Uranus

Discovery

Observations from the Earth

Results from the flyby of the Voyager 2 spacecraft

Uranus’s magnetic field

Uranus’ rotation

Atmospheric temperature

Uranus’ atmosphere

Uranus’s internal structure

Uranus’ rings

Uranian satellites

Observations of Miranda and other satellites

Oberon

Titania

Umbriel

Ariel

Miranda

Puck

Other natural satellites

Resources

Uranus is the seventh planet from the sun. It has a large size (its diameter is almost four times that of Earth) and mass, low mean density, fairly rapid rotation, and well-developed ring (at least 12 components) and natural satellite (at least 28 members) systems. The planet has a strong magnetic field with a large tilt (58.6°) to its rotation axis and offset (0.3 Uranus radius) from its center. Analysis of the observations made by Voyager 2 during its flyby of Neptune in August 1989 shows that Uranus and Neptune are similar in most of these properties and form a subgroup of the Jovian planets. Jupiter and Saturn, which are much larger and more massive, form the other subgroup. More information has been discovered

since the Voyager 2 mission, such as from the Hubble Space Telescope.

Discovery

german-born English astronomer Sir Frederick William Herschel (1738–1822) fortuitously discovered Uranus in 1781; it was the first planet discovered telescopically. It was found to orbit the sun at a mean distance of about 19.2 astronomical units (AU) (2,870,000,000 km), about twice as far from the sun as Saturn (9.54 AU), the most distant planet known before 1781.

Observations from the Earth

Knowledge about Uranus came slowly because of its distance. Even when it is closest, Uranus shows a disk of only 4 in (10 cm) in apparent diameter through a telescope and is 5.7^m apparent magnitude (barely visible to the unaided eye even in the best observing conditions). Herschel discovered Oberon and Titania, the outermost and largest, respectively, satellites of Uranus in 1787. Determination of their orbits around Uranus from observations gave their periods of revolution P and mean distances A from Uranus. This allowed one to determine Uranus’ mass from the general form of Kepler’s third law; it turned out to be 14.5 Earth masses. Using its radius of 15,873 mi (25,560 km), one calculated Uranus’ mean density (its mass divided by its volume), which is 1.27 grams/cm³. This indicated that Uranus is a smaller type of Jovian planet similar to Jupiter and Saturn; they are characterized by large masses and sizes and low mean densities (compared to the Earth), and are inferred to consist largely of gases.

The planes of the orbits of Oberon and Titania were expected to lie in or near the plane of Uranus’ equator, since most other planetary satellites have orbital planes that are in or near the equatorial planes of their planets. When the orbital planes of Oberon and Titania were determined, however, they indicated that the plane of Uranus’ equator is almost perpendicular to the plane of its orbit around the sun (and also to the ecliptic). This is unlike the other planets, whose

equatorial planes are tilted by at most 30° to the planes of their orbits around the sun. This implied that Uranus’ axis (and poles) of rotation lie almost in its orbital plane. This conclusion has been confirmed by observations of Uranus’ rotation from the Voyager 2 spacecraft in 1986. This is the first of several interesting characteristics astronomers have discovered for Uranus, and gives Uranus interesting seasons during its year (period of revolution around the sun), which is 84.1 Earth-years long. (The Uranian seasons will be discussed in more detail below.) The cause of this unusual orientation of Uranus’ rotation axis is still unknown and is now the subject of considerable speculation and theoretical research. One theory is that the orientation of its rotation axis was produced by the collision of an Earth-sized body with Uranus near the end of its formation.

Unexplained perturbations of Uranus’ orbit in the early nineteenth century led to the prediction of the existence of a still more distant large planet, resulting in the discovery of Neptune in 1846. Neptune is the most distant (30.06 AU mean distance from the sun) Jovian planet, and it has several properties (mass, size, rotation period, rings, and magnetic field) like those of Uranus.

Three more satellites of Uranus, closer to it than Titania, were discovered during the 105 years after Neptune’s discovery. They are, in order of closeness to Uranus, Umbriel and Ariel, discovered in 1851 by Lassell (1799–1880), and Miranda, discovered in 1948 by G. P. Kuiper (1905–1973). These discoveries showed that Uranus has a satellite system comparable to those of Jupiter and Saturn, although Titania, its largest (980 mi [1,580 km] diameter) and most massive satellite, and the slightly smaller Oberon, are comparable in size and mass to Saturn’s satellites Iapetus and Rhea rather than to its much larger satellite Titan and Jupiter’s four Galilean satellites. All other satellites of Uranus are smaller and less massive.

The best telescopic observations of Uranus from Earth’s surface show a small, featureless, bluish green disk. Spectroscopic observations show that this color is produced by the absorption of sunlight by methane gas in its atmosphere; this gas is also present in the atmospheres of Jupiter and Saturn. Observations of occultations (similar to eclipses) or stars by Uranus indicated that Uranus’ atmosphere is mostly composed of molecular hydrogen and helium, which are also the main components of the atmospheres of Jupiter and Saturn. The Hubble Space Telescope, in 2001, has photographed more activity in the atmosphere of Uranus as what was photographed by Voyager 2. In 1986, photographs showed a primarily calm and inactive atmosphere.

Observations at infrared wavelengths (that are longer than those of red light), where planets radiate away most of their heat energy, show that Uranus radiates at most only slightly more infrared radiative energy than its atmosphere absorbs from sunlight. Any excess energy originating from Uranus’ interior can be attributed to the decay of radioactive elements, which also produces much of the heating in the Earth’s interior. This is not true for the other Jovian planets, which all emit as much as twice as much infrared energy as their atmospheres absorb from sunlight; this requires another internal energy source for them, which is possibly continuing gravitational contraction.

One of the last major discoveries about the Uranus system from Earth-based observations was made on March 10, 1977, during observations of Uranus’ occultation of the star SAO 158657, when James L. Elliot’s (1943–) group and other observers noticed unexpected dimming of the star’s light before the occultation and again after it. These dimmings were correctly identified with the existence of several faint, thin rings orbiting Uranus well inside Miranda’s orbit, which were hitherto undetected; unlike Saturn’s rings, they are too faint to be directly observed from the Earth’s surface by ordinary methods. Uranus’ rings have been observed several times since then during stellar occultations and by the Voyager 2 spacecraft and the Hubble Space Telescope. The rings are very dark; their albedos (the fraction of the light that falls on them which they reflect) are only about 0.05.

A second major discovery made by Earth-based infrared observations was the detection of waterice on the surfaces of some of Uranus’ satellites.

The seasons of Uranus are interesting in nature. The fact that Uranus’ rotation axis lies almost in the plane of its orbit around the sun means that at some seasons its south pole will be pointed nearly at the sun, and nearly all of its southern hemisphere will be in continuous sunlight (early southern hemisphere summer). At the same time, nearly all of its northern hemisphere will be in continuous night (early northern hemisphere winter). These seasons occurred in 1901 and in late 1985, and will occur next in 2069. The sun was last above Uranus’ equator in December 1965, when it rose at Uranus’ south pole and set at the north pole. The sun will shine continuously on Uranus’ south pole for the next 41.6 years until mid-2007, when it will again be above Uranus’ equator and will set at the south pole and rise at the north pole, which will be in continuous sunlight for the next 42.5 years while the south pole will be in continuous night. The north pole will point closest to the sun in early 2030 (early northern hemisphere summer), and the sun will set there and rise at the south pole in 2050. Calculations show that a horizontal unit surface area, say a square meter, at either pole will receive over a Uranian year about 1.5 times the sunlight that the same surface would receive at Uranus’ equator over a Uranian year (84.1 Earth-years).

