LECTURE 6: THE PLANET URANUS
Uranus ([ˈjʊərənəs] or [jʊˈreɪnəs] [12]) is the seventh planet from the Sun and the third-largest and fourth-most massive planet in the solar system. It is named after the ancient Greek deity of the sky (Uranus, Οὐρανός), the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Uranus was the first planet discovered in modern times. Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers due to its dimness.[13] Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet made using a telescope.
Uranus is similar in internal and atmospheric composition to Neptune, and both have different compositions from those of the larger gas giants Jupiter and Saturn. As such, astronomers sometimes place them in a separate category, the "ice giants". Uranus' atmosphere, while similar to Jupiter and Saturn in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds.[8]
Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun; its north and south poles lie where most other planets have their equators.[14] Seen from Earth, Uranus' rings can appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock, though in 2007 and 2008 the rings appear edge-on. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants.[14] However, terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second.[15]
Uranus had been observed on many occasions prior to its discovery as a planet, but it was generally mistaken for a star. The earliest recorded sighting was in 1690 when John Flamsteed catalogued Uranus as 34 Tauri and observed it at least six times. The French astronomer, Pierre Lemonnier, observed Uranus at least twelve times between 1750 and 1769,[16] including on four consecutive nights.
Sir William Herschel observed the planet on 13 March 1781 while in the garden of his house at 19 New King Street in the town of Bath, Somerset (now the Herschel Museum of Astronomy),[17] but initially reported it (on 26 April 1781) as a "comet".[18] Herschel "engaged in a series of observations on the parallax of the fixed stars",[19] using a telescope of his own design.
He recorded in his journal "In the quartile near ζ Tauri … either [a] Nebulous star or perhaps a comet".[20] On March 17, he noted, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place".[21] When he presented his discovery to the Royal Society, he continued to assert that he had found a comet while also implicitly comparing it to a planet:[22]
Herschel notified the Astronomer Royal, Nevil Maskelyne, of his discovery and received this flummoxed reply from him on April 23: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it".[23]
While Herschel continued to cautiously describe his new object as a comet, other astronomers had already begun to suspect otherwise. Russian astronomer Anders Johan Lexell estimated its distance as 18 times the distance of the Sun from the Earth, and no comet had yet been observed with a perihelion of even four times the Earth–Sun distance.[24] Berlin astronomer Johann Elert Bode described Herschel's discovery as "a moving star that can be deemed a hitherto unknown planet-like object circulating beyond the orbit of Saturn".[25] Bode concluded that its near-circular orbit was more like a planet than a comet.[26]
The object was soon universally accepted as a new planet. By 1783, Herschel himself acknowledged this fact to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System."[27] In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on the condition that he move to Windsor so the Royal Family could have a chance to look through his telescopes.[28]
Maskelyne asked Herschel to "do the astronomical world the faver [sic] to give a name to your planet, which is entirely your own, & which we are so much obliged to you for the discovery of."[29] In response to Maskelyne's request, Herschel decided to name the object Georgium Sidus (George's Star), or the "Georgian Planet" in honour of his new patron, King George III.[30] He explained this decision in a letter to Joseph Banks:[27]
Astronomer Jérôme Lalande proposed the planet be named Herschel in honour of its discoverer.[31] Bode, however, opted for Uranus, the Latinized version of the Greek god of the sky, Ouranos. Bode argued that just as Saturn was the father of Jupiter, the new planet should be named after the father of Saturn.[28][32][33] The earliest citation of the name Uranus in an official publication is in 1823, a year after Herschel's death.[34][35] The name Georgium Sidus or "the Georgian" was still used infrequently (by the British alone) for some time thereafter; the final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.[32]
The preferred pronunciation of the name Uranus among astronomers is [ˈjʊərənəs], with the first syllable stressed and a short a (ūrănŭs);[36] this is more classically correct than the alternate [jʊˈɹeɪ.nəs], with stress on the second syllable and a "long a" (ūrānŭs), which is often used in the English-speaking world.
