir aussi

• Atmosphère
• Atmosphère de Vénus
• Atmosphère terrestre
• Atmosphère de Mars
• Atmosphère de Jupiter