Results from the flyby of the Voyager 2 spacecraft

The Voyager 2 spacecraft was launched from Earth on August 20, 1977. As it flew by Jupiter in July 1979, it was accelerated toward Saturn which, in turn, accelerated Voyager 2 toward Uranus during the August 1981 flyby. Voyager 2 flew by Uranus on a hyperbolic orbit, passing it at a minimum distance of 66,447 mi (107,000 km) from the center of Uranus on January 24, 1986. The observations that Voyager 2 made of the Uranus system from November 4, 1985, to February 26, 1986, added immensely to scientific knowledge about it. Uranus’ rotation axis was pointed less than 8° from the sun at its approach. Because Voyager 2 was approaching Uranus along a path that made about a 35° angle with the line from the sun, Voyager 2 passed through Uranus’ ring and satellite system much like a bullet passing through a bull’s eye target. It could not pass fairly close to any more than two of Uranus’ satellites at most (Figure 1).

The satellites closely approached by Voyager 2 were Miranda and Ariel; the spacecraft passed by these two at distances of 16,146 mi (26,000 km) and 17,388 mi (28,000 km), respectively.

The main discoveries made by Voyager 2 during its encounter with Uranus are the following:

Uranus’s magnetic field

Like Earth and the other Jovian planets, Uranus has a strong magnetic field that arises in its interior. Evidence for Uranus’ magnetic field and magneto-sphere (the region of space where the planet’s magnetic field is dominant over the interplanetary field) was not found until January 22, 1986, two days before closest approach to Uranus, when radio noise from charged particles trapped in its magnetosphere was detected. Voyager 2 crossed into Uranus’ magnetosphere on January 24 and remained inside it for 45 hours. Uranus’ magnetic field was found to be quite strong but very unusual. First, Uranus’ magnetic poles were found to be 58.6° from its poles of rotation, which is much greater than the tilts of the magnetic fields to the poles of rotation found for Earth (11°), Jupiter (9.6°), and Saturn (0°) (Figure 2). second, the center of Uranus’ magnetic field was found to be offset from its center of mass by 0.3 of Uranus’ radius, a much greater offset than those found for the above-named planets. One effect of this magnetic field offset is that the magnetic field strength at the cloud level in

Uranus’ atmosphere is expected to vary by factors of five to ten depending on Uranian latitude and longitude. Radiation belts of charged particles trapped in Uranus’ magnetosphere were detected. They consist mainly of low energy protons and electrons; very few heavy ions are detected. The particle densities in these radiation belts are low compared with those densities in the radiation belts of the Earth and Jupiter, possibly because the large tilt of Uranus’ magnetic field to the interplanetary magnetic field allows the solar wind to make frequent convective sweeps of particles out of Uranus’ radiation belts.

Uranus’ rotation

The fact that Uranus’ magnetic field is tilted to its rotation axis and is offset from its center causes fluctuations of its magnetic field that are associated with the rotation of Uranus’ interior. From measurements of these fluctuations by Voyager 2, the rotation period of Uranus’ interior was found to be 17 hours 14 minutes. This is the first accurate rotation period for Uranus; earlier attempts in the last 100 years to determine its rotation period from spectroscopic and photometric observations gave very diverse, conflicting, and, as scientists now know, incorrect results. Other somewhat different rotation periods found from Voyager 2 observations of cloud features in Uranus’ atmosphere, which range from 16 to 17.5 hours, are caused by winds in Uranus’ atmosphere.

Atmospheric temperature

In the earlier discussion of Uranus’ seasons, it was mentioned that a unit surface at the poles will receive about 1.5 times as much sunlight as the same surface would on Uranus equator over a Uranian year. Based on this, one might expect Uranus’ south polar region, which at the time of the Voyager 2 flyby had been in continuous sunlight for the order of 20 years, to be warmer than its equatorial region, which would be warmer than the north polar region, which had been in prolonged night. Voyager 2 ’s infrared instruments did not observe this; at the level of clouds in Uranus’ atmosphere, the temperature seemed to be the same, about -346° F (-210° C), from the south pole across the equator to the north pole. The most evident temperature change at this level was a 34° F (1° C) decrease centered at about 30° south latitude. This surprising observation shows the great capacity of the enormously thick atmosphere of Uranus (and those of the other Jovian planets) to absorb and transport away almost all the sunlight energy from a region sunlit continuously for decades.

Uranus’ atmosphere

By solar and stellar occultations and visual, infrared, and radio observations from Voyager 2, the structure and circulation of Uranus’ atmosphere has been mapped from just below the cloud layers to its exo-sphere. The main components of the atmosphere are hydrogen and helium, the most abundant elements in the universe. Methane comprises 1–2% of the observable troposphere. Water vapor and ammonia are inferred to be important components of the atmosphere below the clouds, but they have not been detected in the observable part of the troposphere because it is too cold and freezes them out.

Uranus’ upper atmosphere is dominated by hydrogen, mainly molecular. Methane and other hydrocarbons are nearly all frozen out by the underlying cold atmosphere, especially the lower stratosphere (at 428° F [220° C] temperature). Molecular hydrogen is broken down into atomic hydrogen mainly by the absorption of ultraviolet radiation from the sun, and some atomic hydrogen is ionized into free protons and electrons, forming an ionosphere. The temperature of the upper atmosphere increases to 482° F (250° C) at 497 mi (800 km) above the cloud layers, and continues to increase to over 932° F (500° C) at 3,105 mi (5,000 km) above the clouds. Atomic hydrogen becomes the main component of Uranus’ upper atmosphere above 4,658 mi (7,500 km) above the clouds, forming a hydrogen thermal corona in its exosphere that extends at least 15,525 mi (25,000 km) above the clouds (extending through the zone of the rings from 9,936 mi [16,000 km] to 16,146 mi [26,000 km] above the clouds). The source of most of the heating of Uranus’ upper atmosphere is still unknown as is also true for the heating of the upper atmospheres of the Earth, Jupiter, and Saturn.

Uranus’s internal structure

Evidence indicates that Uranus may have a silicate rock core (perhaps rich in iron and magnesium), which is 4,800 km in diameter (approximately 40% of the planet’s mass). The mantle is likely ice or ice-rock mixture (water ice, methane ice, ammonia ice) that may be molten in part (perhaps evidence of convention produced in the magnetic field). Above the mantle is the lower atmosphere, which consists of molecular (gaseous) hydrogen, helium, and traces of other gasses (approximately 10% of planet’s mass). Finally, the upper atmosphere is methane with cloud layers of ammonia or water ice. The magnetic field discovered and mapped by Voyager 2 implies a field generating region in Uranus’ interior which extends out to 0.7 of Uranus radius from the center, and that part of Uranus’ interior is a fluid and has a high internal temperature.

Uranus’ rings

Uranus’ ring particles are dark grey to black and form rings about 2.0 to 10,000 mi (3.2 to 16,000 km) wide and less than one kilometer in average thickness. The rings are in the equatorial plane of Uranus, which is tilted at 98 degrees to the typical planetary attitude of the solar system (i.e., the plane of the orbit of the eight planets). This suggests that the rings formed after the planet was tilted. They are made mostly of ice and rock boulders.

There is a ring hierarchy about Uranus. The inner eight rings are very thin and have no known shepherd satellites. In an outward order, these rings are known as Epsilon, Delta, Gamma, Beta, Alpha, 4-ring, 5-ring, and 6-ring. The two outer rings have variable thickness and have shepherd satellites (Cordelia on the inner edge and Ophelia on the outer edge). The number nine and ten rings, which are further out, are called URI and Epsilon.

The Voyager 2 observations also indicate that the rings contain a large fraction of large particles; the average particle size in the rings was calculated to be between 8 and 28 in (20 and 70 cm). An appreciable amount of micron-sized dust seems to be distributed throughout the ring system. However, this dust is probably transitory; atmospheric drag on the dust particles from Uranus’ hydrogen corona that was mentioned above is expected to decelerate them and cause them to spiral into the denser layers of Uranus’ atmosphere after, at most, a few thousand years. Uranus has 10 known rings that are nearly circular and lie in or nearly in the plane of Uranus’ equator. The Epsilon ring is one of the most distant rings from Uranus, and it is also one of the most elliptical and the widest ones.

