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2,870,990,000 km (19.218 AU) from the Sun, Uranus hangs on the wall of space as a mysterious blue green planet. With a mass of 8.683e25 kg and a diameter of 51,118 km at the equator, Uranus is the third largest planet in our solar system. It has been described as a planet that was slugged a few billion years ago by a large onrushing object, knocked down (never to get up), and now proceeds to roll around an 84-year orbit on its belly. As the strangest of the Jovian planets, the description is accurate. Uranus has a 17 hour and 14 minute day and takes 84 years to make its way about the sun with an axis tilted at around 90° with retrograde rotation. Stranger still is the fact that Uranus' axis is almost parallel to the ecliptic, hence the expression "on its belly".
Uranus is so far away that scientists knew comparatively little about it before NASA's Voyager 2 undertook its historic first encounter with the planet. The spacecraft flew closely past distant Uranus, and came within 81,500 kilometers (50,600 miles) of Uranus's cloudtops on Jan. 24, 1986. Voyager 2 radioed thousands of images and mass amounts of other scientific data about Uranus, its moons, rings, atmosphere, interior and magnetic environment. However, while Voyager has revealed much about the gas giant, many questions remain to be answered.
The history of the planet's discovery is the first we have of its kind; Uranus was the first planet to be discovered with a telescope. The circumstances surrounding the discovery of the object are befitting of the odd planet. The earliest recorded sighting of Uranus was in 1690 by John Flamsteed, but the object was catalogued as another star. On March 13, 1781 Uranus was sighted again by amateur astronomer William Herschel and thought to be a comet or nebulous star. In 1784, Jean-Dominique Cassini, director of the Paris Observatory and prominent professional astronomer, made the following comment:
'A discovery so unexpected could only have singular circumstances, for it was not due to an astronomer and the marvelous telescope…was not the work of an optician; it is Mr. Herschel, a [German] musician, to whom we owe the knowledge of this seventh principal planet.' (Hunt, 35)
Four years passed before Uranus was recognized as a new planet, the first to be discovered in 'modern' times.
The discovery poses an interesting question however. Why Herschel and not someone like Cassini - a director of a prominent Observatory?
It was by no accident that he discovered the first new planet. William Herschel had more than a passing fancy for the telescope. By purchasing the materials and even grinding the lenses himself, he built telescopes (namely reflectors) of exceptional quality for the time period. That same quality afforded Herschel better observational conditions than his contemporaries, and the result was a changed view of astronomy. A new planet had been discovered, and our view of the solar system was never to be the same again.
The atmosphere and geology of the first new planet is fascinating. Uranus is primarily composed of rock and various ices; with only about 15% hydrogen and a little helium - in contrast to the compositions of Jupiter and Saturn, which are mostly hydrogen. Uranus' average temperature is around 60 Kelvin (- 350 degrees Fahrenheit) and the atmosphere is made of 83% hydrogen, 15% helium and 2% methane. The blue color we often see is the result of absorption of red light absorbed by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer. Just below the clouds visible to earthbound observers are enormous quantities of ammonia, hydrogen sulfide, and water. Still deeper inside Uranus, under the crushing weight of the overlying atmosphere, is an invisible rocky surface - discovered only by its subtle tugs on the planet's moons. A big Earth-sized planet is hiding down there, swathed in an immense blanket of air. Like the other gas giants, Uranus has bands of clouds that blow around rapidly. However, they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures. Recent observations made with the Hubble Space Telescope show larger and more pronounced streaks. In the past two years the speculation has been that the difference is due to seasonal effects. The speed of the winds on Uranus is changing, and while that is not exciting for a person inhabiting the Earth and used to its changeable weather, the news is noteworthy for a gas giant. The winds of Jupiter and Saturn have remained constant over time. The winds of Uranus blow at velocities of 40 to 160 meters per second (90 to 360 mph); whereas on Earth, jet streams in the atmosphere only blow at about 50 meters per second (110 mph). Astronomers are excited that these observations could foreshadow dramatic atmospheric changes in the future. Compared with recent pictures from space, black and white drawings of Uranus - rendered by visual astronomers in the early 1900's - "depict a vastly different planet, decorated with bright, broad bands, and even the hint of something that might be a great spot." (Flanagan, 56) Significantly, they were drawn at a time when Uranus was between its solstice and its equinox, the same phase the planet is approaching now.
