Uranus, Named After the Father of the Roman God Saturn, Is the Seventh Planet in the Solar

Uranus Uranus, named after the father of the Roman god Saturn, is the seventh planet in the Solar System and third of the gas giants. It is the third largest planet by diameter, yet fourth most massive. Uranus has the distinction of being the first planet discovered in modern history. Actually, its discovery as a planet almost did not happen. In 1781, the astronomer William Herschel was charting the stars found in the Gemini constellation when he observed a disk-like object. His initial conclusion was that he had discovered a comet and reported his findings as such to the Royal Society of England. However, Herschel was puzzled when he calculated the object's orbit. Instead of the more elliptical path occurring with comets, he found that it was much more circular. This observation, which was confirmed by other astronomers at the time, led Herschel to conclude that he had, in fact, discovered a new planet. Shortly thereafter, it was widely accepted that Herschel had discovered an unknown planet. As a result of his discovery, Herschel was given the privilege of naming the new planet. The name he chose was Georgium Sidus, which is Latin for Georgian Planet. He opted for this name to honor then king of England, George III. This name, however, was not widely accepted, and as a result others began to suggest names. The name Uranus was put forth in the tradition of naming planets after deities in Roman mythology. Over time the scientific community accepted this as the planet's name. At present, the only planetary mission to visit Uranus is Voyager 2. This lone encounter, which occurred in 1986, provided a large amount of data and discoveries. The spacecraft took thousands of pictures of Uranus and its moons and rings. Although the images of the planet showed little other than the uniform blue-green color seen from Earth-based telescopes, other images revealed the presence of ten previously unknown moons and two new rings. At this time, no future missions are scheduled for Uranus. ATMOSPHERE Due to its stark blue appearance, the Uranian atmospheric patterns have been much more difficult to observe than, say, those of Jupiter or even Saturn. Fortunately, the Hubble Space Telescope has provided much more insight into the structural nature of Uranus' atmosphere. Through more advanced imaging technologies than Earth-based telescopes or Voyager 2, Hubble has shown that there are latitudinal bands much like those found on the other gas giants. Additionally, the winds associated with these bands can blow in excess of 576 km/hr. The reason behind the monotonous atmospheric appearance is the composition of the top-most layer of the atmosphere. The visible cloud layers are composed primarily of methane, which absorbs those visible wavelengths corresponding to the color red. Thus, the reflected wavelengths are those of blue and green.

Beneath this outer methane layer, the atmosphere is composed of roughly 83% hydrogen (H2) and 15% helium with trace amounts of methane and acetylene. This composition is similar to that of the other gas giants. Uranus' atmosphere is drastically different in another regard, though. Whereas Jupiter and Saturn's atmospheres are primarily gaseous, Uranus' contains much more ice. This indicates that the Uranian atmosphere is extremely cold. In fact, at approximately -224° C, its atmosphere is the coldest found in the Solar System. What is even more interesting is data indicates that this extreme temperature is constant globally, occurring even on the side that is not sunlit.

INTERIOR Uranus' interior is thought to consist of two layers: a core and mantle. Current models suggest that the core is primarily composed of rock and ice and is approximately .55 times the mass of the Earth. The planet's mantle is believed to be 8.01 x 1024 kg, or about 13.4 times the mass of the Earth. Furthermore, the mantle is composed of water, ammonia and other volatile elements. What distinguishes Uranus' mantle from those of Jupiter and Saturn is that it is icy, though not in the traditional sense. Instead, the ice is very hot and thick. The mantle is 5,111 km thick. What is most surprising about Uranus' interior and one of the most distniguishing features with respect to the other gas giants is that it does not emit more energy than it receives from the Sun. Considering that even Neptune, which is very similar in size to Uranus, produces approximately 2.6 times the amount of heat that it receives from the Sun, scientists are very intrigued by the low heat that Uranus generates. There are two popular theories for this phenomenon. The first says that Uranus was struck by a large body, dispersing into space most of the heat that planets normally retain from their formations. The second theory claims that there is some barrier preventing the internal heat from making its way to the planet's surface.

