Atmosphere of Uranus - AbsoluteAstronomy.com

The atmosphere of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

, like those of the larger gas giants

Gas Giants

Gas Giants were a pop rock band from Tempe, Arizona, formed as a successor project to the Gin Blossoms. The group was known as The Pharaohs when they formed in 1997, but changed their name after their label, A&M Records, merged with Universal Records and the band changed hands, re-signing with...

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

and Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, is composed primarily of hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

and helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. At depth it is significantly enriched in volatiles (dubbed "ices") such as water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, ammonia

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

and methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

. 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

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

.

The Uranian atmosphere can be divided into three layers: the troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; the stratosphere

Stratosphere

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

, spanning altitudes between 50 and 4000 km and pressures of between and the hot thermosphere

Thermosphere

The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...

(or exosphere

Exosphere

The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...

) extending from an altitude of 4,000 km to several Uranian radii from the nominal surface at 1 bar pressure. Unlike Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's, Uranus's atmosphere has no mesosphere

Mesosphere

The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

.

The troposphere hosts four cloud layers: methane clouds at about 1.2 bar

Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

, hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

/ammonia

Ammonia

clouds in 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

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

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

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

, 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

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. Remote sensing capability extends down to roughly 300 km below the 1 bar level, with a corresponding pressure around 100 bar

Bar (unit)

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

Angelo Secchi

-External links:...

and William Huggins

William Huggins

Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...

, who found a number of broad dark bands, which they were unable to identify. They also failed to detect any solar Fraunhofer lines

Fraunhofer lines

In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer . The lines were originally observed as dark features in the optical spectrum of the Sun....

—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

Rupert Wildt

Rupert Wildt was a German-American astronomer.He was born in Munich, Germany, and grew up in that country during World War I and its aftermath. In 1927 he was awarded a Ph.D. from the University of Berlin...

and Vesto Slipher

Vesto Slipher

Vesto Melvin Slipher was an American astronomer. His brother Earl C. Slipher was also an astronomer and a director at the Lowell Observatory....

, who found that the dark bands at 543, 619, 925, 865 and 890 nm belonged to gaseous methane

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

Gerard Kuiper

Gerard Peter Kuiper , Netherlands – December 24, 1973, Mexico City) was a Dutch-American astronomer after whom the Kuiper belt was named.-Early life:...

noticed another diffuse dark band in the spectrum of Uranus at 827 nm, which he failed to identify. In 1952 Gerhard Herzberg

Gerhard Herzberg

Gerhard Heinrich Friedrich Otto Julius Herzberg, was a pioneering physicist and physical chemist, who won the Nobel Prize for Chemistry in 1971, "for his contributions to the knowledge of electronic structure and geometry of molecules, particularly free radicals". Herzberg's main work concerned...

, a future Nobel Prize

Nobel Prize

The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

winner, showed that this band was caused by the weak quadrapole 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

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

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

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

and by ground based telescopes equipped with adaptive optics

Adaptive optics

Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

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 windspeeds on Uranus, known before only from the Voyager 2 observations, and to study the dynamics of the Uranian atmopshere.

Composition

The composition of the Uranian atmosphere is different from that of Uranus as a whole, consisting mainly of molecular hydrogen and helium

Helium

. The helium molar fraction, i.e. the number of helium atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s per molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

of hydrogen/helium, was determined from the analysis of Voyager 2

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

far infrared and radio occultation

Occultation

An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

observations. The currently accepted value is in the upper troposphere, which corresponds to a mass fraction . This value is very close to the protosolar

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

helium mass fraction of , indicating that helium has not settled towards the centre of the planet as it has in the gas giants.

The fourth most abundant constituent of the Uranian atmosphere

Atmosphere

is methane

Methane

, the presence of which has been known for some time as a result of the ground-based spectroscopic

Spectroscopy

Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

observations. Methane possesses prominent absorption band

Absorption band

An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...

s in the visible

Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

and near-infrared, making Uranus aquamarine

Aquamarine (color)

Aquamarine is a color that is a pale bright tint of spring green toned toward cyan. It is named after the mineral aquamarine, a gemstone mainly found in granite rocks...

or cyan

Cyan

Cyan from , transliterated: kýanos, meaning "dark blue substance") may be used as the name of any of a number of colors in the blue/green range of the spectrum. In reference to the visible spectrum cyan is used to refer to the color obtained by mixing equal amounts of green and blue light or the...

in colour. Methane molecules account for about 2.3% of the atmosphere by molar fraction below the methane cloud deck at 1.3 bar

Bar (unit)

; about 10 to 30 times that found in the Sun. The mixing ratio is much lower in the upper atmosphere due to the extremely low temperature at the tropopause

Tropopause

The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...