As of 2005, two more rings have been identified, each by NASA’s Hubble Space Telescope. The larger of the two rings has a diameter that is twice the diameter of the planet’s previously known rings. They are the furthest rings from Uranus and, thus, are often called Uranus’ second system of rings.

Uranian satellites

Uranus’ satellites all lie in the equatorial plane (like the rings). There are several groups of satellites. The inner satellites are irregular dark objects (which may mean they are carbonaceous rock or are methane ice bodies coated with carbon material) under 93 mi (150 km) in diameter. Some of the larger of the inner satellites are Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck. Most of the outer satellites are all rather large satellites (292–982 mi [470 to 1580 km] in diameter) that are locked in 1:1 spin-orbit couples with Uranus. The outer satellites of Miranda, Ariel, Umbriel, Titania, and Oberon are all spherical objects of water ice surrounding rock.

Voyager 2 discovered ten satellites, which all orbit Uranus closer to it than Miranda and are all smaller than Miranda. Surface albedo could be found for only Puck and Cordelia, which are 0.08 and 0.07, respectively, indicating that they are somewhat brighter than the ring particles. The other found satellites by Voyager 2 seem to be dark like Puck, Cordelia, and the rings. Cordelia and Ophelia—two satellites very close to Uranus—seem to serve as shepherd satellites for the Epsilon ring, keeping its particles in the ring by their gravitational perturbations on them, thereby increasing this ring’s orbital stability. Gravitational perturbations produced by several other satellites near the rings may make the rings more stable. Ophelia is slightly more and Cordelia slightly less than two Uranus radii from Uranus’ center; this raises the possibility that the rings were formed by satellites inside Uranus’ Roche limit that were torn to pieces by collisions or by tidal forces produced in them by Uranus.

Observations of Miranda and other satellites

Since Voyager 2 passed fairly close to Miranda, it was possible to determine Miranda’s mass from its perturbation of Voyager 2 ’s hyperbolic orbit past Uranus found from an analysis of the Voyager 2 radio data. Ariel’s mass was then determined from its perturbations of the orbits of Miranda and Voyager 2. Voyager 2 did not approach Umbriel, Titania, and Oberon closely enough for reliable determination of their masses from perturbations of its flyby orbit. Instead their masses are determined from their perturbations of Ariel’s orbit and each other’s orbits using both Voyager 2 observations and Earth-based observations made over many years. Accurate radii were found for all five satellites from Voyager 2 images; this allowed the calculation of mean densities for these satellites. Their mean densities, which range from 1.20 grams/cm³ for Miranda to 1.69 grams/cm³ for Titania, are compatible with their interiors being largely composed of water ice, which was detected earlier on their surfaces.

All five satellites were found to be tidally locked to Uranus, as predicted by theory, so their rotation periods are the same as their orbital periods of revolution. Their rotation axes have become aligned nearly parallel to Uranus’ rotation axis, so that only their southern hemispheres could be imaged. Voyager 2 infrared measurements near the south poles of Miranda and Ariel gave temperatures of -304.6° F (-187° C) and 308.2° F (-189° C), respectively, considerably warmer than the cloud layer of Uranus’ atmosphere, but understandable for a surface that has been in continuous sunlight for about 20 years. None of the satellites show an appreciable atmosphere.

Voyager 2 obtained detailed images of parts of the sunlit surface of all five previously known satellites and also of Puck. Bright and dark (albedo) regions, craters with or without bright ray systems around them, mountains, cliffs, scarps, valleys, canyons, graben, faults, and other geological features are clearly seen on these images. Maps of the parts of the satellite surfaces that have been imaged have been made, and names have been assigned to many surface features. Summaries of the surface features of the six satellites with imaged surface features are given below. The images with the best resolution obtained were Ariel and Miranda because Voyager 2 flew closest to them.

Oberon

oberon, Uranus’ most distant (of the major moons) satellite, has extensive, heavily cratered terrain interrupted by canyons (rift valleys) and scarps. Some craters are surrounded by bright ray systems; others show dark matter on their floors. A prominent feature on Oberon’s limb is a large mountain 7 mi (11 km) high and 28 mi (45 km) wide. Oberon shows the least evidence from modification of its cratered terrain by geologic activity of any Uranian satellite. Image resolution is poor due to Oberon’s distance.

Titania

titania, Uranus’ largest and most massive satellite, shows similar geological features to those found on Oberon. Heavily cratered plains are the most extensive surfaces found here. Oberon also shows a global rift valley network related to global tectonics. Image resolution is somewhat better for Titania than Oberon, since Voyager 2 was closer to Titania. A prominent system of canyons and scarps is the most noticeable class of features on Titania’s surface; shadows indicate some of them may be as deep as 3.7 mi (6 km). Moderately cratered and smooth plains are also observed, which indicate resurfacing. These features indicate that more geologic activity occurred on Titania than on Oberon after the end of the heavy bombardment (crater formation) phase of their histories. Here the resurfacing material should be liquid or icy water, ammonia, or methane, perhaps mixed with rocky material, rather than terrestrial type volcanic lava. Impact and/or internal heating may have melted or softened these ices, which then erupted and flowed over the satellite surfaces, resurfacing them. This phenomenon is called cryovolcanism (sometimes water volcanism), and it may have been important in the geological histories of many of the satellites of the Jovian planets.

Umbriel

with an average albedo of 0.21, Umbriel is the darkest of Uranus’ five largest satellites. It also has a more uniform albedo than Titania and Oberon, showing only a few striking albedo features on the part of its surface imaged by Voyager 2. Umbriel also has extensive, heavily cratered terrain and several groups of canyons, scarps, and lineaments, which seem to be of more recent origin than the craters that they cut. Evidence is also seen of resurfacing in and near several craters, indicating cryovolcanism. Much of its surface may have been resurfaced with dark material of uncertain origin (cryovolcanic, dark ejecta from a crater, or from an external source).

Ariel

ariel is similar in size and mass to Umbriel, but whereas Umbriel is the darkest of Uranus’ five largest satellites, Ariel is the brightest, with a 0.40 average albedo. Ariel’s surface seems to have evolved more like Titania’s surface than that of Umbriel; it shows global-scale faulting (canyons, scarps, and lineaments) and resurfacing by cryovolcanism, but on a more extensive scale than occurred on Titania. Ariel’s most heavily cratered plains show less cratering than the most cratered units on Titania and Oberon. Evidence has been found for the extrusion of ice into Ariel’s surface, filling part of a valley and partially burying an impact crater.

Miranda

miranda shows the most interesting evidence for geologic activity and surface modification among Uranus’ satellites. In addition to old cratered plains, canyons, scarps, lineaments, and valleys, the three coronae Arden, Elsinore, and Inverness exist on its surface; they are its most prominent surface features. They are large, lightly cratered regions of up to 186 mi (300 km) or more extent; Arden shows banded regions, while Elsinore and Inverness show numerous grooves and ridges. A cliff near Miranda’s south pole may be as high as 12 mi (20 km). One plausible model for Miranda’s evolution hypothesizes the following sequence of events. First, Miranda accreted from several smaller bodies near Uranus and nearly in its equatorial plane; impacts during accretion formed the old cratered plains. Some of the canyon systems were formed near the end of accretion. Next, a large body impacted Miranda, forming the Arden basin. Debris was scattered over and ejected from Miranda, and cryovolcanism flooded the basin, forming Arden Corona. Then, the Inverness basin was flooded, forming Inverness Corona. The last main cryovolcanic activity formed Elsinore Corona. This left Miranda’s surface early in its present state, with only a few more recent craters added since then.

Puck

puck’s surface was also imaged by Voyager 2, revealing a cratered surface that is considerably darker (0.08 average albedo) than that for any of the five larger satellites. The Voyager 2 images also show that Puck is almost spherical. The three largest impact craters on Puck are named Bogle, Lob, and Butz.