ORBIT & ROTATION When Uranus was discovered it expanded the radius of the known Solar System by almost a factor of two. What this means is that, on average, Uranus' orbit is about 2.87 x 109 km. The consequence of such an enormous distance is that it takes sunlight around two hours and forty minutes to reach Uranus—that is almost twenty times as long as it takes sunlight to reach the Earth! This huge distance also means that a year on Uranus lasts almost 84 Earth years! At 0.0473, Uranus' orbital eccentricity is just slightly less than that of Jupiter's .0484, making it the fourth most circular orbit of all the planets. The result of Uranus' fairly small orbital eccentricity is that the difference between its perihelion of 2.74 x 109 km and aphelion of 3.01 x 109 km is just 2.71 x 108 km. Perhaps the most interesting thing about Uranus is how odd its rotation is compared to all of the other planets'. The axis of rotation for every planet other than Uranus is roughly perpendicular with their orbital plane. However, Uranus' axis is tilted almost 98°, which effectively means that Uranus rotates on its side. The result of this is that Uranus' North Pole points at the Sun for half of its year, while the South Pole points at the Sun the other half of its year! In other words, it is daytime on one Uranian hemispheres, while it is night time on the other for 42 Earth years at a time. Furthermore, due to this extreme rotation, Uranus does not have days like on other planets—that is, the Sun doesn't rise and set like on other planets. The cause for this highly unusual axial tilt is theorized to be the effect of a large body striking Uranus with such force that it essentially knocked the planet over on its side.

RINGS Although Saturn's rings have been well known for some time, it wasn't until 1977 that the planetary rings surrounding Uranus were discovered. The reason behind this is twofold: their distance from the Earth and their low reflectivity of light. Nonetheless, the Voyager 2 spacecraft identitfied two more on its fly-by mission in 1986, followed by the Hubble Space Telescope discovery of two additional rings in 2005. The total number of known rings currently sits at thirteen, the largest and brightest of which is the epsilon ring. Uranus' rings differ from those found around Saturn in both the size of its particles and the particles' composition. First, the particles making up Saturn's rings are small, with few larger than several meters in diameter, whereas Uranus' rings contain many bodies up to twenty meters in diamter. Second, the particles of Saturn's rings are largely composed of ice. Uranus', however, are composed of both ice and considerable dust and debris. Uranus Facts

Moons of Uranus

Presently, Uranus is known to have 27 moons. Usually, moons are named after mythological figures; however, Uranian moons are named after characters taken from the literary works of either William Shakespeare or Alexander Pope. Although some of Uranus' moons were discovered shortly after the planet's discovery, most were unknown until the mid-twentieth century. The classification of Uranus' moons can be broken down into three groups: the inner moons, the large moons, and the irregular moons. The thirteen inner moons are satellites that, as their names suggest, orbit more closely to Uranus than the others. The official outer boundary for the inner moons is the orbit of the large moon Miranda. The inner moons can be characterized as small (diameter not exceeding 160 km), composed of half water ice and half rock and reflecting very little light. Also, the orbits of these moons are so highly disordered that scientists do not currently know how they avoid colliding with one another. The group of large moons consists of Miranda, Ariel, Umbriel, Titania and Oberon. The largest of these, Titania, has a diameter of 1,578 km, which is about half of the diameter of Earth's moon. These moons are similar in character. All are about half rock and half ice, except for the smallest of these, Miranda, which is predominantly ice. The irregular moons, which orbit at a considerably farther distance than the inner and larger moons, have highly eccentric and retrograde orbits. These moons are thought to be captured asteroids. The first two moons to be discovered, Titania and Oberon, were spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen. In the 1840s, better instruments and a more favorable position of Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next two moons, Ariel and Umbriel, were discovered by William Lassell in 1851.The Roman numbering scheme of Uranus's moons was in a state of flux for a considerable time, and publications hesitated between Herschel's designations (where Titania and Oberon are Uranus II and IV) and William Lassell's (where they are sometimes I and II). With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck. In 1852, Herschel's son John Herschel gave the four then-known moons their names. No other discoveries were made for almost another century. In 1948, Gerard Kuiper at the McDonald Observatory discovered the smallest and the last of the five large, spherical moons, Miranda. Decades later, the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons. Another satellite, Perdita, was retroactively discovered in 1999 after studying old Voyager photographs. Uranus was the last giant planet without any known irregular satellites, but since 1997 nine distant irregular moons have been identified using ground-based telescopes. Two more small inner moons, Cupid and Mab, were discovered using the Hubble Space Telescope in 2003. The moon Margaret was the last Uranian moon discovered as of 2012, and its details were published in October 2003.

SPURIOUS MOONS

After Herschel discovered Titania and Oberon on January 11, 1787, he subsequently believed that he observed four other moons; two on January 18 and February 9, 1790, and two more on February 28 and March 26, 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites, though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in which he discovered Ariel and Umbriel, however, failed to support Herschel's observations; Ariel and Umbriel, which Herschel certainly ought to have seen if he had seen any satellites beside Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. Herschel's four spurious satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania and Oberon), 38.08 days, and 107.69 days (exterior to Oberon). It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of faint stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.