, which lowers the saturation level and causes excess methane to freeze out. Methane appears to be undersaturated in the upper troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

above the clouds having a partial pressure of only 30% of the saturated vapor pressure there. The concentration of less volatile compounds such as ammonia

Ammonia

, water

Water

and hydrogen sulfide

Hydrogen sulfide

in the deep atmosphere is poorly known. However, as with methane, their abundances are probably greater than solar values by a factor of at least 20 to 30, and possibly by a factor of a few hundred.

Knowledge of the isotopic

Isotopic

The word isotopic has a number of different meanings, including:* In the physical sciences, to do with chemical isotopes;* In mathematics, to do with a relation called isotopy.* In geometry, isotopic refers to facet-transitivity....

composition of Uranus's atmosphere is very limited. To date the only known isotope abundance ratio is that of deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

to light hydrogen: , which was measured by the Infrared Space Observatory

Infrared Space Observatory

The Infrared Space Observatory was a space telescope for infrared light designed and operated by the European Space Agency , in cooperation with ISAS and NASA...

(ISO) in the 1990s. It appears to be higher than the protosolar

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

value of measured in Jupiter. The deuterium is found almost exclusively in hydrogen deuteride

Hydrogen deuteride

Hydrogen deuteride is a diatomic molecule composed of the two isotopes of hydrogen: the majority isotope 1H protium and 2H deuterium. Its molecular formula is HD.- Availability in nature :...

molecules which it forms with normal hydrogen atoms.

Infrared spectroscopy, including measurements with Spitzer Space Telescope

Spitzer Space Telescope

The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

(SST), and UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

occultation observations, found trace amounts of various hydrocarbon

Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

s in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by solar UV radiation. They include ethane

Ethane

Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

, acetylene

Acetylene

Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

, methylacetylene

Methylacetylene

Methylacetylene is an alkyne with the chemical formula H3C≡CH. It is a component of MAPP gas along with its isomer 1,2-propadiene , which is commonly used in gas welding...

, diacetylene

Diacetylene

Diacetylene , with the formula C4H2, is a highly unsaturated hydrocarbon that contains three single bonds and two triple bonds. It is the first in the series of polyynes.-Occurrence:...

. Infrared spectroscopy also uncovered traces of water vapour, carbon monoxide

Carbon monoxide

Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

and carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

in the stratosphere, which can only originate from an external source such as infalling dust and comet

Comet

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

Structure

The Uranian atmosphere can be divided into three layers: the troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; the stratosphere

Stratosphere

, spanning altitudes between 50 and 4000 km and pressures between and the thermosphere

Thermosphere

/exosphere

Exosphere

extending from 4000 km to as high as a few Uranus radii from the surface. There is no mesosphere

Mesosphere

Troposphere

The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude. The temperature falls from about 320 K at the base of the troposphere at −300 km to 53 K at 50 km. The temperature at the cold upper boundary of the troposphere (the tropopause) actually varies in the range between 49 and 57 K depending on planetary latitude, with the lowest temperature reached near 25° southern latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

. The troposphere holds almost all of the mass of the atmosphere, and the tropopause region is also responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature

Effective temperature

The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

of .

The troposphere is believed to possess a highly complex cloud structure; water cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s are hypothesised to lie in the pressure range of , ammonium hydrosulfide clouds in the range of , ammonia or hydrogen sulfide clouds at between 3 and 10 bar and finally thin methane

Methane

clouds at . Although Voyager 2

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

directly detected methane clouds at 1.2–1.3 bar via a radio occultation

Occultation

experiment, all other cloud layers remain speculative. The existence of a hydrogen sulfide cloud layer is only possible if the ratio of sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

and nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

abundances (S/N ratio) is significantly greater than its solar value of 0.16. Otherwise all hydrogen sulfide would react with ammonia, producing ammonium hydrosulfide, and the ammonia clouds would appear instead in the pressure range 3–10 bar. The elevated S/N ratio implies depletion of ammonia in the pressure range 20–40 bar, where the ammonium hydrosulfide clouds form. These can result from the dissolution of ammonia within water clouds or in the deep water-ammonia ionic ocean.

