Page Uranus. Uranus's Structure

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1 Page Uranus Uranus, although small compared with Jupiter and Saturn, is much larger than the Earth. Its diameter is about 4 times that of the Earth, and its mass is about 15 Earth masses. Lying approximately 19 AU from the Sun (about twice Saturn's distance), Uranus is difficult to study from Earth, visible only as a blue but featureless disk. Even pictures of it taken by the Voyager 2 spacecraft show few details (fig ), although computer processing of the images shows that it has faint cloud bands. Figure Uranus as pictured from the Voyager 2 spacecraft. Because of Uranus's odd tilt, we are viewing it nearly pole on. Note the lack of clearly defined cloud belts. Uranus was unknown to the ancients, even though it is just visible to the naked eye. It was discovered by Sir William Herschel, a German émigré to England. Herschel, a musician, was at the time only an amateur astronomer interested in hunting comets, a task in which he collaborated with his sister Caroline. In 1781 he observed a pale blue object whose position in the sky changed from night to night. Herschel at first thought he had discovered a comet, but observations over several months showed that the body's orbit was nearly circular, and he therefore concluded that he had found a new planet. For this discovery, King George III named Herschel his personal astronomer, and to honor the king, Uranus was briefly known as Georgium Sidus, or George's Star. In ancient Greek mythology, Uranus was the father of Cronus and was identified as the God of the Heavens. Uranus's Structure Uranus, like Jupiter and Saturn, is rich in hydrogen and its compounds water, ammonia, and methane. We know this from spectra of its atmosphere, which show very strong absorption lines of methane. In

2 fact, it is methane that gives the planet its deep blue color. When sunlight falls on Uranus's atmosphere, the methane gas strongly absorbs the red light. The remaining light, now blue, scatters from cloud particles in the Uranian atmosphere and is reflected into space, as depicted in figure The cloud particles that cause the scattering are thought to be primarily crystals of frozen methane. Such crystals can form in Uranus's atmosphere because, being so far from the Sun, it is extremely cold. Figure Sketch illustrating why Uranus is blue. Methane absorbs red light, removing the red wavelengths from the sunlight that falls on the planet. The surviving light now missing its red colors is therefore predominantly blue. As that light scatters off cloud particles in the Uranian atmosphere and returns to space, it gives the planet its blue color. Astronomers rely on indirect methods to study the interior of Uranus, using, for example, its density and shape. From its mass and radius, astronomers can calculate that Uranus has an average density of about 1.27 grams per cubic centimeter. This density is nearly twice that of Saturn and almost as large as Jupiter's. But a planet's density depends on both the planet's composition and the amount by which its gravity compresses it. Given that some part of the density of Jupiter and Saturn is simply a result of their greater mass, astronomers deduce that Uranus must contain proportionally fewer light elements, such as hydrogen, than those more massive worlds. On the other hand, the density is too low for Uranus to contain much rock or iron material. Astronomers therefore believe it must be composed of material that is light and abundant, such as ordinary water mixed with methane and ammonia. This mix satisfactorily explains both the density and the spectrum of Uranus. Page 271 Confirmation of the abundance of water also comes from studies of the planet's shape. Uranus, like Jupiter and Saturn, rotates moderately fast, with its equator rotating faster than its poles. At its equator, Uranus spins once every 17 hours, bulging the planet's equator. The size of such a bulge depends in part on the planet's gravitational attraction and therefore on how the mass generating that attraction is distributed inside the planet. Thus, astronomers can deduce the density and composition deep inside Uranus from the size of its bulge. Such studies are consistent with the hypothesis that Uranus is composed of mostly water and other hydrogen rich molecules, and that it has a core of rock and iron rich material, as illustrated in figure It is not known whether the core formed first and then attracted the lighter gases and ices that condensed around it, or whether the core formed by heavy material sinking to the center after the planet formed. In fact, some astronomers think Uranus's core may be simply highly compressed water, ammonia, and methane with little rocky material. It appears that Uranus did not attract