Other natural satellites

The following are the other 22 natural satellites of Uranus, along with their designations, dates (imaged) of discovery, and discoverer:

• Juliet (S/1986 U 2, January 3, 1986, Voyager 2)
• Portia (S/1986 U 1, January 3, 1986, Voyager 2)
• Cressida (S/1986 U 3, January 9, 1986, Voyager 2)
• Desdemona (S/1986 U 6, January 13, 1986, Voyager 2)
• Rosalind (S/1986 U 4, January 13, 1986, Voyager 2)
• Belinda (S/1986 U 5, January 13, 1986, Voyager 2)
• Cordelia (S/1986 U 7, January 20, 1986, Voyager 2)
• Ophelia (S/1986 U 8, January 20, 1986, Voyager 2)
• Bianca (S/1986 U 9, January 23, 1986, Voyager 2)
• Caliban (S/1997 U 1, September 6, 1997, Gladman, Nicholson, Burns, Kavelaars)
• Sycorax (S/1997 U 2, October 31, 1997, Gladman, Nicholson, Burns, Kavelaars)
• Perdita (S/1986 U 10, January 18, 1986, Karkoschka via Voyager 2)
• Setebos (S/1999 U 1, July 18, 1999, Kavelaars, Gladman, Holman, Petit, Scholl)
• Stephano (S/1999 U 2, July 18, 1999, Gladman, Holman, Kavelaars, Petit, Scholl)
• Prospero (S/1999 U 3, July 18, 1999, Holman, Kavelaars, Gladman, Petit, Scholl)
• Trinculo (S/2001 U 1, August 13, 2001, Holman, Kavelaars, Milisavljevic)
• Perdita (S/1986 U 10, January 18, 1986 [announced August 25, 2003, Karkoschka via Hubble Space Telescope)
• Ferdinand (S/2001 U 2, August 13, 2001 [announced August 29, 2003], Holman Kavelaars, Milisavljevic [2001]; Sheppard, Jewitt [2003])
• Mab (S/2003 U 1, August 25, 2003 [announced September 25], Showalter, Lissauer)
• Cupid (S/2003 U 2, August 25, 2003 [announced September 25], Showalter, Lissauer)
• Francisco (S/2001 U 3, August 13, 2001 [announced October 2003], Holman, Kavelaars, Milisavljevic, Gladman)
• Margaret (S/2003 U 3, August 29, 2003 [announced October 9], Sheppard, Jewitt)

See also Space probe.

Resources

BOOKS

de Pater, Imke and Jack J. Lissauer. Planetary Sciences Cambridge, UK: Cambridge University Press, 2001.

Irwin, Patrick. Giant Planets of Our Solar System: Atmospheres, Composition, and Structure. Berlin, Germany, and New York: Springer, 2003.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Sobel, Dava. The Planets. New York: Viking, 2005.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

OTHER

Arnett, B. SEDS, University of Arizona. “The Eight Planets, a Multimedia Tour of the Solar System.” (August 25, 2006)<http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html> (November 4, 2006).

Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Welcome to the Planets: Uranus.” <http://pds.jpl.nasa.gov/planets/choices/uranus1.htm> (accessed November 4, 2006).

SpaceKids, National Aeronautics and Space Administration.“Tour the Solar System and Beyond.” <http://spacekids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 14, 2006).

TeachNet-lab.org.“A Tour of the Planets.” <http://www.teachnet-lab.org/miami/2001/salidoi2/a_tour_of_the_planets.htm> (accessed October 14, 2006).

Frederick R. West

David T. King, Jr.

Uranus

Uranus is the seventh planet from the Sun . It has a large size (its diameter is almost four times that of Earth ) and mass , low mean density , fairly rapid rotation , and well-developed ring (11 components) and satellite (15 members) systems. The planet has a strong magnetic field with a large tilt (58.6°) to its rotation axis and offset (0.3 Uranus radius) from its center. Analysis of the observations made by Voyager 2 during its flyby of Neptune in August 1989 shows that Uranus and Neptune are similar in most of these properties and form a subgroup of the Jovian planets; Jupiter and Saturn , much larger and more massive, form the other subgroup.

Discovery

William Herschel (1738–1822) fortuitously discovered Uranus in 1781; it was the first planet discovered telescopically. It was found to orbit the sun at a mean distance of about 19.2 astronomical units (a.u.) (2,870,000,000 km), about twice as far from the sun as Saturn (9.54 a.u), the most distant planet known before 1781.

Observations from Earth

Knowledge about Uranus came slowly because of its distance. Even when it is closest, Uranus shows a disk of only 4 in (10 cm) in apparent diameter through a telescope and is 5.7m apparent magnitude (barely visible to the unaided eye even in the best observing conditions). Herschel discovered Oberon and Titania, the outermost and largest, respectively, satellites of Uranus in 1787. Determination of their orbits around Uranus from observations gave their periods of revolution P and mean distances A from Uranus. This allowed one to determine Uranus' mass from the general form of Kepler's third law; it turned out to be 14.5 Earth masses. Using its radius of 15,873 mi (25,560 km), one calculated Uranus' mean density (its mass divided by its volume ), which is 1.27 grams/cm3. This indicated that Uranus is a smaller type of Jovian planet similar to Jupiter and Saturn; they are characterized by large masses and sizes and low mean densities (compared to Earth), and are inferred to consist largely of gases.

The planes of the orbits of Oberon and Titania were expected to lie in or near the plane of Uranus' equator, since most other planetary satellites have orbital planes that are in or near the equatorial planes of their planets. When the orbital planes of Oberon and Titania were determined, however, they indicated that the plane of Uranus' equator is almost perpendicular to the plane of its orbit around the sun (and also to the ecliptic). This is unlike the other planets, whose equatorial planes are tilted by at most 30° to the planes of their orbits around the sun. This implied that Uranus' axis (and poles) of rotation lie almost in its orbital plane. This conclusion has been confirmed by observations of Uranus' rotation from the Voyager 2 spacecraft in 1986. This is the first of several interesting characteristics we have discovered for Uranus, and gives Uranus interesting seasons during its year (period of revolution around the sun), which is 84.1 Earth years long. The Uranian seasons will be discussed in more detail below. The cause of this unusual orientation of Uranus' rotation axis is still unknown and is now the subject of considerable speculation and theoretical research. One theory is that the orientation of its rotation axis was produced by the collision of an Earth-sized body with Uranus near the end of its formation.

Unexplained perturbations of Uranus' orbit in the early nineteenth century led to the prediction of the existence of a still more distant large planet, resulting in the discovery of Neptune in 1846. Neptune is the most distant (30.06 a.u. mean distance from the sun) Jovian planet, and it has several properties (mass, size, rotation period, rings, and magnetic field) like those of Uranus.

Three more satellites of Uranus, closer to it than Titania, were discovered during the 105 years after Neptune's discovery. They are, in order of closeness to Uranus, Umbriel and Ariel, discovered in 1851 by Lassell (1799–1880), and Miranda, discovered in 1948 by G. P. Kuiper (1905–1973). These discoveries showed that Uranus has a satellite system comparable to those of Jupiter and Saturn, although Titania, its largest (980 mi [1,580 km] diameter) and most massive satellite, and the slightly smaller Oberon, are comparable in size and mass to Saturn's satellites Iapetus and Rhea rather than to its much larger satellite Titan and Jupiter's four Galilean satellites. All other satellites of Uranus are smaller and less massive.

The best telescopic observations of Uranus from Earth's surface show a small, featureless, bluish green disk. Spectroscopic observations show that this color is produced by the absorption of sunlight by methane gas in its atmosphere; this gas is also present in the atmospheres of Jupiter and Saturn. Observations of occultations (similar to eclipses ) or stars by Uranus indicated that Uranus' atmosphere is mostly composed of molecular hydrogen and helium, which are also the main components of the atmospheres of Jupiter and Saturn.