Uranus is the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology. (The Roman equivalent would have been Caelus.) The adjective of Uranus is "Uranian". The element uranium, discovered in 1789, was named in its honour by its discoverer, Martin Klaproth.[37]
Its astronomical symbol is . It is a hybrid of the symbols for Mars and the Sun because Uranus was the Sky in Greek mythology, which was thought to be dminated by the combined powers of the Sun and Mars.[38] Its astrological symbol is , suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name").[31] In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).[39][40]
Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). The intensity of sunlight on Uranus is about 1/400 that of Earth.[41] Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.[24] With time, discrepancies began to appear between the predicted and observed orbits, and in 1841, John Couch Adams first proposed that the differences might be due to the gravitational tug of an unseen planet. In 1845, Urbain Le Verrier began his own independent research into Uranus' orbit. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, at nearly the position predicted by Le Verrier.[42]
The rotational period of the interior of Uranus is 17 hours, 14 minutes. However, as on all giant planets, its upper atmosphere experiences very strong winds in the direction of rotation. In effect, at some latitudes, such as about two-thirds of the way from the equator to the south pole, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.[43]
Uranus' axis of rotation lies on its side with respect to the plane of the solar system, with an axial tilt of 98 degrees. This makes its exchange of seasons completely unlike those of the other major planets. Other planets can be visualized to rotate like tilted spinning tops relative to the plane of the solar system, while Uranus rotates more like a tilted rolling ball. Near the time of Uranian solstices, one pole faces the Sun continually while the other pole faces away. Only a narrow strip around the equator experiences a rapid day-night cycle, but with the Sun very low over the horizon as in the Earth's polar regions. At the other side of Uranus' 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.[44] 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.[45][46]
One result of this axis orientation is that, on average during the 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 which causes this is unknown. The reason for Uranus' 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.[47] Uranus' south pole was pointed almost directly at the Sun at the time of Voyager 2's flyby in 1986. The labeling 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 shall be the pole which points above the invariable plane of the solar system, regardless of the direction the planet is spinning.[48][49] However, a different convention is sometimes used, where a body's north and south poles are defined according to the right-hand rule in relation to the direction of rotation.[50] In terms of this latter coordinate system it was Uranus' north pole which was in sunlight in 1986. Astronomer Patrick Moore, commenting on the issue, summed it up by saying "Take your pick!"[51]
From 1995 to 2006, Uranus' apparent magnitude fluctuated between +5.6 and +5.9, placing it just within the limit of naked eye visibility at +6.5.[6] Its angular diameter is between 3.4 and 3.7 arcseconds, compared with 16 to 20 arcseconds for Saturn and 32 to 45 arcseconds for Jupiter.[6] At opposition, Uranus is visible to the naked eye in dark, un-light polluted skies, and becomes an easy target even in urban conditions with binoculars.[4] In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening. With a large telescope of 25 cm or wider, cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.[52]
Uranus' mass is roughly 14.5 times that of the Earth, making it the least massive of the giant planets, while its density of 1.27 g/cm³ makes it the second least dense planet, after Saturn.[5] Though having a diameter slightly larger than Neptune (roughly four times Earth's), it is less massive.[3] These values indicate that it is made primarily of various ices, such as water, ammonia, and methane.[7] The total mass of ice in Uranus' interior is not precisely known, as different figures emerge depending on the model chosen; however, it must be between 9.3 and 13.5 Earth masses.[7][53] Hydrogen and helium constitute only a small part of the total, with between 0.5 and 1.5 Earth masses.[7] The remainder of the mass (0.5 to 3.7 Earth masses) is accounted for by rocky material.[7]
The standard model of Uranus' structure is that it consists of three layers: a rocky core in the center, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope.[7][54] The core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus'; the mantle comprises the bulk of the planet, with around 13.4 Earth masses, while the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus' radius.[7][54] Uranus' core density is around 9 g/cm³, with a pressure at the core/mantle boundary of 8 million bars (800 GPa) and a temperature of about 5000 K.[53][54] 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.[7][54] This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.[55] The bulk compositions of Uranus and Neptune are very different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants.
