Saturday, May 10, 2008

SIDEBAR: Sir William Herschel

SIDEBAR: William Herschel - The Man Who Discovered Uranus

Musician Born
William Herschel, one of history's greatest astronomers, was born at Hanover, on the 15th of November, 1738. His father, Isaac Herschel, was an accomplished musician and earned an adequate but not wealthy living. While he left few worldly goods to his children, but he more than compensated for this with a splendid inheritance of genius. This was especially true in the case of his fourth child, William, and of a sister, Caroline, several years younger. Later in life, Caroline proved to be a faithful chronicler who recorded many useful details of her brother's life.

This modest family circle dispersed at the outbreak of the Seven Years' War in 1756. The French invaded Hanover, then part of the British dominions. Young William Herschel had already became a regular performer in the regimental band of the Hanoverian Guards, and he abruptly experienced actual warfare in the battle of Hastenbeck. After this disastrous battle, he hid in a ditch throughout the night to consider his traumatic experiences; he concluded that soldiering was not for him. Thus, he deserted his unit and carefully made his way to England. Years later, he was formerly pardoned for this offence. Many years later, Herschel had become the famous astronomer and visited King George at Windsor, the King personally gave his pardon written out his Majesty himself.
During his first few years in England, the young musician had some difficulty in providing for his maintenance . Eventually, at age twenty-two, he became Instructor of Music to the Durham Militia. Shortly afterwards, his talents being more widely recognized, he was appointed as organist at the parish church at Halifax. With much better prospects and the Seven Years' War being over, he ventured to Hanover to see his father. We can imagine the delight with which old Isaac Herschel welcomed his promising son, as well as his parental pride when a concert was given at which some of William's compositions were performed. If the father were so intensely gratified on this occasion, what would his feelings have been could he have lived to witness his son's future career? Unfortunately, Isaac Herschel died many years before his son became an astronomer.

In 1766, Herschel was further promoted to organist in the Octagon Chapel, at Bath. Bath was then, as now, a highly fashionable resort, and many notable personages patronized the rising musician. Herschel had other points in his favor besides his professional skill; his appearance was good, his address was prepossessing, and even his nationality was a distinct advantage, inasmuch as he was a Hanoverian in the reign of King George the Third. On Sundays he played the organ, to the great delight of the congregation, & on week-days he was occupied by giving lessons to private pupils, and in preparation for public performances. He thus came to be busily employed with comfortable means.

Discovered Astronomy
From his earliest youth, Herschel had an unquenchable thirst for knowledge. In studying the theory of music, he was led to mathematics. This led to vast regions of knowledge and eventually to astronomy. More and more this pursuit seems to have engrossed his attention, until at last it had become an absorbing passion, and every spare moment was spent on astronomy.
Herschel started with a small, borrowed telescope; however, he quickly realized he needed a telescope of far greater power, and he determined to make it with his own hands.
Instead of a refracting telescopes with which we are ordinarily familiar, he opted to build a reflector. Reflector's optical power is obtained from a mirror at the bottom of the tube, and the astronomer looks down through the tube TOWARDS HIS MIRROR and thus views the reflection of the stars. The surface has to be hollowed out a little with extreme accuracy; the slightest deviation would prevent efficient performance of the telescope.
Herschel composed a mirror with an alloy mixture of two parts of copper to one part tin; this resulted in an intensely hard material, very difficult to cast but polishes to an extremely fine refecting surface. Herschel actually made a good living making and selling these telescopes. The metallic reflecting mirror has the advantage that, with reasonable care, surface stays bright and untarnished for a much longer period.

In 1774, William first glimpsed the stars with his own instrument. Thereafter, his telescopes came out nightly, sometimes into the small back garden of his house at Bath, and sometimes into the street in front of his hall-door.

