Of all the aspects of the Jovian planets their ring systems are among the most captivating. Galileo first observed what we know as Saturn's rings in July 1610, but it was not until 1655 that Christian Huygens proposed that they are a flattened disk of matter detached from the planet. Finally, in 1857 James Clerk Maxwell showed mathematically that they must consist of numerous tiny bodies in orbit about Saturn. This was experimentally demonstrated in 1895 from Doppler shifts showing that the ring particles move in Keplerian orbits, fastest close to the planet and slower farther away.
It can be shown with reasonable mathematical precision that particles swarming about a planet eventually form a thin ring system in the equatorial plane. This system is produced by the gravitational attraction of the planet and many gravitational interactions of the ring particles with each other. Satellites of the planet play an important role in sculpting the appearance of the ring system and in keeping it from spreading out in the equatorial plane. Also the ring system forms within several planetary radii of the surface of the planet and is not able to form at greater distances. Although the same basic principles underlie the three known ring systems, Saturn's rings are much more elaborate and complex than those of Uranus and Jupiter.
Rings of Saturn
The circular rings lie in a plane coinciding with Saturn's equator. During the 29.5-year period of the planet's revolution around the sun the rings are observed obliquely at different angles from the earth.
Three concentric ri ngs have been known for some time and are labeled A, 8, and C in order of decreasing distance from Saturn. The bright ring 8 is separated from ring A by a space of about 5000 kilometers, called Cassini's division. Next is the semitransparent ring C, the so-called crepe ring, which lies inside the inner edge of the 8 ring. An exceptionally faint 0 ring, which lies inside the inner edge of the C ring, has been found by Voyager investigations. Outside the A ring other faint rings, known as E, F, and C, have been identified. The vertical extent of all the rings is less than a couple of kilometers. Given their immense diameters, they are proportionally thousands of times thinner than a razor blade.
The composition of the ri ng particles is suggested by the way they reflect sunlight. Their infrared reflectivity indicates that they are water ice or at least covered with water ice. The particles are better reflectors of red light than they are of blue-which suggests that some other substance is mixed with the water ice. Ring particles vary in size from a few centimeters up to several meters. Each particle pursues its independent orbit around Saturn in accordance with Kepler's third law. The farther out from the planet, the lower are the particles' speeds where a solid ring would rotate fastest at the farthest point from the planet. The entire ring system lies within the critical distance called the Roche limit, equal to about 2.4 Saturnian radii. This limit is named after the nineteenth-century French mathematician Edouard Roche, who found that inside this limit the gravitational attraction exerted by a planet on two adjacent orbiting particles is larger than the attraction of the two particles for each other. Whether the rings were formed inside the Roche limit by the breakup of a satellite, comet, or other body or whether Saturn's gravitational force prevented primordial particles from coalescing to form a satellite is unknown.
Cassini's division and another known as Encke's division appear dark, suggesti ng an absence of particles. However, high-resolution photographs made by the Voyager spacecraft, as in Figure 9.10, surprised astronomers when they revealed that the three major
rings, A, 8, and C, are made up of hundreds, if not thousands, of very narrow ringlets. Even Cassini's and Encke's divisions are crammed with ringlets, with something like 100 in Cassini's division alone (Figure 9.11). Apparently, particles in Cassini's division do not readily scatter photons in the backward direction so that they appear dark from the sunlit side.
The Voyagers provided evidence that some of the ringlets are not circular, while the F ring has knots, braids, and twists in it-which had not been predicted from gravitational theory. We are not sure what causes this strange behavior. Probably the most unexpected aspect found was wedgeshaped spokes orientated radially out from the planet in the 8 ring. The spokes from the sunlit side, where they appear dark, and looking. back
toward the sun, where they appear bright. They are perplexing in that, if produced somehow by the ring particles, Keplerian motion should dissolve the spokes in a short time; but they are seen to last close to 10 hours. The spokes are a mystery for which we may not have a satisfactory solution for many years.
Jupiter Rings
The notion that jupiter possesses a ring system like that of Saturn was proposed some 20 years ago. Pioneer 11 data were interpreted as consistent with the existence of a system of tiny satellites forming a ring about jupiter. This was at best speculation, and it was only Voyager 1's photograph of the Beehive star cluster that finally revealed the ring. The photograph showed a ring system extending some 0.7 to 0.8 jupiter radii above the cloud tops of the planet. At most the ring is about 30 kilometers thick and 6,000 kilometers wide.
The particles composing the ring appear to be smaller on the average than Saturn's ring particles. Also unlike Saturn's ring particles, those of jupiter's and Uranus's systems are quite dark. Thus they are not water ice or coated with water ice. The evidence suggests that they are probably silicate particles whose origin is.not known.
There is also a diffuse disk, several times fainter than the bright ring, extending inside toward the planet. Surrounding the bright and diffuse rings is a faint halo some 20,000 kilometers thick. Saturn's rings are not embedded in such a halo.
Uranus' Rings
Occasionally a planet will pass between the earth and a star. Such an event is called an occultation (from the Latin word meaning "hiding"). In recent years astronomers have carefully monitored these occultations since the time and place on the earth at which the occultation will be visible can be calculated. It requires a precise knowledge of the planet's orbit to make such a calculation, and the precision with which the prediction is confirmed by the observation in turn tells us how well we really know the orbit.
As the planet begins to occult the star, its atmosphere, which is partially transparent, covers the star first so that there is a gradual dimming of the star. If there were no atmosphere, the star's brightness would remain constant until the opaque body of the planet cut off all light; the change would be sudden, not gradual.
In this manner astronomers aboard the Kuiper Airborne Observatory, an airplane fitted with an infrared telescope, flying high over the Indian Ocean discovered a ring system around Uranus on March 10, 1977. About a half hour before the occultation was to take place, the star's light dimmed unexpectedly for a few seconds, followed by four other dips in brightness minutes later. The sequence was repeated in reverse as the star passed beyond the disk of Uranus on the other side. Since the original discovery of five rings four less prominent rings have been identified; making a total of nine rings.
The rings appear to be very narrow, not more than 10 to 100 kilometers in width, and they lie close to the planet's equatorial plane. Hence the origin of the rings is closely related to that of Uranus since the planet's equatorial plane is almost perpendicular to its orbital plane. Six of the rings appear to be slightly elliptical, with the radii for all the rings lying between 1.6 and 1.95 planetary radii. All the rings are dark and have sharp edges. Since the ring particles are poor reflectors, it is hard to believe that they are coated with water (or ammonia or methane) ice. More likely they are a silicate- or carbon-bearing material.
