Planetary rings, which for centuries were thought to be unique attributes of Saturn, have recently been observed around all the giant planets.17 Surprisingly, each ring system is distinctive. The rings of Jupiter are extremely tenuous and contain significant amounts of short-lived dust, suggesting that they are continually regenerated. Saturn's rings, which are predominantly composed of water ice particles that are centimeters to meters in size, are broad, bright, and opaque; they exhibit the most diversity in their organization and variety. Uranus's narrow, slightly noncircular and nonequatorial bands, composed predominantly of dark boulders, reside within an extensive structured disk of dust that is invisible from Earth. Neptune's rings are distinguished by one ring that contains four prominent arcs restricted to a small range of the circumference. (Table 4.3) summarizes knowledge of the various ring systems.)
All rings lie close to their planet's equatorial plane, and most are within their planet's Roche limit, where tidal forces would tear asunder a self-gravitating fluid body; they also extend out into the planet's magnetosphere and, in the case of Uranus, dip down within the upper reaches of the planetary atmosphere.
Saturn's ring system was first spotted by Galileo Galilei in 1610, but the nature of the rings was not correctly identified until the observations and insight of Christiaan Huygens in the late 1650s. The ring systems of the other giant planets were not discovered until the past 15 years. Uranus's system was first identified in 1977 during a stellar occultation that was best observed from NASA's Kuiper Airborne Observatory. Jupiter's rings were unambiguously seen by Voyager 1 in 1979 but had been inferred earlier from charged particle absorption signatures obtained by Pioneer 10. Neptune's arcs eventually made their presence known in a 1984 stellar occultation observed simultaneously at three ground-based telescopes.
The structure of the rings and their composition differ among the various planets and, to a lesser extent, within each ring system. The most elaborate set of rings, which also contains the widest range of identified processes, is Saturn's system. The general forms of the ring systems surrounding the four giant planets are illustrated in Figure 4.5.
Beyond the gravitational perturbations due to embedded and adjacent small moons (many of which were contemporaneously discovered by the Voyager spacecraft), ring particles interact with the magnetosphere via charging, plasma drag, and dynamical forces with the ambient electromagnetic field. Electrostatic effects may lift small particles off the surfaces of the larger ring particles to create "spokes," the dark, roughly chevron-shaped lanes discovered by Voyager in the midst of Saturn's B-ring. Small particles at the inner edge of rings may experience gas drag from the extended planetary atmosphere.
The size distribution of ring particles extends from submicron dust, through meter- sized particles, to small embedded moons, including the recently discovered Pan, about 10 km in radius. Theoretical expectations, but only limited data, support the idea that ring particles segregate in size, both radially and vertically.
The composition of ring particles is well known only for Saturn. Spectroscopic, thermal, radio, and neutron measurements combine with estimates of mass density to suggest that Saturn's ring particles throughout are almost entirely water ice with just a little contaminant to account for an observed reddening. For the other ring systems, the particles superficially resemble the contiguous small moons; probably these rings contain silicate and, in the cases of Uranus and Neptune, possibly carbonaceous material. In Saturn's rings, color and albedo variations hint at modest compositional differences across various radial regions of the rings.
For Saturn's and Uranus's rings, occultations of spacecraft radio signals at two wavelengths have provided information on particle sizes in the range of roughly 1 cm to 10 m at a number of locations (unfortunately excluding Saturn's B-ring because its high opacity prevented transmission of the signal). The derived differential size distributions of particles whose radii span several orders of magnitude satisfy power laws with indices ranging between 2.5 and 3.5. Smaller particles are inferred from photometry and from the different ring opacities measured in stellar occultations at a spread of wavelengths. The relative fraction of dust differs significantly across the rings; some of the dusty rings have very steep size distributions.
We have a first-order understanding of the dynamical processes in rings, much of it based on previous work in galactic and stellar dynamics. The rings are a kinetic system, in which the deviations from perfect circular, equatorial motion can be considered as random "thermal" velocities in a viscous fluid. Unfortunately, the models are often idealized (e.g., all particles are treated as hard spheres of the same size) and cannot yet predict many phenomena in the detail given by spacecraft observations and Earth-based occultations (e.g., sharp edges or specific wave profiles).
All of the ring systems show many youthful features: Saturn's ice is bright and yet is continually bombarded by dark carbonaceous material from comets; Uranus's rings are narrow and yet should be dragged inward by the planet's atmosphere; Neptune's arcs are constrained to a small range of longitude but should shear apart; and Jupiter's particles are so small that they will be eliminated in much less than a millennium. The angular momentum transferred in asymmetric gravitational interactions between rings and nearby moons should have caused them to spread much further apart over the eons than they are observed to be. Further, the small moons discovered adjacent to the planets by Voyager could not have survived the flux of interplanetary meteoroids for the age of the solar system; in much less time, according to present models, these small moons would be shattered by interplanetary impactors. This realization provides a potential solution to the problem presented by young rings: such impacts may not only destroy the moons, but may also regularly recreate the ring systems that are gradually spreading and being ground to dust. Thus, the moons not only sculpt the rings' structure, but may also be the reservoirs for past and future ring systems. Of course these reservoirs themselves are gradually being depleted.
Our description of the various ring systems remains incomplete, especially in our knowledge of the overall size distribution and the composition of the ring particles (which, in fact, may vary within each system). We need to understand the vast differences among the various planetary ring systems. Do they indicate different origins, different environments, or merely different random outcomes of the same stochastic processes of ring creation and destruction? We need accurate measurements of the three-dimensional morphology of the rings to compare with predictions from present models of ring dynamics so as to refine such models.
Questions about the ages of the rings, their recent origins, and their history have been brought into sharp focus by spacecraft observations of many apparently youthful features as well as by calculations indicating that the present rings could not have persisted for the age of the solar system. Perhaps the most important question is whether our understanding of present processes in planetary rings can be fruitfully compared with similar processes in the early solar nebula to explain the origin of the planets and satellites in a flat disk of interacting particles, dust, and gas. We can also hope to apply this understanding to other flattened, rotating systems like galaxies and accretion disks. Some of the following questions arising from our current understanding of rings include:
To understand planetary rings, three major objectives must be achieved by any exploration program:
Many of the above objectives are likely to be accomplished with Galileo's observations of Jupiter's ring and, especially, with Cassini's extended visit to Saturn. Ground-based observations of stellar occultations can characterize the spatial and temporal variability of the narrow rings of Saturn, Uranus, and Neptune; they may also refine the precession rate of Saturn's rotation pole. In the case of Neptune, a primary goal is to determine the radial structure of the arcs. The limited number of images that Galileo can obtain may require that subsequent missions plan imaging observations to define the ring's structure, particle size distribution, and possible variability. When spacecraft are sent to Uranus and Neptune, ring observations should be taken in the normal course of mission operations.
17. For a comprehensive review, see, for example, Nicholson, P.D., and L. Dones, "Planetary Rings," Reviews of Geophysics (Suppl.) 29:313-327, 1991; Esposito, L.W., "Understanding Planetary Rings," Annual Review of Earth and Planetary Sciences, Vol. 21, 1993; or Greenberg, R., and A. Brahic (eds.), Planetary Rings, University of Arizona Press, Tucson, Ariz., 1984.
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