Uranus

Uranus is the 7th planet in our Solar System, the third gas / largest planet. Like all gas planets, the copious amounts of gas causes wind speeds of hundreds of km/h. It is the only planet in our Solar System that rotates on its side, meaning its axis is less than 45 degrees to horizontal. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the other gas giants. Uranus is the first planet discovered with a telescope. In March 13, 1781 William Herschel thought that he saw a comet or star in his telescope and it ended up being Uranus.

Like the classical planets, Uranus is visible to the naked eye, but it was never recognised as a planet by ancient observers because of its dimness and slow orbit. Sir William Herschel first observed Uranus on 13 March 1781, leading to its discovery as a planet, expanding the known boundaries of the Solar System for the first time in history and making Uranus the first planet classified as such with the aid of a telescope.

The name of Uranus references the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός), the father of Cronus (Saturn) and grandfather of Zeus (Jupiter), which in Latin became Ūranus (IPA: [ˈuːranʊs]). It is the only planet whose English name is derived directly from a figure of Greek mythology. The adjectival form of Uranus is "Uranian". The pronunciation of the name Uranus preferred among astronomers is /ˈjʊərənəs/, with stress on the first syllable as in Latin Ūranus, in contrast to /jʊˈreɪnəs/, with stress on the second syllable and a long a, though both are considered acceptable.

## Axial Tilt

The Uranian axis of rotation is approximately parallel with the plane of the Solar System, with an axial tilt of 97.77° (as defined by prograde rotation). This gives it seasonal changes completely unlike those of the other planets. Near the solstice, one pole faces the Sun continuously and the other faces away. Only a narrow strip around the equator experiences a rapid day–night cycle, but with the Sun low over the horizon. At the other side of Uranus's orbit the orientation of the poles towards the Sun is reversed. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness. Near the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day–night cycles similar to those seen on most of the other planets. Uranus reached its most recent equinox on 7 December 2007.

 Vernal equinox
 Summer solstice
 Autumnal equinox

One result of this axis orientation is that, averaged over the Uranian year, the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism that causes this is unknown. The reason for Uranus's unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus, causing the skewed orientation. Research by Jacob Kegerreis of Durham University suggests that the tilt resulted from a rock larger than the Earth crashing into the planet 3 to 4 billion years ago. Uranus's south pole was pointed almost directly at the Sun at the time of *Voyager 2*'s flyby in 1986. The labelling of this pole as "south" uses the definition currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite is the pole that points above the invariable plane of the Solar System, regardless of the direction the planet is spinning. A different convention is sometimes used, in which a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation.

## Composition

### Interior and Internal heat

Uranus's mass is roughly 14.5 times that of Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times that of Earth. A resulting density of 1.27 g/cm3 makes Uranus the second least dense planet, after Saturn. This value indicates that it is made primarily of various ices, such as water, ammonia, and methane. The total mass of ice in Uranus's interior is not precisely known, because different figures emerge depending on the model chosen; it must be between 9.3 and 13.5 Earth masses. Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses. The remainder of the non-ice mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.

The standard model of Uranus's structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20% of Uranus'; the mantle comprises its bulk, with around 13.4 Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20% of Uranus's radius. Uranus's core density is around 9 g/cm3, with a pressure in the centre of 8 million bars (800 GPa) and a temperature of about 5000 K. The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.

Uranus's internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux. Why Uranus's internal temperature is so low is still not understood. Neptune, which is Uranus's near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun, but Uranus radiates hardly any excess heat at all. The total power radiated by Uranus in the far infrared (i.e. heat) part of the spectrum is 1.06±0.08 times the solar energy absorbed in its atmosphere. Uranus's heat flux is only 0.042±0.047 W/m2, which is lower than the internal heat flux of Earth of about 0.075 W/m2. The lowest temperature recorded in Uranus's tropopause is 49 K (−224.2 °C; −371.5 °F), making Uranus the coldest planet in the Solar System.

One of the hypotheses for this discrepancy suggests that when Uranus was hit by a supermassive impactor, which caused it to expel most of its primordial heat, it was left with a depleted core temperature. This impact hypothesis is also used in some attempts to explain the planet's axial tilt. Another hypothesis is that some form of barrier exists in Uranus's upper layers that prevents the core's heat from reaching the surface. For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport; perhaps double diffusive convection is a limiting facto

### Atmosphere

**Uranus**, just like Jupiter and Saturn, is primarily made out of hydrogen and helium, the difference being that a considerable minority (2.3%) is made out of methane, which is much more than the rest of the atmosphere that isn't hydrogen or helium on Jupiter, Saturn or even Neptune.Uranus's atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C; −371 °F), and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. The upper most clouds are made out of ammonia and hydrogen clouds, the middle clouds are made out of ammonium hydrosulfide and the lowest clouds are made out of water vapor. *This exact same composition can be found on every gas planet in our Solar System.*

The **atmosphere of Uranus** 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 five main layers: the troposphere, between altitudes of −600 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 hot thermosphere (and exosphere) extending from an altitude of 4,056 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, obtained some valuable compositional data. No other missions to Uranus are currently scheduled.