Caroline Herschel - Invaluable Assistant

William was blessed with an extremely capable assistant, Caroline Herschel, William's remarkable sister. Whatever work had to be done she was willing to bear her share in it, or even to toil at it unassisted if possible. She not only managed all his domestic affairs, but gladly assisted in the grinding of the lenses and in the polishing of the mirrors. One polishing stage required the workman's hand on the mirror for many long hours. During these labors, Caroline used to sit by her brother, and enliven the time by reading stories aloud, sometimes pausing to feed him with a spoon while his hands were engaged on the task from which he could not desist for a moment.
When Herschel was at the telescope at night, Caroline sat by him at her desk, pen in hand, ready to write down the notes of the observations as they fell from her brother's lips. The telescope was, of course, in the open air, and as Herschel not infrequently continued his observations throughout the whole of a long winter's night, there were but few women who could have accomplished the task which Caroline so cheerfully executed. From dusk till dawn, when the sky was clear, were Herschel's observing hours, and what this sometimes implied we can realize from the fact that Caroline assures us she had sometimes to desist because the ink had actually frozen in her pen. The night's work over, a brief rest was taken, and while William had his labors for the day to attend to, Caroline carefully transcribed the observations made during the night before, reduced all the figures and prepared everything in readiness for the observations that were to follow on the ensuing evening.

Discoverer of Uranus
In 1781, his greatest achievement took place. Herschel was in the middle of a proejct to closely examine all stars above a certain magnitude. In this great review, Herschel examined thousands of stars without note; however, March 13, 1781, he was pursuing his task among the stars of Gemini and noticed a "star" which possessed a disc, a definite, measurable size, totally different from any thousands of previously examined stars. After a little further observation, he further noted a definite shift of its position relative to the stars. Thus, he initially reported it (on 26 April 1781) as a "comet".[18]
When he presented his discovery to the Royal Society, he continued to assert the discovery was a comet while implicitly comparing it to a planet:[22]The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.
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]

Royal Astronomer
King George the Third, heard of these achievements and became greatly interested. Herschel accepted a royal invitation to Windsor, and he brought his famous telescope to exhibit the new planet to the King. As a result of this meeting, Herschel was able to devote himself exclusively to science for the rest of his life.
The King duly pardoned his desertion from the army, some twenty-five years previously. Further, King conferred on Herschel the title of his Majesty's own astronomer, to assign to him a residence near Windsor, to provide him with a salary, and to furnish such funds as might be required for the erection of great telescopes, and for the conduct of that mighty scheme of celestial observation on which Herschel was so eager to enter. Herschel's capacity for work was greatly enhanced by the aid of his admirable sister, and to her, therefore, the King also assigned a salary, and she was installed as Herschel's assistant in his new post. 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]
With his usual determination, Herschel immediately ceased his musical avocations and began making and erecting the great telescopes at Windsor. For more than thirty years, he and his faithful sister continued their nightly celestial scrutiny. Paper after paper was sent to the Royal Society, describing the thousands, of objects such as double stars; nebulae and clusters, which were first revealed to human gaze during those midnight vigils. To the end of his life, he pursued every possible opportunity with unparalleled success. However, his initial great feat, the discovery of Uranus, remained his most noteworthy achievement.
Herschel married when considerably advanced in life; yet, he lived to find his only son, afterwards Sir John Herschel, treading worthily in his footsteps and attaining renown as an astronomical observer, second only to that of his father. The elder Herschel died in 1822, and his illustrious sister Caroline then returned to Hanover, where she lived for many years to receive the respect and attention which were so justly hers. She died at a very advanced age in 1848.

Uranus Facts

Uranus is the seventh planet from the Sun and the third-largest and fourth-most massive planet in the solar system. Uranus was the first planet discovered in modern times. Though 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 discovered it on March 13, 1781, to expand the known boundaries of the solar system for the first time in modern history. This was also the first discovery of a planet with a telescope.