Tables of Moons

The Uranian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light blue and bolded. Irregular moons with prograde orbits are shown in light grey, those with retrograde orbits in dark grey. Key ‡ ♠ Major Retrograde moons moons Uranian moons Semi- Orde Mass Orbital Inclinatio Eccentricit Discover Label Pronunciatio Diameter major Discoverer r Name Image (×1018 period n y y [e] n (km)[f] axis [2] [d] (key) kg)[g] (d)[35][h] (°)[35] [36] year [2] (km)[35] 40 ± 6 Terrile 1 VI Cordelia 0.044 49,770 0.335034 0.08479° 0.00026 1986 kɔrˈdiːliə (50 × 36) (Voyager 2) 43 ± 8 Terrile 2 VII Ophelia 0.053 53,790 0.376400 0.1036° 0.00992 1986 ɵˈfiːliə (54 × 38) (Voyager 2) 51 ± 4 Smith 3 VIII Bianca 0.092 59,170 0.434579 0.193° 0.00092 1986 biːˈɒŋkə (64 × 46) (Voyager 2) 80 ± 4 Synnott 4 IX Cressida 0.34 61,780 0.463570 0.006° 0.00036 1986 ˈkrɛsɨdə (92 × 74) (Voyager 2) Desdemon ˌdɛzdɨ 64 ± 8 Synnott 5 X 0.18 62,680 0.473650 0.11125° 0.00013 1986 a ˈmoʊnə (90 × 54) (Voyager 2) 94 ± 8 Synnott 6 XI Juliet 0.56 64,350 0.493065 0.065° 0.00066 1986 ˈdʒuːli.ɨt (150 × 74) (Voyager 2) 135 ± 8 Synnott 7 XII Portia 1.70 66,090 0.513196 0.059° 0.00005 1986 ˈpɔrʃə (156 × 126) (Voyager 2) Synnott 8 XIII Rosalind 72 ± 12 0.25 69,940 0.558460 0.279° 0.00011 1986 ˈrɒzəlɨnd (Voyager 2) Showalter 9 XXVI Cupid ˈkjuːpɨd ~18 0.003 74,800 0.618 0.1° 0.0013 2003 and I 8 Lissauer

90 ± 16 Synnott 10 XIV Belinda 0.49 75,260 0.623527 0.031° 0.00007 1986 bɨˈlɪndə (128 × 64) (Voyager 2)

Karkoschak 11 XXV Perdita ˈpɜrdɨtə 30 ± 6 0.018 76,400 0.638 0.0° 0.0012 1999 a (Voyager 2)

Synnott 12 XV Puck 162 ± 4 2.90 86,010 0.761833 0.3192° 0.00012 1985 ˈpʌk (Voyager 2)

Showalter 13 XXVI Mab ˈmæb ~25 0.01 97,700 0.923 0.1335° 0.0025 2003 and Lissauer 471.6 ± 1.4 65.9 ± 14 V ‡Miranda (481 × 468 129,390 1.413479 4.232° 0.0013 1948 Kuiper mɨˈrændə 7.5 × 466) 1,157.8 ± 1.2 1,353 15 I ‡Ariel (1162 × 191,020 2.520379 0.260° 0.0012 1851 Lassell ˈɛəriəl ± 120 1156 × 1155) Key ‡ ♠ Major Retrograde moons moons

1,169.4 ± 1,172 16 II ‡Umbriel 266,300 4.144177 0.205° 0.0039 1851 Lassell ˈʌmbriəl 5.6 ± 135

3,527 17 III ‡Titania 1,576.8 ± 1. 435,910 8.705872 0.340° 0.0011 1787 Herschel tɨˈtɑːnjə ± 90 2

3,014 18 IV ‡Oberon 1,522.8 ± 5. 583,520 13.46323 0.058° 0.0014 1787 Herschel ˈoʊbərɒn ± 75 2 9

Holman et 19 XXII ♠Francisco ~22 0.007 4,276,000 −266.56 147.459° 0.1459 2003[i] frænˈsɪskoʊ al. 2 Gladman et 20 XVI ♠Caliban ~72 0.25 7,230,000 −579.50 139.885° 0.1587 1997 ˈkælɨbæn al. Gladman et 21 XX ♠Stephano ~32 0.022 8,002,000 −676.50 141.873° 0.2292 1999 ˈstɛfənoʊ al. Holman et 22 XXI ♠Trinculo ~18 0.003 8,571,000 −758.10 166.252° 0.2200 2001 ˈtrɪŋkjʊloʊ al. 9 12,179,00 Nicholson 23 XVII ♠Sycorax ~150 2.30 −1283.4 152.456° 0.5224 1997 ˈsɪkəræks 0 et al. Sheppard 14,345,00 24 XXIII Margaret ~20 0.005 1694.8 51.455° 0.6608 2003 and ˈmɑrɡərɨt 0 4 Jewitt 16,418,00 Holman et 25 XVIII ♠Prospero ~50 0.085 −1992.8 146.017° 0.4448 1999 ˈprɒspəroʊ 0 al. 17,459,00 Kavelaars 26 XIX ♠Setebos ~48 0.075 −2202.3 145.883° 0.5914 1999 ˈsɛtɨbʌs 0 et al. 20,900,00 Holman et 27 XXIV ♠Ferdinand ~20 0.005 −2823.4 167.346° 0.3682 2003[i] ˈfɜrdɨnænd 0 al. 4