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 error and frustration. Uranus is a relatively faint object, and its visible angular diameter is smaller than 5″. 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. 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.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. The composition of the Uranian atmosphere is different from that of Uranus as a whole, consisting 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 0.152±0.033 in the upper troposphere, which corresponds to a mass fraction 0.262±0.048. This value is very close to the protosolar helium mass fraction of 0.2741±0.0120, indicating that helium has not settled towards the centre of the planet as it has in the gas giants.

The third 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 colour. Below the methane cloud deck at 1.3 bar methane molecules account for about 2.3% of the atmosphere by molar fraction; 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, which lowers the saturation level and causes excess methane to freeze out. Methane appears to be undersaturated in the upper troposphere 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, water and 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 composition of Uranus's atmosphere is very limited. 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. It appears to be higher than the protosolar value of (2.25±0.35)×10−5 measured in Jupiter. The deuterium is found almost exclusively in hydrogen deuteride molecules which it forms with normal hydrogen atoms. Infrared spectroscopy, including measurements with Spitzer Space Telescope (SST), and UV occultation observations, found trace amounts of complex hydrocarbons in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by 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 are likely to originate from an external source such as infalling dust and comets.

*Deep into the atmosphere of **Uranus**, the extreme pressures may break apart the methane molecules into carbon, which rain down as diamonds.* Beyond that comes the mantle, which is a liquid mess of water, ammonia and methane ices. The core is rocky and small, *made out of iron and nickel*, unlike the 3 other gas planets.

## Moons

Uranus has 27 moons, but almost none that are mainstream-popular, like Triton, Europa, Io or Ganymede. *The moons are named after characters in Shakespeare's plays, some are from old English literature, with an exception for Margaret.* The moons, especially the largest, are very dark, smaller and lighter than our Moon and are primarily out of rock and ice. The largest and most "popular" moons are Titania, Oberon, Umbriel, Ariel and Miranda. The Uranian satellite system is the least massive among those of the giant planets; the combined mass of the five major satellites would be less than half that of Triton (largest moon of Neptune) alone. The largest of Uranus's satellites, Titania, has a radius of only 788.9 km (490.2 mi), or less than half that of the Moon, but slightly more than Rhea, the second-largest satellite of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus's satellites have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light). They are ice–rock conglomerates composed of roughly 50% ice and 50% rock. The ice may include ammonia and carbon dioxide.

Among the Uranian satellites, Ariel appears to have the youngest surface with the fewest impact craters and Umbriel's the oldest. Miranda has fault canyons 20 km (12 mi) deep, terraced layers, and a chaotic variation in surface ages and features. Miranda's past geologic activity is thought to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a former 3:1 orbital resonance with Umbriel. Extensional processes associated with upwelling diapirs are the likely origin of Miranda's 'racetrack'-like coronae. Ariel is thought to have once been held in a 4:1 resonance with Titania. As of 2005 Uranus is known to have nine irregular moons, which orbit it at a distance much greater than that of Oberon, the furthest of the large moons. All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation. The diagram illustrates the orbits of those irregular moons discovered so far. The moons above the X axis are prograde, those beneath are retrograde. The radius of the Uranian Hill sphere is approximately 73 million km.

Uranus's irregular moons range in size from 120–200 km (Sycorax) to about 20 km (Trinculo). Unlike Jupiter's irregulars, Uranus's show no correlation of axis with inclination. Instead, the retrograde moons can be divided into two groups based on axis/orbital eccentricity. The inner group includes those satellites closer to Uranus (a < 0.15 rH) and moderately eccentric (~0.2), namely Francisco, Caliban, Stephano, and Trinculo. The outer group (a > 0.15 rH) includes satellites with high eccentricity (~0.5): Sycorax, Prospero, Setebos, and Ferdinand. The intermediate inclinations 60° < i < 140° are devoid of known moons due to the Kozai instability. In this instability region, solar perturbations at apoapse cause the moons to acquire large eccentricities that lead to collisions with inner satellites or ejection. The lifetime of moons in the instability region is from 10 million to a billion years. Margaret is the only known irregular prograde moon of Uranus, and it currently has the most eccentric orbit of any moon in the Solar System, though Neptune's moon Nereid has a higher mean eccentricity. As of 2008, Margaret's eccentricity is 0.7979.

## Rings

**Uranus** has rings, *however, they are thin and dark*. Unlike Saturn, *which as long, thick, visible rings*, **Uranus** appears to have lines. This, however, can be changed if one views it in forward-scattered light. **Uranus** has 13 rings: 1986UR, 6, 5, 4, $ \alpha ,\beta ,\eta ,\gamma ,\delta ,\lambda ,\epsilon ,\nu $ and $ \mu $. They reach out from a couple thousand km to as far as around 90,000 km. *Nowhere near Saturn distances, but we can't really compare something like Uranus' rings to the magnificence of Saturn.* **Uranus**'s rings are composed of macroscopic particles and dust. In fact, there are multiple dust bands in between various rings, as well as 1986UR and $ \zeta $ being known as **dusty rings**.

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located twice as far from Uranus as the previously known rings. These new rings are so far from Uranus that they are called the "outer" ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13. In April 2006, images of the new rings from the Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other one red. One hypothesis concerning the outer ring's blue colour is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light. In contrast, Uranus's inner rings appear grey.

Jupiter and Neptune also have rings but their rings are *extremely* faint. While Uranus's rings are also faint, they are thicker, denser, *brighter* and definitely more interesting than those of Jupiter and Neptune, *although definitely not as interesting, dense and glorious as those of Saturn.*