There is more to the puzzling features of Uranus than changing winds. Data from Voyager 2 indicates that Uranus' magnetic field is not centered on the midway point of the planet and is tilted at nearly 60 degrees with respect to the axis of rotation. The magnetic field of Uranus - which is roughly comparable to that of Earth's - is not produced by an iron core like other planets. The magnetic field source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent. The magnetic fields of Earth and other planets are believed to arise from electrical currents produced in their molten cores, but if Uranus possessed one, it would be too small and too deep for it to create such a magnetic field. As with Mercury, Earth, Jupiter and Saturn, there is a magnetic tail extending millions of miles behind Uranus. Voyager measured the tail to be at least 10 million kilometers (6.2 million miles) behind the planet. The extreme tilt of the magnetic axis, combined with the tilt of the rotational axis, causes the field lines in this cylindrical magnetic tail to be wound into a corkscrew shape that spins like a lawn sprinkler across the galaxy.
The exotic magnetosphere of Uranus is contrasted by the planet's rather mundane ring system. Like the other gas planets, Uranus has rings. They are very dark in color like Jupiter's, but more like Saturn's rings in size and composition with both fine dust and large particles ranging up to 10 meters in diameter. There are 11 known rings, all relatively faint, the brightest of which is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered - which was of considerable importance since we now know that rings are a more common feature of planets than first thought, and not a peculiarity of Saturn alone. All nine of the previously known rings of Uranus were photographed and measured by Voyager 2, as were other new rings and ringlets in the Uranian system. These observations showed that while Uranus's rings shared similarities with the systems of Jupiter and Saturn, they are also distinctly different. Radio measurements from Voyager 2 showed the outermost ring, the epsilon, to be composed mostly of ice boulders several feet across. However, a very tenuous distribution of fine dust also seems to be spread throughout the ring system.
Incomplete rings and the varying opacity in several of the main rings leads scientists to believe that the ring system may be relatively young and did not form at the same time as Uranus. The 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. To date, two new rings have been positively identified. The first, 1986 U1R, was detected between the outermost of the previously known rings - epsilon and delta - at a distance of 50,000 kilometers (31,000 miles) from Uranus's center. It is a narrow ring like the others. The second, designated 1986 U2R, is a broad region of material perhaps 3,000 kilometers (1,900 miles) across and just 39,000 kilometers (24,000 miles) from the planet's center. The number of known rings may eventually grow as a result of observations by the Voyager 2 photopolarimeter instrument. The sensor revealed what may be a large number of narrow rings - or possibly incomplete rings or ring arcs - as small as 50 meters (160 feet) in width. The individual ring particles are not very reflective, which explains why some have remained unseen. At least one ring, the epsilon, was found to be gray, an unusual color. This ring is surprisingly deficient in particles smaller than the approximate size of a beach ball - the average ring contains smaller dust sized (relatively) particles. This may be due to the atmospheric drag from the planet's extended hydrogen atmosphere, which may siphon smaller particles and dust from the ring. The sharp edge of the epsilon ring indicates that the ring is less than 150 meters (500 feet) thick and that particles near the outer edge are less than 30 meters (100 feet) in diameter. Important clues to Uranus's ring structure may come from the discovery that two small moons - Cordelia and Ophelia - straddle the epsilon ring. This finding hints that small moonlets may be responsible for confining or deflecting material into rings and keeping it from escaping into space. Astronomers expected to find 18 such satellites, but only two were photographed.
The satellites of Uranus form two distinct classes: the 10 small very dark inner ones discovered by Voyager 2 and the five large outer ones. They all have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic). Voyager 2 obtained clear, high-resolution images of each of the five large moons of Uranus known before the encounter: Miranda, Ariel, Umbriel, Titania and Oberon. The two largest, Titania and Oberon, are about 1,600 kilometers (1,000 miles) in diameter, roughly half the size of Earth's Moon. The smallest, Miranda, is only 500 kilometers (300 miles) across, or just one-seventh the lunar size.