The exact location of the upper two cloud layers is somewhat controversial. As was said above, the methane clouds were directly detected by Voyager 2 at 1.2–1.3 bar by radio occultation. This result was later confirmed by an analysis of the Voyager 2 limb

Limb

Limb can refer to:*Limb , an appendage of a human or animal*Limb darkening, in astronomy, the appearance of the border of the disk of a celestial body*Limb Music, a record label*Limb , a compilation album by Foetus...

images. 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). However a recent analysis of the spectroscopic data in the wavelength range 1–2.3 μm placed the methane cloudtops at 2 bar, and the top of the lower clouds at 6 bar. This contradiction may be resolved when new data on methane absorption in Uranus's atmosphere are available. The optical depth of the two upper cloud layers varies with latitude: both become thinner at the poles as compared to the equator, though the methane cloud layer's optical depth has a local maximum at 45°S, where the southern polar collar is located (see below).

The troposphere is very dynamic, exhibiting strong zonal winds, bright methane clouds, a dark spot and noticeable seasonal changes.

Stratosphere

The stratosphere

Stratosphere

is the middle layer of the Uranian atmosphere, in which temperature generally increases with altitude from 53 K in the tropopause

Tropopause

to between 800 and 850 K at in the thermosphere

Thermosphere

. The heating of the stratosphere is caused by the downward heat conduction

Heat conduction

In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....

from the hot thermosphere, as well as by absorption of solar UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

and IR

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

radiation by methane and the complex hydrocarbons formed from methane photolysis. The methane enters the stratosphere through the cold tropopause, where its mixing ratio relative to molecular hydrogen is about 3, three times below saturation. The mixing ratio decreases to about 10⁻⁷ at an altitude corresponding to the pressure of 0.1 mbar.

Hydrocarbons other than methane are present in a relatively narrow layer between 160 and 320 km in altitude, corresponding to a pressure range of 10 to 0.1 mbar and temperatures of between 100 and 130 K. The most abundant stratospheric hydrocarbons after methane are acetylene

Acetylene

and ethane

Ethane

, with mixing ratio

Mixing ratio

In chemistry and physics, the dimensionless mixing ratio is defined as the abundance of one component of a mixture relative to that of all other components...

s of around . Heavier hydrocarbons like methylacetylene

Methylacetylene

Methylacetylene is an alkyne with the chemical formula H3C≡CH. It is a component of MAPP gas along with its isomer 1,2-propadiene , which is commonly used in gas welding...

and diacetylene

Diacetylene

Diacetylene , with the formula C4H2, is a highly unsaturated hydrocarbon that contains three single bonds and two triple bonds. It is the first in the series of polyynes.-Occurrence:...

have mixing ratios of about 10⁻¹⁰—three orders of magnitude lower. The temperature and hydrocarbon mixing ratios in the stratosphere vary with time and latitude. Complex hydrocarbons are responsible for the cooling of the stratosphere, especially acetylene, having a strong emission line at the wavelength of 13.7 μm.

In addition to hydrocarbons, the stratosphere contains carbon monoxide, as well as traces of water vapor and carbon dioxide. The mixing ratio of carbon monoxide—3—is very similar to that of the hydrocarbons, while the mixing ratios of carbon dioxide and water are about 10⁻¹¹ and 8, respectively. These three compounds are distributed relatively homogeneously in the stratosphere and are not confined to a narrow layer like hydrocarbons.

Ethane, acetylene and diacetylene condense in the colder lower part of stratosphere forming haze

Haze

Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...

layers with an optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

of about 0.01 in visible light. Condensation occurs at approximately 14, 2.5 and 0.1 mbar for ethane, acetylene and diacetylene, respectively. The concentration of hydrocarbons in the Uranian stratosphere is significantly lower than in the stratospheres of the other giant planets—the upper atmosphere of Uranus is very clean and transparent above the haze layers. This depletion is caused by weak vertical mixing

Mixing (physics)

In physics, a dynamical system is said to be mixing if the phase space of the system becomes strongly intertwined, according to at least one of several mathematical definitions. For example, a measure-preserving transformation T is said to be strong mixing ifwhenever A and B are any measurable...

, and makes Uranus's stratosphere less opaque

Opacity (optics)

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

and, as a result, colder than those of the other giant planets. The hazes, like their parent hydrocarbons, are distributed unevenly across Uranus; at the solstice of 1986 they were concentrated near the sunlit pole, making it dark in ultraviolet light.

Thermosphere and ionosphere

The outermost layer of the Uranian atmosphere, extending for thousands of kilometres, is the thermosphere

Thermosphere

, which has a uniform temperature around 800 to 850 K. This is much higher than, for instance, the 420 K observed in the thermosphere of Saturn. The heat sources necessary to sustain such high temperatures are not understood, since neither solar FUV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

/EUV

Extreme ultraviolet

Extreme Ultraviolet radiation is high-energy ultraviolet radiation, generally defined to be electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 120 nm down to 10 nm, and therefore having photons with energies from 10 eV up to 124 eV...

radiation nor auroral activity can provide the necessary energy. The weak cooling efficiency due to the depletion of hydrocarbons in the stratosphere may contribute to this phenomenon. In addition to molecular hydrogen, the thermosphere contains a large proportion of free hydrogen atom

Hydrogen atom

A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively-charged proton and a single negatively-charged electron bound to the nucleus by the Coulomb force...

s, while helium is thought to be absent here, because it separates diffusively at lower altitudes.

