At first blink, one might suppose that all the known planetary ring systems are pretty much alike. For example, essentially all the ring particles at each of the four giant planets orbit their respective planet very close to that planet's equator. All are composed primarily of small particles, with a major fraction of the ring particles having typical sizes of a centimeter or less. Most of the rings are very thin compared with their radial extent, two major exceptions being Saturn's outermost ring, called the E ring, and Jupiter's innermost ring, called the Halo ring (see Figure 1.3). Both the E ring and the Halo ring are composed of tiny particles, most with diameters of about a micrometer, or about the size of the particles in cigarette smoke. Particles with such small sizes (and correspondingly small mass) may be subject to other forces which, over the long term, may substantially alter their orbits around the central planet. The possible nature of those forces will be discussed in later chapters.
All of the known ring systems are relatively young compared with the ages of their respective planets. The estimated ages of the four giant planets is about 4.5 x 109 years, close to the estimated ages of both the Earth and the Sun. A number of factors seem to indicate that dynamic processes within the four ring systems limit their lifetimes to no more than 1% (and perhaps closer to 0.1%) of the ages of the planets. Rings composed of dust-sized particles may have lifetimes that are much shorter,
perhaps as short as a human lifetime. We either have the fortune to be living at precisely the time when rings exist for each of the giant planets, an unlikely scenario, or each of the rings is replenished by ongoing processes. In later chapters, we will try to outline some of these replenishment processes, which vary somewhat from planet to planet.
Each of the ring systems seems to exhibit a structure which has the appearance of a series of concentric ringlets, with easily observable radial structure and little or no observable azimuthal variations. This radial sorting of ring particles is primarily the result of gravitational interaction with nearby planetary satellites, although there are a variety of ways this interaction affects the structure. Sometimes it causes sharp inner or outer boundaries of a ring. At other times gaps in an otherwise (radially) continuous ring are created. Gravitational forces from nearby satellites can even result in effectively "corrugating" the rings (bending waves) or causing tightly wound spiral variations in the ring particle population (density waves). We will attempt to explain these effects (at least those that are well understood) in terms that do not require the reader to have an extensive background in celestial dynamics or a keen understanding of higher mathematics. All in all, the creation and molding of planetary ring systems is both complex and fascinating.
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