All the jovian planets have rings. The ring system surrounding Uranus was discovered in 1977, when astronomers observed it passing in front of a bright star, momentarily dimming its light. Such a stellar occultation (Figure 13.15) happens a few times per decade and allows astronomers to measure planetary structures that are too small and faint to be detected directly. The 1977 observation was actually aimed at studying the planet's atmosphere by watching how it absorbed starlight. However, 40 minutes before and after Uranus itself occulted the star, the flickering starlight revealed the presence of a set of rings. The discovery was particularly exciting because, at the time, only Saturn was known to have rings. Jupiter's rings went unseen until Voyager 1 arrived there in 1979, and those of Neptune were unambiguously detected only in 1989, by Voyager 2.
Figure 13.15 Occultation of starlight allows astronomers to detect fine detail on a distant planet. The rings of Uranus were discovered using this technique.
The ground-based observations revealed the presence of a total of nine thin rings. The main rings, in order of increasing radius, are named Alpha, Beta, Gamma, Delta, and Epsilon, and they range from 44,000 to 51,000 km from the planet's center. All lie within the Roche limit of Uranus, which is about 62,000 km from the planet's center. A fainter ring, known as the Eta ring, lies between the Beta and Gamma rings, and three other faint rings, known as 4, 5, and 6, lie between the Alpha ring and the planet itself. In 1986, Voyager 2 discovered two more even fainter rings, one between Delta and Epsilon and one between ring 6 and Uranus. The main rings are shown in Figure 13.16. More details on the rings are provided in Table 13.3.
Uranus's rings are quite different from those of Saturn. Whereas Saturn's rings are bright and wide with relatively narrow gaps between them, the rings of Uranus are dark, narrow, and widely spaced. With the exception of the Epsilon ring and the diffuse innermost ring, the rings of Uranus are all less than about 10 km wide, and the spacing between them ranges from a few hundred to about a thousand kilometers. However, like Saturn's rings, all Uranus's rings are less than a few tens of meters thick (that is, measured in the direction perpendicular to the ring plane).
| TABLE 13.3 The Rings of Uranus | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
*Most of Uranus's rings are so thin that there is little difference between their inner and outer radii. |
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Figure 13.16 The main rings of Uranus, as imaged by Voyager 2. All the rings known before Voyager 2's arrival can be seen in this photo. From the inside out, they are 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, and Epsilon. Resolution is about 10 km, which is just about the width of most of these rings. The two rings discovered by Voyager 2 are too faint to be seen here.
The density of particles within the rings themselves is comparable to that found in Saturn's A and B rings. The particles that make up Saturn's rings range in size from dust grains to boulders, but in the case of Uranus, the particles show a much smaller spread—few if any are smaller than a centimeter or so in diameter. The ring particles are also considerably less reflective than Saturn's ring particles, possibly because they are covered with the same dark material as Uranus's moons. The Epsilon ring (shown in detail in Figure 13.17), exhibits properties a little like those of Saturn's F ring. It is slightly eccentric (its eccentricity is 0.008) and of variable width, although no braids are found. It also appears to be composed of ringlets.
Figure 13.17 A close-up of Uranus's Epsilon ring, showing some of its internal structure. The width of the ring averages 30 km; special image processing has magnified the resolution to about 100 m.
Like the F ring of Saturn, Uranus's narrow rings require shepherding satellites to keep them from diffusing away. In fact, the theory of shepherd satellites was first worked out to explain the rings of Uranus, which had been detected by stellar occultation even before Voyager 2's Saturn encounter. Thus, the existence of the F ring did not come as quite such a surprise as it might have otherwise! Presumably, many of the small inner satellites of Uranus play some role in governing the appearance of the rings. Voyager 2 detected the shepherds of the Epsilon ring, Cordelia and Ophelia (see Figure 13.18). Many other, undetected, shepherd satellites must also exist.
Figure 13.18 These two small moons, discovered by Voyager 2 in 1986 and now named Cordelia and Ophelia, tend to "shepherd" Uranus's Epsilon ring, keeping it from diffusing away into space.
As shown in Figure 13.19 and presented in more detail in Table 13.4, Neptune is surrounded by five dark rings. Three are quite narrow, like the rings of Uranus; the other two are quite broad and diffuse, more like Jupiter's ring. The dark coloration probably results from radiation darkening, as discussed earlier in the context of the moons of Uranus. All the rings lie within Neptune's Roche limit. The outermost (Adams) ring is noticeably clumped in places. From Earth we see not a complete ring but only partial arcs—the unseen parts of the ring are simply too thin (unclumped) to be detected. The connection between the rings and the planet's small inner satellites has not yet been firmly established, but many astronomers believe that the clumping is caused by shepherd satellites.
| TABLE 13.4 The Rings of Neptune | |||||||||||||||||||||||||||||||||||||||||
*Three of Neptune's rings are so thin that there is little difference between their inner and outer radii. **Lassell and Arago were originally identified as a single ring. |
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Figure 13.19 Neptune's faint rings. In this long-exposure image, the planet (center) is heavily overexposed and has been artificially blotted out to make the rings easier to see. One of the two faint rings lies between the inner bright ring (Leverrier) and the planet. The other lies between the two bright rings.
Although all the jovian worlds have ring systems, the rings themselves differ widely from planet to planet. Is there some "standard" way in which rings form around a planet? And is there a standard manner in which ring systems evolve? Or do the processes of ring formation and evolution depend entirely on the particular planet in question? If, as now appears to be the case, ring systems are relatively short-lived, their formation must be a fairly common event. Otherwise, we would not expect to find rings around all four jovian planets at once. There are also many indications that the individual planetary environment plays an important role in determining a ring system's appearance and longevity. Although many aspects of ring formation and evolution are now understood, it must be admitted that no comprehensive theory yet exists.