Observations at infrared wavelengths (that are longer than those of red light ), where planets radiate away most of their heat energy , show that Uranus radiates at most only slightly more infrared radiative energy than its atmosphere absorbs from sunlight. Any excess energy originating from Uranus' interior can be attributed to the decay of radioactive elements, which also produces much of the heating in Earth's interior . This is not true for the other Jovian planets, which all emit as much as twice as much infrared energy as their atmospheres absorb from sunlight; this requires another internal energy source for them, which is possibly continuing gravitational contraction.

One of the last major discoveries about the Uranus system from Earth-based observations was made on March 10, 1977, during observations of Uranus' occultation of the star SAO 158657, when J. L. Elliot's (1943–) group and other observers noticed unexpected dimming of the star's light before the occultation and again after it. These dimmings were correctly identified with the existence of several faint, thin rings orbiting Uranus well inside Miranda's orbit which were hitherto undetected; unlike Saturn's rings, they are too faint to be directly observed from Earth's surface by ordinary methods. Uranus' rings have been observed several times since then during stellar occultations and by the Voyager 2 spacecraft. The rings are very dark; their albedos (the fraction of the light that falls on them which they reflect) are only about 0.05.

A second major discovery made by Earth-based infrared observations was the detection of water ice on the surfaces of some of Uranus' satellites.

Let us now return to the seasons of Uranus. The fact that Uranus' rotation axis lies almost in the plane of its orbit around the sun means that at some season its south pole will be pointed nearly at the sun, and nearly all of its southern hemisphere will be in continuous sunlight (early southern hemisphere summer), while nearly all of its northern hemisphere will be in continuous night (early northern hemisphere winter). These seasons occurred in 1901 and in late 1985, and will occur next in 2069. The sun was last above Uranus' equator in December 1965, when it rose at Uranus' south pole and set at the north pole; the sun will then shine continuously on Uranus' south pole for the next 41.6 years until mid-2007, when it will again be above Uranus' equator and will set at the south pole and rise at the north pole, which will be in continuous sunlight for the next 42.5 years while the south pole will be in continuous night. The north pole will point closest to the sun in early 2030 (early northern hemisphere summer), and the sun will set there and rise at the south pole in 2050. Calculations show that a horizontal unit surface area, say a square meter, at either pole will receive over a Uranian year about 1.5 times the sunlight that the same surface would receive at Uranus' equator over a Uranian year (84.1 Earth years).

Results from the flyby of the Voyager 2 spacecraft

The Voyager 2 spacecraft was launched from Earth on August 20, 1977. As it flew by Jupiter in July 1979, it was accelerated toward Saturn which, in turn, accelerated Voyager 2 toward Uranus during the August 1981 flyby. Voyager 2 flew by Uranus on a hyperbolic orbit, passing it at a minimum distance of 66,447 mi (107,000 km) from the center of Uranus on January 24, 1986. The observations that Voyager 2 made of the Uranus system from November 4, 1985 to February 26, 1986 added immensely to our knowledge about it. Because Uranus' rotation axis was pointed less than 8° from the Sun then, and because Voyager 2 was approaching Uranus along a path that made about a 35° angle with the line from the Sun, Voyager 2 passed through Uranus' ring and satellite system much like a bullet passing through a "bull's eye target" and could not pass fairly close to any more than two of Uranus's satellites at most.

The satellites closely approached by Voyager 2 were Miranda and Ariel; the spacecraft passed by these two at distances of 16,146 mi (26,000 km) and 17,388 mi (28,000 km), respectively.

The main discoveries made by Voyager 2 during its encounter with Uranus are the following:

Uranus's magnetic field

Like Earth and the other Jovian planets, Uranus has a strong magnetic field which arises in its interior. Evidence for Uranus' magnetic field and magnetosphere (the region of space where the planet's magnetic field is dominant over the interplanetary field) was not found until January 22, 1986, two days before closest approach to Uranus, when radio noise from charged particles trapped in its magnetosphere was detected. Voyager 2 crossed into Uranus' magnetosphere on January 24 and remained inside it for 45 hours. Uranus' magnetic field was found to be quite strong but very unusual. First, Uranus' magnetic poles were found to be 58.6° from its poles of rotation, which is much greater than the tilts of the magnetic fields to the poles of rotation found for Earth (11°), Jupiter (9.6°), and Saturn (0°). Second, the center of Uranus' magnetic field was found to be offset from its center of mass by 0.3 of Uranus' radius, a much greater offset than those found for the above-named planets. One effect of this magnetic field offset is that the magnetic field strength at the cloud level in Uranus' atmosphere is expected to vary by factors of five to 10 depending on Uranian latitude and longitude . Radiation belts of charged particles trapped in Uranus' magnetosphere were detected. They consist mainly of low energy protons and electrons; very few heavy ions are detected. The particle densities in these radiation belts are low compared with those densities in the radiation belts of Earth and Jupiter, possibly because the large tilt of Uranus' magnetic field to the interplanetary magnetic field allows the solar wind to make convective sweeps of particles out of Uranus' radiation belts fairly frequently.

Uranus' rotation

The fact that Uranus' magnetic field is tilted to its rotation axis and is offset from its center causes fluctuations of its magnetic field that are associated with the rotation of Uranus' interior. From measurements of these fluctuations by Voyager 2, the rotation period of Uranus' interior was found to be 17 hours 14 minutes. This is the first accurate rotation period for Uranus; earlier attempts in the last 100 years to determine its rotation period from spectroscopic and photometric observations gave very diverse, conflicting, and, as we now know, incorrect results. Other somewhat different rotation periods found from Voyager 2 observations of cloud features in Uranus' atmosphere, which range from 16 to 17.5 hours, are caused by winds in Uranus' atmosphere.

Atmospheric temperature

In the earlier discussion of Uranus' seasons, it was mentioned that a unit surface at the poles will receive about 1.5 times as much sunlight as the same surface would on Uranus equator over a Uranian year. Based on this, one might expect Uranus' south polar region, which at the time of the Voyager 2 flyby had been in continuous sunlight for the order of 20 years, to be warmer than its equatorial region, which would be warmer than the north polar region, which had been in prolonged night. Voyager 2's infrared instruments did not observe this; at the level of clouds in Uranus' atmosphere, the temperature seemed to be the same, about -346°F (-210°C), from the south pole across the equator to the north pole. The most evident temperature change at this level was a 34°F (1°C) decrease centered at about 30° south latitude. This surprising observation shows the great capacity of the enormously thick atmosphere of Uranus (and those of the other Jovian planets) to absorb and transport away almost all the sunlight energy from a region sunlit continuously for decades.

Uranus' atmosphere

By solar and stellar occultations and visual, infrared, and radio observations from Voyager 2, the structure and circulation of Uranus' atmosphere has been mapped from just below the cloud layers to its exosphere. The main components of the atmosphere are hydrogen and helium, the most abundant elements in the universe. Methane comprises 1–2% of the observable troposphere. Water vapor and ammonia are inferred to be important components of the atmosphere below the clouds, but they have not been detected in the observable part of the troposphere because it is too cold and freezes them out.

Uranus' upper atmosphere is dominated by hydrogen, mainly molecular. Methane and other hydrocarbons are nearly all frozen out by the underlying cold atmosphere, especially the lower stratosphere (at 428°F [220°C] temperature). Molecular hydrogen is broken down into atomic hydrogen mainly by the absorption of ultraviolet radiation from the sun, and some atomic hydrogen is ionized into free protons and electrons, forming an ionosphere. The temperature of the upper atmosphere increases to 482°F (250°C) at 497 mi (800 km) above the cloud layers, and continues to increase to over 932°F (500°C) at 3,105 mi (5,000 km) above the clouds. Atomic hydrogen becomes the main component of Uranus' upper atmosphere above 4,658 mi (7,500 km) above the clouds, forming a hydrogen thermal corona in its exosphere that extends at least 15,525 mi (25,000 km) above the clouds (extending through the zone of the rings from 9,936 mi [16,000 km] to 16,146 mi [26,000 km] above the clouds). The source of most of the heating of Uranus' upper atmosphere is still unknown as is also true for the heating of the upper atmospheres of Earth, Jupiter, and of Saturn.

