Chapter 7 The Outer Planets

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208 Chapter 7 The Outer Planets WEB LINK 7.20 Uranus’s Vital Statistics Neptune’s Vital Statistics Mass 8.68 × 10 25 kg (14.5 M ) Equatorial radius 25,559 km (4.01 R ) Average density 1290 kg/m3 (0.234 Earth density) Orbital eccentricity Sidereal period of 0.047 revolution (year) Average distance 84.0 Earth years from Sun 2.87 × 109 Equatorial km (19.2 AU) rotation period Interior sidereal 16 h 30 min (retrograde) rotation period 17 h 14 min (retrograde) Albedo (average) 0.66 FIGURE 7-28 Uranus, Earth, and Neptune These images of Uranus, the Earth, and Neptune are to the same scale. Uranus and Neptune are quite similar in mass, size, and chemical composition. Both planets are surrounded by thin, dark rings, quite unlike Saturn’s, which are broad and bright. The clouds on the right of Uranus are each the size of Europe. (NASA) Some molecules can join together in long, repeating molecular chains to form substances called polymers, of which plastics are the best known example. Many of the hydrocarbons and carbon-nitrogen compounds in Titan’s atmosphere can form such polymers. Scientists hypothesize that droplets of lighter polymers remain suspended in Titan’s atmosphere to form a mist, while heavier polymer particles settle down onto Titan’s surface. Using the Keck I telescope in 1999, astronomers observed infrared emissions from Titan’s surface. The bright areas in Figure 7-27 are inferred Earth Mass 1.02 × 1026 kg (17.1M) Equatorial radius 24,764 km (3.88 R) Average density 1640 kg/m3 (0.297 Earth density) Orbital eccentricity Sidereal period of 0.009 revolution (year) Average distance 164.8 Earth years from Sun 4.50 × 109 Equatorial km (30.1 AU) rotation period Interior sidereal 19 h 6 min rotation period 16 h 7 min Albedo (average) 0.62 to be islands or continents of rock and ice, while the dark areas are believed to be ethane oceans. We have little reason to suspect that life exists on Titan, however; its surface temperature of 95 K (–288ºF) is prohibitively cold. Nevertheless, a more detailed study of the chemistry of Titan may shed light on the origins of life on Earth. Just as we have recently revisited Jupiter, we are sending back a spacecraft to Saturn. Launched in October 1997, spacecraft Cassini is scheduled to go into orbit in July 2004
209 Chapter 7 The Outer Planets on a four-year mission to study the ringed planet and the bodies that orbit it. Like Galileo at Jupiter, Cassini will deploy a probe, named Huygens, that will descend into Saturn’s atmosphere. URANUS Uranus and its largest moons are so far from the Sun (19.2 AU) that from Earth they appear to be a small cluster of stars. No wonder they seem fixed in the heavens: A Uranian year equals 84 Earth years. Since its discovery in 1781, Uranus has only orbited the Sun just over 2 1 /2 times. 7-12 Uranus sports a hazy atmosphere and clouds Until early 1996, observations of Uranus, the fourth most massive planet in our solar system, revealed few notable features in the visible part of the spectrum. Even the 1986 visit of Voyager 2 to Uranus showed a remarkably featureless world. It took the Hubble Space Telescope’s infrared camera to find what Voyager’s visible light camera could not: Uranus has a system of belts WEB LINK 7.21 ANIMATION 7.2 Winter at north pole North pole 98° Northern spring, southern autumn Summer at south pole South pole FIGURE 7-29 Exaggerated Seasons on Uranus Uranus’s axis of rotation is tilted so steeply that it lies nearly in the plane of its orbit. Seasonal changes on Uranus are thus greatly exaggerated. For example, during Sun and zones. Its hydrogen atmosphere has traces of methane along with a high-altitude haze under which are clear air and ever-changing methane clouds that dwarf the typical cumulus clouds we see on Earth. Uranus’s clouds are towering and huge, each typically as large as Europe (Figure 7-28). Along with the rest of the atmosphere, the clouds go around the planet once every 16 1 /2 hours. Uranus contains 14 1 /2 times as much mass as the Earth and is 4 times bigger in diameter (see Figure 7-28). Its outer layers are composed predominantly of gaseous hydrogen and helium. The temperature in the upper atmosphere of the planet is so low (about 73 K, or –330ºF) that the methane and water there condense to form clouds of ice crystals. Because methane freezes at a lower temperature than water, methane forms higher clouds over Uranus. Methane efficiently absorbs red light, giving Uranus its blue-green color. Earth-based observations show that Uranus rotates once every 17 hours 14 minutes on an axis of rotation that lies very nearly in the plane of the planet’s orbit. Uranus’s rotation axis is inclined 98º from a line perpendicular to its plane of orbit. Therefore, it is one of only three planets with retrograde rotation (Venus and Pluto are the other two). As Uranus orbits the Sun, its north and south poles alternately point almost directly toward or directly away from the Sun, producing exaggerated seasons (Figure 7-29). In the North pole Northern autumn, southern spring Summer at north pole South pole Winter at south pole midsummer at Uranus’s south pole, the Sun appears nearly overhead for many Earth years, while the planet’s northern regions are subjected to a long, continuous winter night. Half an orbit later, the seasons are reversed.
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Chapter 7 The Outer Planets

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