3 as large an amount of hydrogen and helium gas as Jupiter and Saturn when it formed. Its composition can be explained if it formed primarily from planetesimals rich in ices of water, methane, and ammonia. Astronomers consequently often describe Uranus (and Neptune as well) as ice giants, although it should be stressed that the central temperature is probably about 5000 K, so there is no ice inside them today. Figure Artist's view of a recently suggested model for the interior of Uranus. Note how different it is from Jupiter and Saturn, which both contain large regions of liquid and metallic hydrogen. Uranus's Odd Tilt Uranus's rotation axis is tipped so that its equator is nearly perpendicular to its orbit. That is, it spins nearly on its side, as illustrated in figure Moreover, the orbits of Uranus's moons are similarly tilted. They orbit Uranus in its equatorial plane, and as a result their orbits are also tilted at approximately 90 with respect to the planet's orbit. Some astronomers therefore hypothesize that during its formation, Uranus was struck by an enormous planetesimal whose impact tilted the planet and splashed out material to create its family of moons. Other astronomers think Uranus was tilted by gravitational tugs exerted on it by neighboring planets (especially Saturn). Figure Sketch of the odd tilt of Uranus and its satellites, perhaps caused by a huge impact early in the planet's history. Because of this tilt, when Uranus's north pole points toward the Sun (its northern summer), the Sun will be above the horizon for many Earth years. Likewise, the other pole will be in night for a corresponding period. On the other hand, during the Uranian spring and fall, the Sun rises and sets approximately every 17 hours. Size of bodies and orbits are not to scale. The seasons of Uranus Regardless of what caused it, the strong tilt of its axis gives Uranus an odd pattern of day and night. For part of its orbit, one pole is in perpetual day and the other pole is in perpetual night. Thus, sunlight

4 heats the planet very unevenly, perhaps explaining why Uranus lacks the cloud bands seen on the other giant, gaseous planets. The lack of cloud bands may be temporary, however. Recent observations with an infrared telescope on Earth reveal an odd dark marking in Uranus's atmosphere, perhaps a feature that fades and reappears. ANIMATION The seasons of Uranus Page 272 Uranus's Rings and Moons Uranus is encircled by a set of narrow rings (fig ). The rings, like those of Saturn, are composed of particles ranging from dust grains to objects perhaps a meter or so in diameter, moving in individual orbits. The Uranian rings are very dark, more easily seen from scattered light from sunlight shining through the rings, implying that they are not made of, or coated with, ice like the bright rings of Saturn; instead, they may be rich in carbon particles or organic molecules. Some of Uranus's rings are very narrow and are held in place by shepherding satellites, as occurs for some of Saturn's rings. Figure The rings of Uranus were imaged by Voyager 2 from behind the planet (left) and in front (right), illustrating that in addition to thin bright rings, there is dark dust spread between the rings. There is a mismatch between the two views of the outermost ring because it has an elliptical orbit. Uranus has five fairly large moons (fig ) and about 20 smaller ones. Like the moons of Jupiter and Saturn, they form a regular system and are probably composed mainly of ice and rock. Many of the Uranian moons are heavily cratered, but Miranda, the smallest of the five large moons, has a surface unlike that of any other known Solar System body (fig ). The surface is broken into distinct areas that seem to bear no relation to one another. One region is wrinkled, while an adjacent region has small hills and craters, rather like our Moon. Miranda's patchwork appearance leads some astronomers to think

5 it may have been shattered by impact with another large body, with the pieces having been subsequently drawn back together by their mutual gravity, giving this peculiar moon its jumbled appearance. Alternatively, the curious surface might have been caused by rising and sinking motions driven by heat in Miranda's interior. Regardless of the cause, Miranda has some extremely curious and unexplained surface features, such as a set of cliffs, visible in figure 10.29, that are twice the height of Mount Everest. Figure The five largest Uranian moons. They are named for characters in English literature. Titania and Oberon are the queen and king of the fairies in Shakespeare'sA Midsummer Night's Dream. Ariel and Miranda are characters in Shakespeare's The Tempest. Umbriel is a sprite in Alexander Pope's poem The Rape of the Lock, in which Ariel also appears. The smaller Uranian moons are also named for characters from Shakespeare, such as Puck and Cordelia. ANIMATION Miranda Figure Miranda, an extremely puzzling moon, as observed by the Voyager 2 spacecraft. Note the enormous cliffs glinting in the sunlight at the bottom of the picture. Miranda