Uranus's internal structure

Evidence indicates that Uranus may have a silicate rock core (perhaps rich in iron and magnesium ), which is 4,800 km in diameter (approximately 40% of the planet's mass). The mantle is likely ice or ice-rock mixture (water ice, methane ice, ammonia ice) that may be molten in part (perhaps evidence of convention produced in the magnetic field). Above the mantle is the lower atmosphere, which consists of molecular (gaseous) hydrogen, helium, and traces of other gasses (approximately 10% of planet's mass). Finally, the upper atmosphere is methane with cloud layers of ammonia or water ice. The magnetic field discovered and mapped by Voyager 2 implies a field generating region in Uranus' interior which extends out to 0.7 of Uranus radius from the center, and that part of Uranus' interior is a fluid and has a high internal temperature.

Uranus's rings

Uranus' ring particles are dark grey to black and form rings about 1–37 mi (2–60 km) wide and less than one kilometer thick. The rings are in the equatorial plane of Uranus, which is tilted at 98 degrees to the typical planetary attitude of the solar system (i.e., the plane of the orbit of the inner eight planets). This suggests that the rings formed after the planet was titled.

There is a ring hierarchy about Uranus. The inner eight rings are very thin and have no known shepard satellites. In an outward order, these rings are known as Delta, Gamma, Eta, Beta, Alpha, 4-ring, 5-ring, and 6-ring. The two outer rings have variable thickness and have shepard satellites (Cordelia on the inner edge and Ophelia on the outer edge). The outer rings are URI and Epsilon.

The Voyager 2 observations also indicate that the rings contain a large fraction of large particles; the average particle size in the rings was calculated to be between 8 and 28 in (20-70 cm). An appreciable amount of micron-sized dust seems to be distributed throughout the ring system. However, this dust is probably transitory; atmospheric drag on the dust particles from Uranus' hydrogen corona that was mentioned above is expected to decelerate them and cause them to spiral into the denser layers of Uranus' atmosphere after, at most, a few thousand years. Uranus has 10 known rings that are nearly circular and lie in or nearly in the plane of Uranus' equator. The Epsilon ring is not only the most distant ring from Uranus, but it is also the most elliptical and the widest one.

Uranian satellites

Uranus' satellites all lie in the equatorial plane (like the rings). There are several groups of satellites. The inner satellites, of which there are ten, are irregular dark objects (which may mean they are carbonaceous rock or are methane ice bodies coated with carbon material) under 93 mi (150 km) in diameter. The inner satellites are Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck. The outer satellites are all rather large satellites (292–982 mi [470–1580 km] in diameter) that are locked in 1:1 spin-orbit couples with Uranus. The outer satellites (Miranda, Ariel, Umbriel, Titania, and Oberon) are all spherical objects of water ice surrounding rock.

Voyager 2 discovered 10 satellites which all orbit Uranus closer to it than Miranda and are all smaller than Miranda. Surface albedo could be found for only Puck and Cordelia, which are 0.08 and 0.07, respectively, indicating that they are somewhat brighter than the ring particles. The other eight newly found satellites seem to be dark like Puck, Cordelia, and the rings. Cordelia and Ophelia, the two satellites closest to Uranus, seem to serve as "shepherd satellites" for the Epsilon ring, keeping its particles in the ring by their gravitational perturbations on them, thereby increasing this ring's orbital stability. Gravitational perturbations produced by several other satellites near the rings may make the rings more stable. Ophelia is slightly more and Cordelia slightly less than two Uranus radii from Uranus' center; this raises the possibility that the rings were formed by satellites inside Uranus' Roche limit that were torn to pieces by collisions or by tidal forces produced in them by Uranus.

Observations of Miranda and other satellites

Since Voyager 2 passed fairly close to Miranda, it was possible to determine Miranda's mass from its perturbation

of Voyager 2's hyperbolic orbit past Uranus found from an analysis of the Voyager 2 radio data. Ariel's mass was then determined from its perturbations of the orbits of Miranda and Voyager 2. Voyager 2 did not approach Umbriel, Titania, and Oberon closely enough for reliable determination of their masses from perturbations of its flyby orbit. Instead their masses are determined from their perturbations of Ariel's orbit and each other's orbits using both Voyager 2 observations and Earth-based observations made over many years. Accurate radii were found for all five satellites from Voyager 2 images; this allowed the calculation of mean densities for these satellites. Their mean densities, which range from 1.20 grams/cm3 for Miranda to 1.69 grams/cm3 for Titania, are compatible with their interiors being largely composed of water ice, which was detected earlier on their surfaces. All five satellites were found to be tidally locked to Uranus, as predicted by theory, so their rotation periods are the same as their orbital periods of revolution. Their rotation axes have become aligned nearly parallel to Uranus' rotation axis, so that only their southern hemispheres could be imaged. Voyager 2 infrared measurements near the south poles of Miranda and Ariel gave temperatures of -304.6°F (-187°C) and -308.2°F (-189°C), respectively, considerably warmer than the cloud layer of Uranus' atmosphere, but understandable for a surface that has been in continuous sunlight for about 20 years. None of the satellites show an appreciable atmosphere.

Voyager 2 obtained detailed images of parts of the sunlit surface of all five previously known satellites and also of Puck. Bright and dark (albedo) regions, craters with or without bright ray systems around them, mountains , cliffs, scarps, valleys, canyons, graben, faults, and other geological features are clearly seen on these images. Maps of the parts of the satellite surfaces that have been imaged have been made, and names have been assigned to many surface features. Summaries of the surface features of the six satellites with imaged surface features are given below. The images with the best resolution obtained were Ariel and Miranda because Voyager 2 flew closest to them.

Oberon

Oberon, Uranus' most distant and second largest satellite, has extensive, heavily cratered terrain interrupted by canyons (rift valleys) and scarps. Some craters are surrounded by bright ray systems; others show dark matter on their floors. A prominent feature on Oberon's limb is a large mountain 7 mi (11 km) high and 28 mi (45 km) wide. Oberon shows the least evidence from modification of its cratered terrain by geologic activity of any Uranian satellite. Image resolution is poor due to Oberon's distance.

Titania

Titania, Uranus' largest and most massive satellite, shows similar geological features to those found on Oberon. Heavily cratered plains are the most extensive surfaces found here. Oberon also shows a global rift valley network related to global tectonics . Image resolution is somewhat better for Titania than Oberon, since Voyager 2 was closer to Titania. A prominent system of canyons and scarps is the most noticeable class of features on Titania's surface; shadows indicate some of them may be as deep as 3.7 mi (6 km). Moderately cratered and smooth plains are also observed, which indicate resurfacing. These features indicate that more geologic activity occurred on Titania than on Oberon after the end of the heavy bombardment (crater formation) phase of their histories. Here the resurfacing material should be liquid or icy water, ammonia, or methane, perhaps mixed with rocky material, rather than terrestrial type volcanic lava. Impact and/or internal heating may have melted or softened these ices, which then erupted and flowed over the satellite surfaces, resurfacing them. This phenomenon is called cryovolcanism (sometimes "water volcanism"), and it may have been important in the geological histories of many of the satellites of the Jovian planets.

Umbriel

With an average albedo of 0.21, Umbriel is the darkest of Uranus' five largest satellites. It also has a more uniform albedo than Titania and Oberon, showing only a few striking albedo features on the part of its surface imaged by Voyager 2. Umbriel also has extensive, heavily cratered terrain and several groups of canyons, scarps, and lineaments which seem to be of more recent origin than the craters which they cut. Evidence is also seen of resurfacing in and near several craters, indicating cryovolcanism. Much of its surface may have been resurfaced with dark material of uncertain origin (cryovolcanic, dark ejecta from a crater, or from an external source).