While the model considered above is more or less standard, it is not unique; other models also satisfy observations. For instance, if substantial amounts of hydrogen and rocky material are mixed in the ice mantle, the total mass of ices in the interior will be lower, and, correspondingly, the total mass of rocks and hydrogen will be higher. Presently available data does not allow us to determine which model is correct.[53] The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers.[7] However for the sake of convenience an oblate spheroid of revolution, where pressure equals 1 bar (100 kPa), is designated conditionally as a ‘surface’. It has equatorial and polar radii of 25,559 ± 4 and 24,973 ± 20 km, respectively.[3] This surface will be used throughout this article as a zero point for altitudes.
Uranus' internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low thermal flux.[56][15] Why Uranus' internal temperature is so low is still not understood. Neptune, which is Uranus' near twin in size and composition, radiates 2.61 times as much energy into space as it receives from the Sun.[15] Uranus, by contrast, 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.[57][8] In fact, Uranus' 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.[57] The lowest temperature recorded in Uranus' tropopause is 49 K (−224 °C), making Uranus the coldest planet in the Solar System.[57][8]
Hypotheses for this discrepancy include that when Uranus was "knocked over" by the supermassive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat, leaving it with a depleted core temperature.[58] Another hypothesis is that some form of barrier exists in Uranus' upper layers which prevents the core's heat from reaching the surface.[7] For example, convection may take place in a set of compositionally different layers, which may inhibit the upward heat transport.[8][57]
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.[8] Remote sensing capability extends down to roughly 300 km below the 1 bar (100 kPa) level, with a corresponding pressure around 100 bar (10 MPa) and temperature of 320 K.[59] The tenuous corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure.[60] The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of −300 and 50 km and pressures from 100 to 0.1 bar; (10 MPa to 10 kPa) the stratosphere, spanning altitudes between 50 and 4000 km and pressures of between 0.1 and 10–10 bar (10 kPa to 10 µPa), and the thermosphere/corona extending from 4,000 km to as high as 50,000 km from the surface.[8] 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.[8] The helium molar fraction, i.e. the number of helium atoms per molecule of gas, is 0.15 ± 0.03[10] in the upper troposphere, which corresponds to a mass fraction 0.26 ± 0.05.[8][57] This value is very close to the protosolar helium mass fraction of 0.275 ± 0.01,[61] indicating that helium has not settled in the center of the planet as it has in the gas giants.[8] The third most abundant constituent of the Uranian atmosphere is methane (CH4).[8] Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color.[8] Methane molecules account for 2.3% of the atmosphere by molar fraction below the methane cloud deck at the pressure level of 1.3 bar (130 kPa); this represents about 20 to 30 times the carbon abundance found in the Sun.[8][9][62] The mixing ratio[e] 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.[63] 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.[8][64] In addition to methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet (UV) radiation.[65] They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), diacetylene (C2HC2H).[63][66][67] Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.[67][66][68]
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude.[8] The temperature falls from about 320 K at the base of the nominal troposphere at −300 km to 53 K at 50 km.[59][62] The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between 49 and 57 K depending on planetary latitude.[8][56] The tropopause region is responsible for the vast majority of the planet’s thermal far infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.[56][57]
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 (5 to 10 MPa), ammonium hydrosulfide clouds in the range of 20 to 40 bar (2 to 4 MPa), ammonia or hydrogen sulfide clouds at between 3 and 10 bar (0.3 to 1 MPa) and finally directly detected thin methane clouds at 1 to 2 bar (0.1 to 0.2 MPa).[8][59][69][9] The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.[15]