The thermosphere and upper part of the stratosphere contain a large concentration of ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s and electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s, forming the ionosphere

Ionosphere

The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

of Uranus. Radio occultation observations by the Voyager 2 spacecraft showed that the ionosphere lies between 1,000 and 10,000 km altitude and may include several narrow and dense layers between 1,000 and 3,500 km. The electron density in the Uranian ionosphere is on average , reaching to as high as in the narrow layers in the lower ionosphere. The ionosphere is mainly sustained by solar UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

radiation and its density depends on the solar activity. The auroral activity on Uranus is not as powerful as at Jupiter and Saturn and contributes little to the ionization. The high electron density may be in part caused by the low concentration of hydrocarbon

Hydrocarbon

s in the stratosphere.

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 . The total emitted power of is 1–2 W—an order of magnitude higher than that the near-infrared hydrogen quadrupole

Quadrupole

A quadrupole or quadrapole is one of a sequence of configurations of—for example—electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.-Mathematical...

emissions. Trihydrogen cation functions as one of main coolers of the ionosphere.

The upper atmosphere of Uranus is the source of the far ultraviolet (90–140 nm) emissions known as dayglow or electroglow, which, like the 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

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

of atomic and molecular hydrogen excited by solar radiation, with a possible contribution from photoelectrons.

Hydrogen corona

The upper part of the thermopshere, where the mean free path

Mean free path

In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...

of the molecules exceeds the scale height

Scale height

In various scientific contexts, a scale height is a distance over which a quantity decreases by a factor of e...

, is called the exosphere

Exosphere

. The lower boundary of the Uranian exosphere, the exobase, is located at a height of about 6,500 km, or 1/4 of the planetary radius, above the surface. The exosphere is unusually extended, reaching as far as several Uranian radii from the planet. It is made mainly of hydrogen atoms and is often called the hydrogen corona

Corona

A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

of Uranus. The high temperature and relatively high pressure at the base of the thermosphere explain in part why Uranus's exosphere is so vast. The number density of atomic hydrogen in the corona falls slowly with the distance form the planet, remaining as high a few hundred atoms per cm³ at a few radii from Uranus. The effects of this bloated exosphere include a drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

on small particles orbiting Uranus, causing a general depletion of dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

in the Uranian rings, which may contaminate the upper atmosphere of the planet.

Dynamics

Uranus has a relatively bland appearance, lacking broad colorful bands and large clouds prevalent on Jupiter and Saturn. No discrete features were observed in Uranus's atmosphere before 1986. 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. The northern boundary of the cap was located at about −45° of latitude. The brightest zonal band was located near the edge of the cap at −50° to −45° and was then called a polar collar. The southern polar cap, which existed at the time of the solstice

Solstice

A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

in 1986, faded away in 1990s. After the equinox in 2007, the southern polar collar started to fade away as well, while the northern polar collar located at 45° to 50° latitude (first appeared in 2007) have grown more conspicuous since then.

The atmosphere of Uranus is relatively calm compared to those of other giant planets. Only a limited number of small bright clouds at middle latitudes in both hemispheres and one Uranus Dark Spot have been observed since 1986. One of those bright cloud features, located at –34° of latitude, probably existed continuously since at least 1986. Nevertheless, the Uranian atmosphere has rather strong zonal winds blowing in the retrograde (counter to the rotation) direction near the equator, but switching to the prograde direction poleward of ±20° latitude. The wind speeds are from −50 to −100 m/s at the equator increasing to 240 m/s near 50° latitude in the northern hemisphere. The wind profile may be slightly asymmetric with winds stronger in the southern hemisphere, although it may be a seasonal effect as this hemisphere was continuously illuminated by the Sun before 2007.

Uranus exhibits a considerable seasonal variation over its 84 year orbit. It is generally brighter near solstices and dimmer at equinoxes. The variations are largely caused by changes in the viewing geometry: a bright polar region comes into view near solstices, while the dark equator is visible near equinoxes. Still there exist some intrinsic variations of the reflectivity of the atmosphere: periodically fading and brightening polar caps as well as appearing and disappearing polar collars.

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