Chapter 7 The Outer Planets

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206 Chapter 7 The Outer Planets actually consists of hundreds upon hundreds of closely spaced thin bands, or ringlets, of particles (Figure 7-22b). Furthermore, myriad dust-sized particles fill the Cassini and Encke divisions. The Voyager cameras also sent back the first high-quality pictures of the F ring, a thin set of ringlets just beyond the outer edge of the A ring. Two tiny satellites following orbits on either side of the F ring serve to keep the ring intact (Figure 7-23). The outer of the two satellites orbits Saturn at a slower speed than do the ice particles in the ring. As the ring particles pass near it, they receive a tiny, backward gravitational tug, which slows them down, causing them to fall into orbits a bit closer to Saturn. Meanwhile, the inner satellite orbits the planet faster than the F ring particles. Its gravitational force pulls them forward and nudges the particles into a higher orbit. The combined effect of these two satellites is to focus the icy particles into a well-defined, narrow band about 100 km wide. Because of their confining influence, these two moons, Prometheus and Pandora, are called shepherd satellites or shepherd moons. Among the most curious features of the F ring is that the ringlets are sometimes braided (Figure 7-24) and sometimes separate. Saturn’s shell of liquid metallic hydrogen produces a planetwide magnetic field that apparently affects its rings. Saturn’s slower rotation and much smaller volume of liquid metallic hydrogen produce a surface magnetic field only about 2 /3 as strong as Earth’s surface field. Data from spacecraft show that Saturn’s magnetosphere contains radiation Prometheus Pandora F ring A ring Encke division FIGURE 7-23 The F Ring and Its Two Shepherds Two tiny satellites, Prometheus and Pandora, each measuring about 50 km across, orbit Saturn on either side of the F ring. The gravitational effects of these two shepherd satellites confine the particles in the F ring to a band about 100 km wide. (NASA) belts similar to those of Earth. Furthermore, dark spokes move around Saturn’s rings (Figure 7-25); these are believed to be created by the magnetic field, which lifts charged particles out of the plane in which the rings orbit. Spreading the particles out decreases the light scattered from them and therefore makes the rings appear darker. FIGURE 7-24 Braided F Ring This photograph from Voyager 1 FIGURE 7-25 Spokes in Saturn’s Rings Believed to be caused by shows several strands, each measuring roughly 10 km across, that Saturn’s magnetic field temporarily lifting particles out of the ring plane, comprise the F ring. The total width of the F ring is about 100 km. these dark regions move around the rings like the spokes on a rotating (NASA) wheel. (NASA)
207 Chapter 7 The Outer Planets 7-11 Titan has a thick, opaque atmosphere rich in nitrogen, methane, and other hydrocarbons Only 7 of Saturn’s 30 known moons are spherical. The rest are oblong, suggesting that they are captured asteroids. The 12 moons discovered in 2001 move in clumps, indicating that they may be pieces of a larger moon that was broken up by impacts. Saturn’s largest moon, Titan, is second in size to Ganymede among the moons of the solar system, and the only moon to have a dense atmosphere. About 10 times more gas lies above each square meter of Titan’s surface than lies above each square meter of the Earth. Christiaan Huygens discovered Titan in 1655, the same year he proposed that Saturn has rings. By the early 1900s, several scientists had begun to suspect that Titan might have an atmosphere, because it is cool enough and massive enough to retain heavy gases. Because of its atmosphere, Titan was a primary target for the Voyager missions. To everyone’s disappointment, the Voyagers spent hour after precious hour sending back featureless images (Figure 7-26). Unexpectedly, Titan’s thick cloud cover completely blocked any view of its surface. The same dense haze allows little sunlight to penetrate—that moon’s surface must be a dark, gloomy place. Voyager data indicated that roughly 90% of Titan’s atmosphere is nitrogen. Most of this nitrogen probably formed from the breakdown of ammonia (NH 3 ) by the Sun’s ultraviolet radiation into hydrogen and nitrogen atoms. Because Titan’s gravity is too weak to retain hydrogen, this gas has escaped into space, leaving behind ample nitrogen. The unbreathable atmosphere is about 4 times as dense as Earth’s. The second most abundant gas on Titan is methane, a major component of natural gas. Sunlight interacting with methane induces chemical reactions that produce a variety of other carbon-hydrogen compounds, or hydrocarbons. For example, spacecraft have detected small amounts of ethane Titan Titan’s shadow FIGURE 7-26 Titan These views of Titan were taken by Voyager 2. Very few features are visible in the thick, unbroken haze that surrounds this large satellite. The main haze layer is located nearly 300 km above Titan’s surface. (Left: JPL/NASA; center: NASA; right: Erich Karkoschka, LPL/STScI/NASA) WEB LINK 7.19 (C 2 H 6 ), acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), and propane (C 3 H 8 ) in Titan’s atmosphere. Ethane, the most abundant of these compounds, condenses into droplets as it is produced and falls to Titan’s surface to form a liquid. Enough ethane may exist to create rivers, lakes, and even oceans on Titan (Figure 7-27). Nitrogen combines with these hydrocarbons to produce other compounds. Although one of these compounds, hydrogen cyanide (HCN), is a poison, some of the others are the building blocks of life’s organic molecules. FIGURE 7-27 Surface Features on Titan The Keck I Telescope, using an infrared camera, was able to peer through Titan’s haze (see Figure 7-26). The dark region, emitting little infrared, may be a sea of liquid methane or more complex hydrocarbons, while the brighter regions may be rock and water ice. The white circle indicates Titan’s circumference. (Lawrence Livermore National Laboratory)
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Chapter 7 The Outer Planets

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