Ariel

Ariel is similar in size and mass to Umbriel, but whereas Umbriel is the darkest of Uranus' five largest

satellites, Ariel is the brightest, with a 0.40 average albedo. Ariel's surface seems to have evolved more like Titania's surface than that of Umbriel; it shows global-scale faulting (canyons, scarps, and lineaments) and resurfacing by cryovolcanism, but on a more extensive scale than occurred on Titania. Ariel's most heavily cratered plains show less cratering than the most cratered units on Titania and Oberon. Evidence has been found for the extrusion of ice into Ariel's surface, filling part of a valley and partially burying an impact crater .

Miranda

Miranda shows the most interesting evidence for geologic activity and surface modification among Uranus' satellites. In addition to old cratered plains, canyons, scarps, lineaments, and valleys, the three coronae Arden, Elsinore, and Inverness exist on its surface; they are its most prominent surface features. They are large, lightly cratered regions of up to 186 mi (300 km) or more extent; Arden shows banded regions, while Elsinore and Inverness show numerous grooves and ridges. A cliff near Miranda's south pole may be as high as 12 mi (20 km). One plausible model for Miranda's evolution hypothesizes the following sequence of events. First, Miranda accreted from several smaller bodies near Uranus and nearly in its equatorial plane; impacts during accretion formed the old cratered plains. Some of the canyon systems were formed near the end of accretion. Next, a large body impacted Miranda, forming the Arden basin . Debris was scattered over and ejected from Miranda, and cryovolcanism flooded the basin, forming Arden Corona. Then the Inverness basin was flooded, forming Inverness Corona. The last main cryovolcanic activity formed Elsinore Corona. This left Miranda's surface early in its present state, with only a few more recent craters added since then.

Puck

Puck's surface was also imaged by Voyager 2, revealing a cratered surface that is considerably darker (0.08 average albedo) than that for any of the five larger satellites. The Voyager 2 images also show that Puck is almost spherical. The three largest impact craters on Puck are named Bogle, Lob, and Butz.

See also Planetary atmospheres; Space probe.

Resources

books

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge Univ. Press, 1999.

Bergstrahlh, Jay T., Ellis D. Miner, and Mildred S. Matthews, eds. Uranus. Tucson: University of Arizona Press, 1991.

de Pater, Imke and Jack J. Lissauer. Planetary Sciences Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

periodicals

Beatty, J.K. "Voyager 2's Triumph." Sky & Telescope 72, no. 4 (October 1986): 336-342.

Cuzzi, Jeffrey, and Larry Esposito. "The Rings of Uranus." Scientific American (July 1987): 52-66.

Dowling, T. "Big Blue: Twin Worlds of Uranus and Neptune." Astronomy 18, no. 10 (October 1990): 42-53.

other

Arnett, B. SEDS, University of Arizona. "The Nine Planets, a Multimedia Tour of the Solar System." (November 6, 2002) [cited February 8, 2003]. <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.

Frederick R. West

David T. King, Jr.

Uranus

Uranus was the first planet to be discovered that had not been known since antiquity. Although Uranus is just bright enough to be seen with the naked eye, and in fact had appeared in some early star charts as an unidentified star, English astronomer William Herschel was the first to recognize it as a planet in 1781.

The planet's benign appearance gives no hint of a history fraught with catastrophe: Sometime in Uranus's past, a huge collision wrenched the young planet. As a result, the rotation pole of Uranus is now tilted more than 90 degrees from the plane of the planet's orbit. Uranus travels in a nearly circular orbit at an average distance of almost 3 billion kilometers (1.9 billion miles) from the Sun (about nineteen times the distance from Earth to the Sun).

A Somewhat Small Gas Planet

The composition of Uranus is similar to that of the other giant planets^* and the Sun, consisting predominantly of hydrogen (about 80 percent) and helium (15 percent). The remainder of Uranus's atmosphere is methane (less than 3 percent), hydrocarbons (mixtures of carbon, nitrogen, hydrogen, and oxygen), and other trace elements. Uranus's color is caused by the methane, which preferentially absorbs red light, rendering the remaining reflected light a greenish-blue color.

Like Jupiter and Saturn, Uranus is a gas planet, although a somewhat small one (at its equator, its radius is about 25,559 kilometers [15,847 miles]). We see the outermost layers of clouds, which are probably composed of icy crystals of methane. Below this layer of clouds, the atmosphere gets thicker and warmer. Deep within the center of Uranus, at extremely high pressure, a core of rocky material is hypothesized to exist, with a mass almost five times that of Earth.

One of the more puzzling aspects of Uranus is the lack of excess heat radiating from its interior. In comparison, the other three giant planets radiate significant excess heat. Astronomers believe that this excess heat is left over from the time of the planets' formation and from continuing gravitational contraction . Why then does Uranus have none? Scientists theorize that perhaps the heat is there but is trapped by layers in the atmosphere, or perhaps the event that knocked Uranus over on its side somehow caused much of the heat to be released early in the planet's history.

Magnetic Field

When the Voyager 2 spacecraft flew by Uranus in 1986, it detected a magnetic field about fifty times stronger than that of Earth. In a surprising twist, the magnetic field's source was not only offset from the center of the planet to the outer edge of the rocky core, but it was also tilted nearly 60 degrees from the planet's rotation axis. From variations in the magnetic field strength detected by Voyager 2, scientists determined that the planet's internal rotation period was 17.2 hours. The winds in the visible cloud layers have rotation periods ranging from about 16 to 18 hours depending on latitude, implying that wind speeds reach 300 meters per second (670 miles per hour) for some regions.

The Moons of Uranus

Within six years of the discovery of Uranus, two moons were discovered. They were subsequently named Titania and Oberon. It was more than sixty years before the next two Uranian moons, Ariel and Umbriel, were discovered. Nearly a century elapsed before Miranda was discovered in 1948, bringing the total of Uranus's large moons to five. Little was known about their surface structure or history until the Voyager 2 spacecraft returned detailed images of the surfaces of these moons.

On Miranda, huge geologic features dominate the small moon's landscape, indicating that some kind of intense heating must have occurred in the past. It is not yet clear whether a massive collision disturbed the small moon, which then reassembled into the current jumble, or whether, in the past, tidal interactions with other moons produced heat to melt and modify the surface, as is the case for Jupiter's moon Io. Oberon, the outermost major moon, shows many large craters, some with bright rays . Titania has fewer large craters, indicating that its surface has been "wiped clean" by resurfacing sometime in the moon's past. Ariel has the youngest surface of the major moons, based on cratering rates. Umbriel is much darker and smoother. Its heavily cratered surface is probably the oldest of the satellites.

In 1986, Voyager 2 discovered ten additional moons, with Puck being the largest. Voyager 2 images of Puck showed it to be an irregularly shaped body with a mottled surface. Voyager 2 did not venture close enough to the other small moons to learn much about them. Since 1986, six more tiny moons have been discovered around Uranus, bringing its total to twenty-one. Little is known about these moons other than their sizes and orbits.

Rings and Seasons

In 1977, astronomers discovered that Uranus has a ring system. Voyager 2 studied the rings in detail when it flew by Uranus in 1986. There are nine well-defined rings, plus a fainter ring and a wider fuzzy ring. Unlike the broad system of Saturnian rings, the main Uranian rings are narrow. The rings are not perfectly circular and also vary in width. Like the rings of Saturn, the Uranian rings are thought to be composed mainly of rocky material (ranging in size from dust particles to house-sized boulders) mixed with small amounts of ice.

The atmosphere of Uranus has often been called bland, and even boring. These epithets are a consequence of fate and unfortunate timing. It was fate that caused the early collision of Uranus with a large body, creating the planet's extreme axial tilt, which in turn created extreme seasons. It was unfortunate timing that the Voyager 2 encounter (which gave us our highest resolution pictures) occurred at peak southern summer, when we had a view of only the southern half of the planet. Historically, this season is when Uranus has appeared blandest in the past.

