Discovery – On March 13, 1781, while continuing a project to chart all stars down to the eighth magnitude, William Herschel observed that one of the “stars” that came into view appeared as a small disk. At first, Herschel suspected that the object was a comet, but other astronomers quickly showed that it was a planet orbiting the Sun beyond Saturn. This was the first planet to be discovered that was not known in ancient times.
After Herschel’s discovery, astronomers looked through records of previous observations and found some 20 cases where Uranus had been seen. However, it had been mistaken for a star, since earlier observers did not have telescopes as good as Herschel’s and could not see the disk.
Spacecraft Exploration – NASA’s Voyager2 is the only spacecraft to have visited the planet. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986.
Location – Uranus is the seventh planet from the Sun, with an average distance of approximately 1,780,000,000 miles. Its discovery doubled the size of the known solar system.
Size – Uranus measures 31,693 miles in diameter.
Orbital Period – 84.01 years.
Rotational period – 17.24 hours (retrograde).
Inclination of rotation axis – 97.86º 2. Consequently, for part of Uranus’ orbit, one pole faces the Sun continually while the other pole faces away. At the other side of its orbit, the orientation of the poles towards the Sun is reversed. Between these two extremes of the planet’s orbit, the Sun rises and sets around the equator 3.
Knocked Over – It is thought that an impact by an Earth-sized protoplanet may have caused the high inclination of Uranus’ rotation axis. However, there is no detailed theory yet that satisfactorily describes such an encounter and its effects.
Number of Satellites – 27 known moons.
Name – Herschel originally named this new planet “Georgium Sidus” (George’s star) after his king and patron, George III of England. When it was pointed out that sidus means star and not planet, he renamed it the Georgian Planet. However, this name was not acceptable outside of Britain. By the suggestion of Johann Bode, the ancient tradition for planetary names was continued by using the name Uranus, the Latinized version of the Greek god of the sky (Ouranos).
Composition – Uranus’ atmosphere is composed of hydrogen (83%), helium (15%), and methane (2%). The remainder is comprised of trace amounts of hydrogen deuteride, ethane, water, and hydrocarbons. These hydrocarbons formed through photochemical reactions.
Ammonia – Unlike Jupiter and Saturn, ammonia is not observed in the upper atmosphere of Uranus. Because of the low temperature (about -350 F) associated with Uranus’ upper atmosphere, ammonia cannot exist in a gaseous state.
Structure – Voyager’s encounter with Uranus showed an atmosphere blue-green in color with almost no discernible features. However, it appears that the atmospheric blandness is due to seasonal effects. As Uranus approaches the position in its orbit where the Sun is directly over the equator, pronounced weather effects have become apparent. For example, in 2006, the Hubble Space Telescope obtained the first images of dark spots within the planet’s atmosphere 4. The few clouds detected within the planet’s atmosphere are composed of condensed methane, and like Jupiter and Saturn, exhibit a banded pattern parallel to latitude lines caused by Uranus’ rapid rotation and lack of a solid (obstructive) surface 4. In addition, Voyager revealed that there is a tenuous haze in Uranus’ upper atmosphere. The color of Uranus’ atmosphere is due to the absorption of red light by methane gas, so the sunlight reflected off the clouds has a blue-green hue 5.
Composition – If Uranus had the same composition as Jupiter and Saturn, it should have a lower density than that of Saturn, since it has only about 15% of its mass. Instead, Uranus has about the same density as Jupiter – 1.27 g/cm3. This indicates that Uranus contains a higher proportion of heavier elements. Rock and ice are the likely candidates for the dense material. Apparently, Uranus acquired a less massive envelope of solar nebula gas than Jupiter and Saturn, and therefore has a relatively higher proportion of rock and ice.
Structure 6 – Modeling of Uranus’ internal structure suggests that it consists of three layers: a core of silicate rock (which may include some nickel-iron), an icy mantle, and a gaseous hydrogen/helium envelope (atmosphere). The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid is sometimes called a water-ammonia ocean.
Diamonds – Research suggests that the extreme pressure and temperature deep within Uranus may break up methane molecules (CH4), with the carbon atoms condensing into crystals of diamond that rain down through the mantle like hailstones. Other research suggests that the base of the mantle may comprise an ocean of liquid diamond, with floating solid “diamond-bergs”.