The 10 new moons discovered by Voyager bring the total number of known Uranian satellites to 15. The largest of the newly detected moons, named Puck, is about 150 kilometers (about 90 miles) in diameter, larger than most asteroids. Preliminary analysis shows that the five large moons are ice-rock conglomerates like the satellites of Saturn. The large Uranian moons appear, in fact, to be about 50 percent water ice, 20 percent carbon and nitrogen-based materials, and 30 percent rock. Their surfaces, almost uniformly dark gray in color, display varying degrees of geologic history. Very ancient, heavily cratered surfaces are apparent on some of the moons, while others show strong evidence of internal geologic activity. Huge fault systems and canyons that indicate an active geologic history, for example, mark Titania. These features may be the result of tectonic movement in its crust. Ariel has the brightest and possibly, the geologically youngest surface in the Uranian moon system. It is largely devoid of craters greater than 50 kilometers (30 miles) in diameter. This indicates that low-velocity material within the Uranian system itself peppered the surface, helping to obliterate larger, older craters. Ariel also appears to have undergone a period of intense activity that lead to many fault valleys, and what appear to be extensive flows of icy material. Where many of the larger valleys intersect, their surfaces are smooth; this could indicate that the valley floors have been covered with younger icy flows. Umbriel is ancient and dark, appears to have undergone little geologic activity. Large craters pockmark its surface. The darkness of Umbriel's surface may be due to a coating of dust and small debris somehow created nearby and confined to the vicinity of that moon's orbit. The outermost of the moons discovered before Voyager, Oberon, also has an old, heavily cratered surface with little evidence of internal activity other than some unknown dark material apparently covering the floors of many craters.
Miranda , innermost of the five large moons, is one of the strangest bodies yet observed in the solar system. Voyager images, which showed some areas of the moon at resolutions of a kilometer or less, consists of huge fault canyons as deep as 20 kilometers (12 miles), terraced layers and a mixture of old and young surfaces. The younger regions may have been produced by incomplete differentiation of the moon, a process in which upwelling of lighter material surfaced in limited areas. Alternatively, Miranda may be a conglomerate of material from an earlier time when the moon was fractured into pieces by a violent impact. Given Miranda's small size and low temperature (-335 degrees Fahrenheit or -187 Celsius), the degree and diversity of the tectonic activity on this moon have surprised scientists. It is believed that an additional heat source such as tidal heating caused by the gravitational tug of Uranus must have been involved. In addition, some means must have mobilized the flow of icy material at low temperatures.
The Voyager 2 flyby mission has made plenty information available on the satellites, ring system, atmosphere and geology of Uranus. It would even appear that our knowledge of the gas giant is nearly complete. Yet, Uranus remains a stubbornly mysterious planet that jealously guards its secrets. Many questions taunt the talented astronomers of our day and there is yet much to be learned about the celestial blue green oddball that hangs on its side in outer space. Why is its axis so unusually tilted? Was it due to a massive collision? Why does Uranus have so much less hydrogen and helium than Jupiter and Saturn? Is it simply because its smaller? Or because it is farther from the Sun? What causes the unusual magnetic field? And sadly, just how many times has Miranda been blown to bits only to coalesce again? These questions and many more remain to encourage the continued study of such a planet. Perhaps in a few years and a couple of space probes later, we will have answers that are more concrete.
Source: Life, Nov 1989 v12 n13 p105(9).
Title: Fantastic voyage: the wondrous legacy of our first grand tour of the planets. (Project Voyager)
Author: Brad Darrach
Source: Astronomy, July 1997 v25 n7 p50(6).
Title: Observer of the gas giants. (astronomer Heidi Hammel's research on outer planets) (includes related information on observation of outer planets)(Interview)
Author: Ruth Flanagan.
Source: Science News, Nov 9, 1985 v128 p292(1).
Title: Uranus: strange encounter looming.
Author: J. Eberhart
Source: Science, April 24, 1998 v280 n5363 p570(3).
Title: Clouds of high contrast on Uranus.
Author: Erich Karkoschka
Source: Discover, April 1994 v15 n4 p 86(6).
Title: Secrets of the rings. (of the large planets)
Author: Dava Sobel
Source: Science News, Oct 20, 1990 v138 n16 p248(2).
Title: Locating Uranus' auroras: painstaking analyses reveal an outer glow.
Author: Jonathan Eberhart
Source: Astronomy, March 1995 v23 n3 p34(8).
Title: The first new planet. (Uranus)
Author: Carl Sagan
Source: Science News, May 14, 1988 v133 n20 p310(1).
Title: The high-pressure world of Uranus.
Author: Ivars Peterson
Editors: Jay T. Bergstralh
Ellis D. Miner
Mildred Shapely Matthews
Uranus and the Outer Planets
Editor: Gary Hunt