Exploration of Uranus

The exploration of Uranus has, to date, been solely through telescopes and NASA's Voyager 2 spacecraft, which made its closest approach to Uranus on January 24, 1986. Voyager 2 discovered 10 previously unknown moons, studied the planet's cold atmosphere, and examined its ring system, discovering two new rings. It also imaged Uranus' five large moons, revealing that their surfaces are covered with impact craters and canyons. A number of dedicated exploratory missions to Uranus have been proposed, but as of 2012 none have been formally approved.

Voyager 2

Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers (50,600 mi) of the planet's cloud tops. This was the probe's first solo planetary flyby, since Voyager 1 ended its tour of the outer planets at Saturn's moon Titan. Uranus is the third largest planet in the Solar System. It orbits the Sun at a distance of about 2.8 billion kilometers (1.7 billion miles) and completes one orbit every 84 years. The length of a day on Uranus as measured by Voyager 2 is 17 hours, 14 minutes. Uranus is distinguished by the fact that it is tipped on its side. Its unusual position is thought to be the result of a collision with a planet-sized body early in the Solar System's history. Given its odd orientation, with its polar regions exposed to sunlight or darkness for long periods, scientists were not sure what to expect at Uranus. The presence of a magnetic field at Uranus was not known until Voyager's arrival. The intensity of the field is roughly comparable to that of Earth's, though it varies much more from point to point because of its large offset from the center of Uranus. The peculiar orientation of the magnetic field suggests that the field is generated at an intermediate depth in the interior where the pressure is high enough for water to become electrically conducting. Voyager 2 found that one of the most striking influences of the sideways position of the planet is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet. Radiation belts at Uranus were found to be of an intensity similar to those at Saturn. The intensity of radiation within the belts is such that irradiation would quickly darken (within 100,000 years) any methane trapped in the icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the moons and ring particles, which are almost uniformly gray in color. A high layer of haze was detected around the sunlit pole, which also was found to radiate large amounts of ultraviolet light, a phenomenon dubbed "electroglow". The average temperature of the atmosphere of the planet is about 59 kelvins. Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature at the cloud tops. Voyager found 10 new moons, bringing the total number to 15 at the time. Most of the new moons are small, with the largest measuring about 150 kilometers (about 90 miles) in diameter. The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet seen in the Solar System. Detailed images from Voyager's flyby of the moon showed huge oval structures termed coronae flanked by faults as deep as 20 kilometers (12 mi), terraced layers, and a mixture of old and young surfaces. One theory holds that Miranda may be a reaggregation of material from an earlier time when the moon was fractured by a violent impact. The five large moons appear to be ice-rock conglomerates like the satellites of Saturn. Titania is marked by huge fault systems and canyons indicating some degree of geologic, probably tectonic, activity in its history. Ariel has the brightest and possibly youngest surface of all the Uranian moons and also appears to have undergone geologic activity that led to many fault valleys and what seem to be extensive flows of icy material. Little geologic activity has occurred on Umbriel or Oberon, judging by their old and dark surfaces. All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the same time as Uranus. Particles that make up the rings may be remnants of a moon that was broken by a high- velocity impact or torn up by gravitational effects. Voyager 2 also discovered two new rings.

Proposed Missions

A number of missions to Uranus have been proposed. Scientists from the Mullard Space Science Laboratory in the United Kingdom have proposed the joint NASA–ESA Uranus Pathfinder mission to Uranus. A call for a medium-class (M-class) mission to the planet to be launched in 2022 was submitted to the ESA in December 2010 with the signatures of 120 scientists from across the globe. The ESA caps the cost of M-class missions at €470 million. Another mission to Uranus, dubbed HORUS, was designed by the Applied Physics Laboratory of Johns Hopkins University. The proposal is for a nuclear-powered orbiter carrying a set of instruments, including an imaging camera, spectrometers and a magnetometer. The mission would launch in April 2021, arriving at Uranus 17 years later. The minimum duration of the HORUS mission is two years. In 2009, a team of planetary scientists from NASA's Jet Propulsion Laboratory advanced possible designs for a solar-powered Uranus orbiter. The most favorable launch window for such a probe would be in August 2018, with arrival at Uranus in September 2030. The science package may include magnetometers, particle detectors and, possibly, an imaging camera. In the 2011 decadal survey for the future potential of planetary exploration, the United States National Research Council recommended a Uranus orbiter and probe. However, this mission is considered to be lower- priority than future missions to Mars and the Jovian system.