Naming Uranus
It is named after the ancient Greek deity of the sky (Uranus, Οὐρανός), the father of Kronos (Saturn) and grandfather of Zeus (Jupiter). Though Herschel discovered this planet, he did not choose this name.
Then Astronomer Royal, Nevil Maskelyne, asked Herschel to name his discovery. 29] In response, Herschel chose the name, Georgium Sidus (George's Star), in honor of his patron, King George III.[30] Astronomer Jérôme Lalande proposed the planet be named Herschel in honour of its discoverer.[31]
Johann Bode (recall Bode's Law), suggested "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 finally switched to Uranus in 1850.[32]

Naming the Moons of Uranus
These names are 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 first two moons to be discovered, Titania and Oberon, were spotted by William Herschel on March 13, 1787. Two more, Ariel and Umbriel, were discovered by William Lassell in 1851.

By 1851, Herschel's son John Herschel had became a well known astronomer; and he gave the four then-known moons their names. For a man who made his reputation by counting and plotting the locations of thousands of stars, astronomer John Herschel had a poetic heart. In 1851, he named the four largest moons of Uranus. Instead of picking characters from mythology, he turned to the works of William Shakespeare and Alexander Pope. He named Oberon and Titania for the king and queen of the fairies from Shakespeare's A Midsummer Night's Dream. Ariel and Umbriel were spirits in Pope's The Rape of the Lock.

In 1948 Gerard Kuiper discovered the fifth and last large moon, Miranda. Miranda (pronounced /mɨˈrændə/ mə-ran'-də) is the smallest and innermost of Uranus' five major moons. It was discovered by Gerard Kuiper on 1948-02-16 at McDonald Observatory. It was named after Miranda from William Shakespeare's play The Tempest by Kuiper in his report of the discovery.[1]

Orbit and rotation
Uranus revolves around the Sun once every 84 Earth years with an average radius of about 20 AU. Its orbital elements were first calculated in 1783 by Pierre-Simon Laplace.[24]; however, discrepancies were eventually observed 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; this led to the discovery of yet another planet. In 1845, Urbain Le Verrier began his own independent research into Uranus' orbit and predicted the new planet's location. On September 23, 1846, Johann Gottfried Galle located a new planet, later named Neptune, very close to the this predicted location.[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]

Axial tilt
The most obvious distinguishing feature of Uranus is its unusual axis of rotation; Uranus lies on its side with respect to the plane of the solar system, with an axial tilt of 98 degrees. Thus, its seasons are completely unlike the other major planets. While other planets rotate like tilted spinning tops, Uranus rotates more like a tilted rolling ball. During Uranian solstices, one pole faces the Sun while the other faces away. In a narrow strip around the equator, one would experiences a rapid day-night cycle with the Sun very low over the horizon (like Earth's polar regions). This polar orientation reverses at opposite sides of the orbit; thus, 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]
This unusual axis orientation causes the polar regions of Uranus to receive much more Solar energy than its equatorial regions. In spite of this, Uranus is hotter at its equator than at its poles, and we don't yet know the reason.
The cause of Uranus' strange axial tilt is also unknown, but we speculate 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.

Another suprising fact, though undiscovered til relatively recently in human history, Uranus is sometimes visible to the unaided eye. 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, it is a pale cyan disk with distinct limb darkening. With a large telescope (25 cm or wider), cloud patterns, as well as some of the larger satellites, such as Titania and Oberon, may be visible.[52]

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. 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]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]

These factors 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 rocky material.[7]
We currently classify Uranus as three layers:

  • rocky core in the center - Core is relatively small, with a mass of only 0.55 Earth masses and a radius less than 20 percent Uranus.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]
  • icy mantle in the middle - Mantle comprises the bulk of the planet, with around 13.4 Earth masses,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]
  • outer gaseous hydrogen/helium envelope.[7][54] - Upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20 percent of Uranus' radius.[7][54]

Internal Heat
Uranus' internal heat appears much lower than the other giant planets; in astronomical terms, it has a low thermal flux.[56][15] We still don't know why Uranus' internal temperature is so low. Compare to Uranus' near twin in size and composition, Neptune, which radiates 2.61 times as much energy into space as it receives from the Sun.[15] In contrast, 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.[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]
Possible reasons for low temperatures in Uranus:

  • Impactor: Perhaps Uranus was "knocked over" by a supermassive impactor to cause its extreme axial tilt; furthermore, perhaps this event also caused it to expel most of its primordial heat and leave a depleted core temperature.[58]
  • Barrier: Perhaps something in Uranus's upper layers blocks 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]

There is no well-defined solid surface within Uranus's interior; however, we define its atmosphere.[8] as the outermost part of Uranus' gaseous envelope accessible to remote sensing. Uranus atmosphere has a different composition from inner Uranus; it consists 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 the hydrocarbon, 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]
race amounts of various other hydrocarbons are in the upper atmosphere of Uranus; these 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 Uranus atmosphere can be divided into three layers:

  1. 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] )
  2. Stratosphere. 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 that form in this part of the atmosphere as a result of methane photolysis.[63][65] Heating from the hot thermosphere may also be significant.[70][71] 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] The most abundant hydrocarbons are acetylene and ethane with mixing ratios of around 10−7 relative to hydrogen, which is similar to the mixing ratios of methane and carbon monoxide 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][70]
  3. Thermosphere and corona has a uniform temperature around 800 to 850 K.[8][70] 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][70] 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][70] This extended corona is a unique feature of Uranus.[70] 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.[70][72] The ionosphere is mainly sustained by solar UV radiation and its density depends on the solar activity.[73] Auroral activity is not as significant as at Jupiter and Saturn.[70][74]

Planetary rings

Uranus has a faint planetary ring system, the second ring system to be discovered in the Solar System after Saturn's.[75] William Herschel claimed to have seen rings at Uranus in 1789 (see below); however, noone else noted them until March 10, 1977 when serendipitously discovered on by scientists using the Kuiper Airborne Observatory for a routine study of Uranus's atmosphere. They observed the occultation of the star SAO 158687 behind Uranus; however, they analyzed their data and found the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet.[77] These rings were directly imaged by Voyager 2 in 1986.[14] to discover two more faint rings.
In December 2005, the Hubble Space Telescope detected two new rings. The largest is twice the distance from the planet of the previously known rings. Thus, these new rings are now called the "outer" ring system. Same image also shows two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. This brings the total number of Uranian rings to 13.[78]
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.[79][80] 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.[79][81] The planet's inner rings appear grey.[79] William Herschel's observations of Uranian rings in the 18th century have been largely discounted; however, these current events do seem to bear out them out. The first mention of a Uranian ring system comes from Herschel's notes detailing observations of Uranus: "February 22, 1789: A ring was suspected".[82] He 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.[79] Herschel's notes were published in a Royal Society journal in 1797. Since there have been no further observations of Uranian rings between 1797 and 1977, many have wondered how Herschel could have seen anything of the sort while hundreds of other astronomers saw nothing. Still, Herschel's descriptions now seem remarkably accurate. (These include: ring's size relative to Uranus, its changes as Uranus travelled around the Sun, and its colour.[83])

Magnetic Field
The magnetic field of Uranus as seen by Voyager 2, in 1986. S and N are magnetic south and north poles. 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 show the magnetic field to be 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]
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 100 electronvolts) 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.[70] However, unlike Jupiter's, Uranus' aurorae seem to be insignificant for the energy balance of the planetary thermosphere.[74]

Uranus's atmospheric dynamics are remarkably bland when compared 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 49o K, making Uranus the coldest planet in the Solar System, colder than Neptune which is a full 10 AUs more distant from Sol.[57][8]

Banded structure, winds and clouds
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.[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 first dark spot observed on Uranus. Image obtained by ACS on HST in 2006.
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]

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.[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.[71] 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]

Uranus has 27 known
natural satellites.[104] 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]

Primary Source: Uranus - Wikipedia, the free encyclopedia


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