Magnetic Field – Deep inside Jupiter and Saturn, convection creates and sustains powerful dynamos in the liquid metallic hydrogen zones, which in turn generate immense magnetic fields. Although weaker, Uranus also has a magnetic field, indicating its interior must consist of a layer of electrically conductive fluid undergoing convection. However, Uranus is too small to achieve the high pressure and temperature required for the formation of liquid metallic hydrogen. Therefore, what material generates Uranus’ magnetic field? Through simulations, researchers have shown that Uranus’ internal pressure and temperature is sufficient to ionize water and ammonia within the mantle, thus causing a modest increase in their electric conductivity.
Offset and Tilted – Experience with the Earth, Jupiter, and Saturn, suggested that the axis of a planetary magnetic field should always be roughly aligned with the axis of the planet’s rotation 7. However, Uranus’ magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60º with respect to the axis of rotation 8. One hypothesis for it being offset and tilted is that, unlike the magnetic fields of Earth, Jupiter, and Saturn, which are generated within or near their cores, Uranus’ magnetic field is generated at relatively shallow depths within the water-ammonia ocean. Another possible explanation is that the ocean of liquid diamond in Uranus’ interior causes these anomalies within the magnetic field.
Internal Heat – Uranus does not radiate more heat than it receives from the Sun like the other Jovian planets. In fact, it radiates less than 10% of the amount of heat that it receives. Hypotheses for this discrepancy include: (1) when Uranus was “knocked over” by the massive impactor which caused its extreme axial tilt, the event also caused it to expel most of its primordial heat and (2) some form of barrier exists in Uranus’ upper layers which prevent the core’s heat from reaching the surface.
Discovery 10 – On March 10, 1977, the rings of Uranus were discovered quite unexpectedly. An experiment called stellar occultation (the passage of an object in front of a star) was being used by a team of scientists to study Uranus’ atmosphere. However, the star began to wink out about 40 minutes before it first should have been affected by the upper atmosphere of the planet. Several times the star dimmed dramatically for intervals that lasted from 2 to 8 seconds, then returned to full brightness. Evidently, something was briefly blocking the starlight. At first it was thought a swarm of small satellites was the cause. However, as Uranus moved beyond the star, exactly the same sequence of occultations occurred, but in reverse order. What had been discovered was a series of rings.
Structure and Composition 11 – The Voyager 2 spacecraft confirmed the existence of Uranus’ ring system. Images taken by the spacecraft revealed eleven narrow rings (separated by broad gaps), ranging from 1 mile to about 30 miles in width. Voyager 2 images also revealed that the “Epsilon” ring (the brightest) is herded into shape by two small satellites, Cordelia and Ophelia 12. The particles of these eleven rings are dark, possibly composed of (or at least coated by) carbonaceous material. The rings particles are fairly large (up to about 30 feet in diameter) with a mixture of dust.
Outer Ring System – In December 2005, images taken by the Hubble Space Telescope revealed two previously unknown rings (bringing the total number of known rings up to thirteen). These two rings are so far from the planet that they are being called Uranus’ “outer ring system”. The outermost ring is blue in color and possibly composed of dust-sized particles of water-ice originating from small impacts of a minor moon named Mab 13.
Origin – Uranus’ ring system is thought to be relatively young, no more than 600 million years old. It probably originated from the collisional fragmentation of a number of moons that once existed around the planet; a process that is most likely still occurring with the collision of moonlets to renew the rings.
Major Moons 14 – Uranus has five major moons. In order of increasing distance from the planet, they are: Miranda, Ariel, Umbriel, Titania, and Oberon (names chosen from characters from the works of Shakespeare and Alexander Pope). Oberon, Titania, Ariel, and Umbriel are similar in size, about 700 to 1000 miles in diameter. Miranda is considerably smaller, with a diameter of only about 300 miles. The densities of the five moons range from 1.3 to 1.6 g/cm3. Based on each moon’s size and density, their internal compositions are similar, about half rock and half ice. Uranus’ major moons occupy orbits lying in the plane of its equator. Thus, their orbits share the unusual inclination of the planet itself, and must have formed from a protosatellite nebula after Uranus was tipped over on its side.
Miranda 15 – Miranda has one of the most extreme and varied topographies of any object in the Solar System. Miranda’s surface is comprised of two terrains: (1) an old, heavily cratered surface that is similar to the lunar highlands, and therefore is probably more than 4 billion years old and (2) a terrain criss-crossed by huge faults, with patchwork regions of broken terrain indicating intense geological activity in the moon’s past. The only plausible heat source to drive such activity is tidal energy. If so, it must have been a transient event, as Miranda is not now active.
