Everything about Uranus totally explained
Uranus (or ) 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, ), the father of
Kronos (
Saturn) and grandfather of
Zeus (
Jupiter). Uranus was the first planet discovered in
modern times. Though it's visible to the naked eye like the five
classical planets, it was never recognized as a planet by ancient observers due to its dimness. 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 and
Neptune have internal and
atmospheric compositions different 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. 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), but initially reported it (on
26 April 1781) as a "
comet". Herschel "engaged in a series of observations on the parallax of the fixed stars", 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". On
March 17, he noted, "I looked for the Comet or Nebulous Star and found that it's a Comet, for it has changed its place". When he presented his discovery to the
Royal Society, he continued to assert that he'd found a comet while also implicitly comparing it to a planet:
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've not yet seen any coma or tail to it".
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. 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". Bode concluded that its near-circular orbit was more like a planet than a comet.
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'd the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System." 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.
Naming
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're so much obliged to you for the discovery of." 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. He explained this decision in a letter to Joseph Banks: 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. The earliest citation of the name Uranus in an official publication is in 1823, a year after Herschel's death. 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 didn't switch to
Uranus until 1850. this is more classically correct than the alternate
[jʊˈɹeɪ.nəs], with stress on the second syllable and a "long a" (ūr
ānŭs),
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.
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 dominated by the combined powers of the Sun and Mars. 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").
Orbit and rotation
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. Its orbital elements were first calculated in 1783 by
Pierre-Simon Laplace.
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.
Axial tilt
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. 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.
| Northern hemisphere |
Year |
Southern hemisphere |
| Winter solstice |
1902, 1986 |
Summer solstice |
| Vernal equinox |
1923, 2007 |
Autumnal equinox |
| Summer solstice |
1944, 2028 |
Winter solstice |
| Autumnal equinox |
1965, 2049 |
Vernal equinox |
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. 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. 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. 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!"
Visibility
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. 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. 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.
Physical characteristics
Internal structure
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. Though having a diameter slightly larger than Neptune (roughly four times Earth's), it's less massive. The total mass of ice in Uranus' interior isn't precisely known, as different figures emerge depending on the model chosen; however, 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 ice mantle isn't in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water,
ammonia and other
volatiles. 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 isn't 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 doesn't allow us to determine which model is correct. This surface will be used throughout this article as a zero point for
altitudes.
Internal heat
Uranus'
internal heat appears markedly lower than that of the other giant planets; in astronomical terms, it has a low
thermal flux. Another hypothesis is that some form of barrier exists in Uranus' upper layers which prevents the core's heat from reaching the surface. The tenuous
corona of the atmosphere extends remarkably over two planetary radii from the nominal surface at 1 bar pressure. There is no
mesosphere.
Composition
The composition of the Uranian atmosphere is different from the composition of Uranus as a whole, consisting as it does mainly of
molecular hydrogen and
helium. in the upper troposphere, which corresponds to a mass fraction . indicating that helium hasn't settled in the center of the planet as it has in the gas giants. The abundances of less volatile compounds such as
ammonia,
water and
hydrogen sulfide in the deep atmosphere are poorly known. However they're probably also higher than solar values. 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.
Troposphere
The troposphere is the lowest and densest part of the atmosphere and is characterized by a decrease in temperature with altitude. 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. The tropopause region is responsible for the vast majority of the planet’s thermal
far infrared emissions, thus determining its
effective temperature of .
The troposphere is believed to possess a highly complex cloud structure;
water clouds are hypothesised to lie in the pressure range of (5 to 10 MPa),
ammonium hydrosulfide clouds in the range of (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 (0.1 to 0.2 MPa). The troposphere is a very dynamic part of the atmosphere, exhibiting strong winds, bright clouds and seasonal changes, which will be discussed below.
Upper atmosphere
The middle layer of the Uranian atmosphere is the
stratosphere, where temperature generally increases with altitude from 53 K in the
tropopause to between 800 and 850 K at the base of the
thermosphere. The heating of the stratosphere is caused by absorption of solar
UV and
IR radiation by
methane and other
hydrocarbons, Heat is also conducted from the hot thermosphere. 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 m
bar (1000 to 10 kPa) and temperatures of between 75 and 170 K.
The outermost layer of the Uranian atmosphere is the thermosphere and
corona, which has a uniform temperature around 800 to 850 K. The ionosphere is mainly sustained by solar UV radiation and its density depends on the
solar activity.
Auroral activity is insignificant as compared to Jupiter and Saturn.
Planetary rings
Uranus has a faint
planetary ring system, composed of dark particulate matter up to ten meters in diameter. The rings were directly imaged when
Voyager 2 passed Uranus in 1986. In April 2006, images of the new rings with the
Keck Observatory yielded the colours of the outer rings: the outermost is blue and the other red.
One hypothesis concerning the outer ring's blue colour is that it's composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light. The planet's inner rings appear grey.
Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case.
Magnetic field
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. Neptune has a similarly displaced and tilted magnetic field, suggesting that this may be a common feature of ice giants.
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. Overall, the structure of the magnetosphere of Uranus is different from that of
Jupiter's and more similar to that of Saturn's.
Uranus' magnetosphere contains
charged particles:
protons and
electrons with small amount of
ions. The particle population is strongly affected by the Uranian moons that sweep through the magnetosphere leaving noticeable gaps. Uranus has relatively well developed
aurorae, which are seen as bright arcs around both magnetic poles. 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. 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).
Seasonal variation
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. Observations included record-breaking wind speeds of 229 m/s (824 km/h) and a persistent thunderstorm referred to as "Fourth of July fireworks". Why this sudden upsurge in activity should be occurring isn't fully known, but it appears that Uranus' extreme
axial tilt results in extreme
seasonal variations in its weather. A similar periodic variation, with maxima at the solstices, has been noted in
microwave measurements of the deep troposphere begun in the 1960s.
Stratospheric temperature measurements beginning in 1970s also showed maximum values near 1986 solstice.
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. During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole wasn't always so dim. while the northern hemisphere demonstrates increasing activity,
Formation
Many argue that the differences between the ice giants and the gas giants extend to their formation. 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. However, more recent simulations, which take into account
planetary migration, seem to be able to form Uranus and Neptune near their present locations. The five main satellites are
Miranda,
Ariel,
Umbriel,
Titania and
Oberon. The moons are ice-rock conglomerates composed of roughly fifty percent ice and fifty percent rock. The ice may include
ammonia and
carbon dioxide.
Extensional processes associated with upwelling
diapirs are likely the origin of the moon's 'racetrack'-like
coronae. Similarly, Ariel is believed to have once been held in a 4:1 resonance with Titania.
Exploration
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,
[ It also studied the magnetic field, its irregular structure, its tilt and its unique corkscrew magnetotail brought on by Uranus' sideways orientation.][ 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.][Further Information]
Get more info on 'Uranus'.
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