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.[60] The heating of the stratosphere is caused by absorption of solar UV and IR radiation by methane and other hydrocarbons,[70] which form in this part of the atmosphere as a result of methane photolysis.[65] Heat is also conducted from the hot thermosphere.[70] 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 (1000 to 10 kPa) and temperatures of between 75 and 170 K.[63][66] The most abundant hydrocarbons are methane, acetylene and ethane with mixing ratios of around 10−7 relative to hydrogen. The mixing ratio of carbon monoxide is similar at these altitudes.[63][66][68] Heavier hydrocarbons and carbon dioxide have mixing ratios three orders of magnitude lower.[66] The abundance ratio of water is around 7×10−9.[67] Ethane and acetylene tend to condense in the colder lower part of stratosphere and tropopause (below 10 mBar level) forming haze layers,[65] which may be partly responsible for the bland appearance of Uranus. However, the concentration of hydrocarbons in the Uranian stratosphere above the haze is significantly lower than in the stratospheres of the other giant planets.[63][71]
The outermost layer of the Uranian atmosphere is the thermosphere and corona, which has a uniform temperature around 800 to 850 K.[8][71] The heat sources necessary to sustain such a high value are not understood, since neither solar far UV and extreme UV radiation nor auroral activity can provide the necessary energy. The weak cooling efficiency due to the lack of hydrocarbons in the stratosphere above 0.1 mBar pressure level may contribute too.[60][71] In addition to molecular hydrogen, the thermosphere-corona contains a large proportion of free hydrogen atoms. Their small mass together with the high temperatures explain why the corona extends as far as 50,000 km or two Uranian radii from the planet.[60][71] This extended corona is a unique feature of Uranus.[71] Its effects include a drag on small particles orbiting Uranus, causing a general depletion of dust in the Uranian rings.[60] The Uranian thermosphere, together with the upper part of the stratosphere, corresponds to the ionosphere of Uranus.[62] Observations show that the ionosphere occupies altitudes from 2,000 to 10,000 km.[62] The Uranian ionosphere is denser than that of either Saturn or Neptune, which may arise from the low concentration of hydrocarbons in the stratosphere.[71][72] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[73] Auroral activity is insignificant as compared to Jupiter and Saturn.[71][74]
Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter.[14] It was the second ring system to be discovered in the Solar System after Saturn's.[75] Thirteen distinct rings are presently known, the brightest being the epsilon ring. Uranus’ rings are probably quite young; gaps in their circumference as well as differences in their opacity suggest that they did not form with Uranus. The matter in the rings may once have been part of a moon which was shattered by a high-speed impact or tidal forces.[75][76]
Prior to the arrival of Voyager 2, no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery. Before 1986, astronomers had expected the magnetic field of Uranus to be in line with the solar wind, since it would then align with the planet's poles that lie in the ecliptic.[84]
Voyager's observations revealed that the magnetic field is peculiar, both because it does not originate from the planet's geometric center, and because it is tilted at 59° from the axis of rotation.[84][85] In fact the magnetic dipole is shifted from the center of the planet towards the south rotational pole by as much as one third of the planetary radius.[84] This unusual geometry results in a highly asymmetric magnetosphere, where the magnetic field strength on the surface in the southern hemisphere can be as low as 0.1 gauss (10 µT), whereas in the northern hemisphere it can be as high 1.1 gauss (110 µT).[84] The average field at the surface is 0.23 gauss (23 µT).[84] In comparison, the magnetic field of Earth is roughly as strong at either pole, and its "magnetic equator" is roughly parallel with its physical equator.[85] The dipole moment of Uranus is 50 times that of Earth.[84][85] Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.[85] One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giant planets, which are generated within their cores, the ice giants' magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean.[55][86]
Despite its curious alignment, in other respects the Uranian magnetosphere is like those of other planets: it has a bow shock located at about 23 Uranian radii ahead of it, a magnetopause at 18 Uranian radii, a fully developed magnetotail and radiation belts.[84][85][87] Overall, the structure of the magnetosphere of Uranus is different from that of Jupiter's and more similar to that of Saturn's.[84][85] Uranus' magnetotail trails behind the planet into space for millions of kilometers and is twisted by the planet's sideways rotation into a long corkscrew.[84][88]