As Uranus continues its eighty-four-year-long progression around the Sun, its equatorial region is now receiving sunlight again, and parts of its northern hemisphere are being bathed in solar radiation for the first time in decades. Today, images from the Hubble Space Telescope are revealing multiple bright cloud features and stunning banded structures on Uranus. It is fascinating to speculate how Uranus will appear to us by the time it reaches equinox in 2007.

see also Exploration Programs (volume 2); Herschel Family (volume 2); NASA (volume 3); Neptune (volume 2); Robotic Exploration of Space (volume 2).

Heidi B. Hammel

Bibliography

Bergstralh, Jay T., Ellis D. Miner, and Mildred Shapley Matthews, eds. Uranus. Tucson, AZ: University of Arizona Press, 1991.

Miner, Ellis D. Uranus: The Planet, Rings, and Satellites, 2nd ed. Chichester, UK: Praxis Publishing, 1998.

Standage, Tom. The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. New York: Walker and Company, 2000.

^*There are four giant planets in the solar system: Jupiter, Saturn, Uranus, and Neptune

Uranus

Uranus, the seventh planet from the Sun, was probably struck by a large object at some point in its history. The collision knocked the planet sideways, giving it a most unique orbit. Unlike the other planets, whose axes are generally upright on their orbits, Uranus rotates on its side with its axis in the plane of its orbit.

It takes the planet slightly more than 84 Earth years to complete one revolution around the Sun and almost 18 Earth hours to complete one rotation about its axis. Because Uranus's poles—and not its equator—face the Sun, each pole is in sunlight for 42 continuous Earth years.

Discovery of the planet

Uranus was discovered in 1781 by German astronomer William Herschel (1738–1822) during a survey of the stars and planets. At first, Herschel thought he had spotted a comet, but the object's orbit was not as elongated as a comet's would normally be. It was more circular, like that of a planet. Six months later, Herschel became convinced that this body was indeed a planet. The new planet was given two tentative names before astronomers decided to call it Uranus, the mythological father of Saturn.

Uranus is about 1.78 billion miles (2.88 billion kilometers) from the Sun, more than twice as far from the Sun as Saturn, its closest neighbor. Thus, the discovery of Uranus doubled the known size of the solar system.

Uranus is 31,800 miles (51,165 kilometers) in diameter at its equator, making it the third largest planet in the solar system (after Jupiter and Saturn). It is four times the size of Earth. Similar to Jupiter, Saturn, and Neptune, Uranus consists mostly of gas. Its pale blue-green, cloudy atmosphere is made of 83 percent hydrogen, 15 percent helium, and small amounts of methane and hydrocarbons. Uranus gets its color because the atmospheric methane absorbs light at the red end of the visible spectrum and reflects light at the blue end. Deep down into the planet, a slushy mixture of ice, ammonia, and methane surrounds a rocky core.

Voyager 2 mission to Uranus

Most of what is known about Uranus was discovered during the 1986 Voyager 2 mission to the planet. The Voyager 2 space probe left Earth in August 1977. It first visited Jupiter in July 1979, then Saturn in August 1981.

Voyager 2 collected information on Uranus during the first two months of 1986. At its closest approach, on January 24, it came within 50,600 miles (81,415 kilometers) of the planet. Among its most important findings were ten previously undiscovered moons (bringing the total to fifteen) and two new rings (bringing the total to eleven). Voyager also made the first accurate determination of Uranus's rate of rotation and found a large and unusual magnetic field. Finally, it discovered that despite greatly varying exposure to sunlight, the planet is about the same temperature all over: about −346°F (−210°C).

Uranus's moons

Before Voyager 2 's visit, scientists believed Uranus had just five moons: Titania, Oberon, Umbriel, Ariel, and Miranda. After a rash of discoveries in the late 1990s, it is now known that Uranus has a complex system of twenty-one natural satellites, each with distinctive features (many of the moons are named for characters in plays by English dramatist William Shakespeare) The five previously discovered moons of Uranus range in diameter from about 980 miles (1,580 kilometers) to about 290 miles (470 kilometers). The largest of the newly discovered moons is 99 miles (160 kilometers) in diameter, just larger than an asteroid. The smallest is a mere 12.5 miles (20 kilometers) wide. The moons

fall into three distinct classes: the original five large ones; the eleven small, very dark inner ones uncovered by Voyager 2; and the five newly discovered much more distant ones.

Voyager 2 determined that the five largest moons are made mostly of ice and rock. While some are heavily cratered and others have steep cliffs and canyons, a few are much flatter. This discovery suggests varying amounts of geologic activity on each moon, such as lava flows and the shifting of regions of lunar crust.

Uranus's rings

The original nine rings of Uranus were discovered only nine years before Voyager 2 's visit. It is now known that Uranus has eleven rings plus ring fragments consisting of dust, rocky particles, and ice. The eleven rings lie between 23,500 and 31,700 miles (38,000 and 51,000 kilometers) from the planet's center. The extremely dark rings range in size from less than 1 mile to 60 miles (0.5 to 95 kilometers) wide.

[See also Solar system ]

Uranus

Nationality/Culture

Greek

Pronunciation

YOOR-uh-nuhs

Alternate Names

Caelus (Roman), Aeon (Roman)

Appears In

Hesiod's Theogony

Lineage

Son of Gaia

Character Overview

Uranus, who represented the sky, was one of the original deities (gods and goddesses) of Greek mythology . He was the son of Gaia (pronounced GAY-uh), the earth, who also became his wife. Together they had many children, including the Titans and the Cyclopes (pronounced sigh-KLOH-peez). He was eventually overthrown by his son Cronus (pronounced KROH-nuhs).

Major Myths

Uranus detested the children he had with Gaia. As soon as they were born, he forced them into Tartarus (pronounced TAR-tur-uhs), a dark place deep beneath the surface of the earth. This caused Gaia great pain. She asked her children to stop Uranus, but only her son Cronus came to her aid. Cronus cut off his father's sex organs with a flint-bladed sickle and threw them into the sea. According to myth, Aphrodite (pronounced af-ro-DYE-tee) was born from the foam where they landed. Uranus became the sky that surrounds the earth, and Cronus replaced his father as king of the universe. But Cronus was later defeated by his son Zeus (pronounced ZOOS) who, together with Hera (pronounced HAIR-uh) and other Olympian gods, overthrew the Titans and took their place ruling the universe.

Uranus in Context

To the ancient Greeks, the mention of the sickle having a flint blade was significant, and puts the era of the old gods into a proper context. Flint, an easily splintered stone, was used in the creation of tools and weapons long before humans had mastered the art of metalworking. Ancient cultures throughout Greece and the surrounding regions were effectively creating bronze tools and weapons at least two thousand years before the writings of mythographers like Hesiod, and in fact were mastering the creation of iron and steel products by that time. For the myth of Uranus to specifically mention a flint blade, then, reflects either the original age of the myth or an attempt to illustrate that the tales of the old gods took place in the very distant past of the ancient Greeks.

Key Themes and Symbols

One of the central themes in the myth of Uranus is the overthrow of the heavenly hierarchy. This is shown in Cronus's rebellion against Uranus, as well as by the prediction that Cronus will suffer the same fate as his father. For the ancient Greeks, Uranus represented the most ancient of beliefs and traditions, two generations removed from their own beliefs in the Olympian gods.

Uranus in Art, Literature, and Everyday Life

Though he lies at the heart of an ancient Greek creation myth, Uranus was seldom worshipped and only occasionally depicted in art. Roman artists sometimes showed a slighdy different version of Uranus, known as Aeon and considered to be the god of time, standing over Gaia and holding the wheel of the zodiac (another representation of time). In 1781, when the sixth planet of our solar system was discovered by William Herschel, it was named Uranus in honor of the oldest god.

Read, Write, Think, Discuss

Most of the planets in our solar system are named after gods and goddesses from Greek and Roman mythology. For several, their names were determined based on similarities to their mythical counterparts. Based on what you know about classical mythology, can you figure out why the planets Mercury, Jupiter, Saturn, and Uranus were named the way they were? You may have to do some research about the planets in order to find the answers.

SEE ALSO Cronus; Titans