Verona Rupes 16 – Of Miranda’s series of faults, the most spectacular is an enormous scarp called Verona Rupes. It is the tallest known cliff in the solar system, ranging from 3 to over 6 miles high.
Coronae 17 – Miranda’s broken terrain consists of three large grooved structures referred to as coronae, which may be the result of “warm” ice upwelling from the moon’s interior. One model holds that when Miranda accreted, it was a uniform mixture of rock and ice. Because of internal heating, the small moon began to differentiate. Masses of dense rock sank towards the center, and plumes of lighter ice rose past the margins of the rock and eventually breached the surface as volcanic liquids to create the coronae. However, differentiation ceased, never to be completed. If it had been completed, more ice would have risen to the surface and the coronae would have been smoothed over.
Ariel 18 – Ariel has the brightest and youngest surface of Uranus’ moons. Moderately cratered plains are broken up by an extensive system of fractures and faults, which may be related to global expansion – the freezing of water causing the interior to expand. Ariel’s past geologic activity is believed to have been driven by tidal heating caused by orbital resonance with other Uranian moons.
Canyons – Some of these fractures and faults form canyons, as deep as the Grand Canyon 19. The floors of these canyons and portions of the plains have been resurfaced by smooth material, probably produced by cryovolcanic eruptions of slushy mixtures of water and ammonia ice. A water-ammonia mixture does not require as high a melting point as pure liquid water to flow; a necessary property for the extremely cold temperature associated with Ariel.
Umbriel 20 – Umbriel has the darkest surface of Uranus’ moons. However, the process by which its surface was darkened remains unknown. Umbriel’s surface also appears to have been shaped primarily by impacts. However, the presence of canyons suggests early tectonic processes. In addition, Umbriel may have undergone a tectonically driven resurfacing event that obliterated its older surface.
Wunda – Wunda is a large crater measuring about 80 miles in diameter, located near the equator of Umbriel. The crater has a large ring of bright material on its floor, which appears to be a deposit of fresh ice.
Titania 21 – Titania’s surface appears somewhat similar to, though a little older than, the surface of Ariel. It has many small, but few large, craters. Titania apparently underwent an active phase in which cyrovolcanism erased the early craters. Since then, more craters have formed, but few are as large as those that were erased. There is a multi-ringed basin about 200 miles in diameter, near the moon’s equator, named Gertrude. Titania’s surface is also intersected by scarps and canyons, which includes Messin Chasm 22, the most prominent canyon measuring over 900 miles long and extending from Titania’s equator to almost the south pole.
Formation – The initial accretional heating together with radioactive decay were probably strong enough to melt the ice if some antifreeze like ammonia was present. This melting may have led to the separation of ice from rock and the formation of a rocky core surrounded by an icy mantle. A water-ammonia subterranean ocean may have formed at the core-mantle boundary. If the temperature sufficiently dropped, this ocean would have subsequently froze, causing global expansion, and thus the formation of the scarps and canyons.
Oberon 23 – Oberon’s surface is the most heavily cratered, and therefore the oldest of Uranus’ moons. Its crater density approaches saturation – when the formation of new craters is balanced by destruction of old ones. Many large craters are surrounded by bright impact ejecta consisting of relatively fresh ice. The floors of the largest craters (Hamlet, Othello and Macbeth), appear to be covered with very dark material (possibly carbon-rich) deposited after their formation. Voyager images revealed a peak nearly 7 miles high near the limb of Oberon, which may be the central peak of an impact basin. Oberon’s surface is also intersected by a system of canyons, which, however, are less widespread than those found on Titania. Furthermore, Oberon may have formed in a very similar manner as Titania.
Minor Moons 24 – To date, Uranus has 22 minor moons. These moons may have been initially asteroids captured by the planet’s gravity and/or fragments from intersatellite collisions. They are irregular in shape and appear quite dark. The dark coloration suggests that either icy materials at their surfaces darkened by some sort of aging process involving methane ice, or the material is an undifferentiated mixture of ice, silicate, and carbonaceous material. If the latter explanation is correct, these small moons preserve some of the most chemically primitive material in the outer solar system.