Uranus' magnetosphere contains charged particles: protons and electrons with small amount of H2+ ions.[85][87] No heavier ions have been detected. Many of these particles probably derive from the hot atmospheric corona.[87] The ion and electron energies can be as high as 4 and 1.2 megaelectronvolts, respectively.[87] The density of low energy (below 1 kiloelectronvolt) ions in the inner magnetosphere is about 2 cm−3.[89] The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps.[87] The particle flux is high enough to cause darkening or space weathering of the moon’s surfaces on an astronomically rapid timescale of 100,000 years.[87] This may be the cause of the uniformly dark colouration of the moons and rings.[76] Uranus has relatively well developed aurorae, which are seen as bright arcs around both magnetic poles.[71] However, unlike Jupiter's, Uranus' aurorae seem to be insignificant for the energy balance of the planetary thermosphere.[74]
Uranus' atmosphere is remarkably bland in comparison to the other gas giants, even to Neptune, which it otherwise closely resembles.[15] When Voyager 2 flew by Uranus in 1986, it observed a total of ten cloud features across the entire planet.[14][90] One proposed explanation for this dearth of features is that Uranus' internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus' tropopause is 49 K, making Uranus the coldest planet in the Solar System, colder than Neptune.[57][8]
In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[14] Their boundary is located at about −45 degrees of latitude. A narrow band straddling the latitudinal range from −45 to −50 degrees is the brightest large feature on the visible surface of the planet.[14][91] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[92] Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. However, at the beginning of the twenty-first century, when the northern polar region came into view, Hubble Space Telescope (HST) and Keck telescope observed neither a collar nor a polar cap in the northern hemisphere.[91] So Uranus appears to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[91]
In addition to large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[14] In all other respects Uranus looked like a dynamically dead planet in 1986. However in the 1990s the number of the observed bright cloud features grew considerably.[15] The majority of them were found in the northern hemisphere as it started to become visible.[15] The common explanation of this fact is that bright clouds are easier to identify in the dark part of the planet, whereas in the southern hemisphere the bright collar masks them.[93] Nevertheless there are differences between the clouds of each hemisphere. The northern clouds are smaller, sharper and brighter.[94] They appear to lie at a higher altitude.[94] The lifetime of clouds spans several orders of magnitude. Some small clouds live for hours, while at least one southern cloud has persisted since Voyager flyby.[15][90] Recent observation also discovered that cloud-features on Uranus have a lot in common with those on Neptune, although the weather on Uranus is much calmer.[15] The dark spots common on Neptune had never been observed on Uranus before 2006, when the first such feature was imaged.[95]
The tracking of numerous cloud features allowed determination of zonal winds blowing in the upper troposphere of Uranus.[15] At the equator winds are retrograde, which means that they blow in the reverse direction to the planetary rotation. Their speeds are from −100 to −50 m/s.[15][91] Wind speeds increase with the distance from the equator, reaching zero values near ±20° latitude, where the troposphere's temperature minimum is located.[56][15] Closer to the poles, the winds shift to a prograde direction, flowing with the planet's rotation. Windspeeds continue to increase reaching maxima at ±60° latitude before falling to zero at the poles.[15] Windspeeds at −40° latitude range from 150 to 200 m/s. Since the collar obscures all clouds below that parallel, speeds between it and the southern pole are impossible to measure.[15] In contrast, in the northern hemisphere maximum speeds as high as 240 m/s are observed near +50 degrees of latitude.[15][91][96]
For a short period from March to May 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance.[94][97] Observations included record-breaking wind speeds of 229 m/s (824 km/h) and a persistent thunderstorm referred to as "Fourth of July fireworks".[90] On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus' surface, giving astronomers more insight into the planet's atmospheric activity.[95] Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus' extreme axial tilt results in extreme seasonal variations in its weather.[46][98] Determining the nature of this seasonal variation is difficult because good data on Uranus' atmosphere has existed for less than 84 years, or one full Uranian year. A number of discoveries have however been made. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.[99] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[100] Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.[70] The majority of this variability is believed to occur due to changes in the viewing geometry.[93]
