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.