However there are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[98] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.[99] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.[98] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[98] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[92] while the northern hemisphere demonstrates increasing activity,[90] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[94]
The mechanism of physical changes is still not clear.[98] Near the summer and winter solstices, Uranus' hemispheres lie alternately either in full glare of the Sun's rays or facing deep space. The brightening of the sunlit hemisphere is thought to result from the local thickening of the methane clouds and haze layers located in the troposphere.[92] The bright collar at −45° latitude is also connected with methane clouds.[92] Other changes in the southern polar region can be explained by changes in the lower cloud layers.[92] The variation of the microwave emission from the planet is probably caused by a changes in the deep tropospheric circulation, because thick polar clouds and haze may inhibit convection.[101] Now that the spring and autumn equinoxes are arriving on Uranus, the dynamics are changing and convection can occur again.[90][101]
Many argue that the differences between the ice giants and the gas giants extend to their formation.[102][103] The Solar System is believed to have formed from a giant rotating ball of gas and dust known as the presolar nebula. As it condensed, it formed into a disc with a slowly collapsing Sun in the middle.[102][103] Much of the nebula's gas, primarily hydrogen and helium, formed the Sun, while the dust grains collected together to form the first protoplanets. As the planets grew, some of them eventually accreted enough matter for their gravity to hold onto the nebula's leftover gas.[102][103] The more gas they held onto, the larger they became; the larger they became, the more gas they held onto until a critical point was reached, and their size began to increase exponentially. The ice giants, with only a few Earth masses of nebular gas, never reached that critical point.[103][104][102] Current theories of solar system formation have difficulty accounting for the presence of Uranus and Neptune so far out from Jupiter and Saturn. They are too large to have formed from the amount of material expected at that distance. Rather, some scientists expect that both formed closer to the Sun but were scattered outward by Jupiter.[102] However, more recent simulations, which take into account planetary migration, seem to be able to form Uranus and Neptune near their present locations.[103]
Uranus has 27 known natural satellites.[104] The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope.[54][105] The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.[54] The Uranian satellite system is the least massive among the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton alone.[5] The largest of the satellites, Titania, has a radius of only 788.9 km, or less than half that of the Moon, but slightly more than Rhea, the second largest moon of Saturn, making Titania the eighth largest moon in the Solar System. The moons have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel (in green light).[14] The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include ammonia and carbon dioxide.[106][76]
Among the satellites, Ariel appears to have the youngest surface with the fewest impact craters, while Umbriel's appears oldest.[14][76] Miranda possesses fault canyons 20 kilometers deep, terraced layers, and a chaotic variation in surface ages and features.[14] Miranda's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently, probably as a result of a formerly present 3:1 orbital resonance with Umbriel.[107] Extensional processes associated with upwelling diapirs are likely the origin of the moon's 'racetrack'-like coronae.[108][109] Similarly, Ariel is believed to have once been held in a 4:1 resonance with Titania.[110]
In 1986, NASA's Voyager 2 visited Uranus. This visit is the only attempt to investigate the planet from a short distance and no other visits are currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers of the planet's cloud tops, before continuing its journey to Neptune. Voyager 2 studied structure and chemical composition of the atmosphere,[62] discovered 10 new moons and studied the planet's unique weather, caused by its axial tilt of 97.77°; and examined its ring system.[111][14] It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus' sideways orientation.[84] It made the first detailed investigations of its five largest moons, and studied all nine of the system's known rings, discovering two new ones.[76][14]
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