Overview

The Voyager 1 and 2 encounters opened the eyes of the world to the fascinating complexity of Saturn's rings. For the more than two decades since, the Voyager images have remained fixed in our minds as if frozen in a lightning flash, and it hasn't been until recently that Cassini has shown that these observations represent snapshots of a very changeable, dynamic system—with many features in the rings varying on timescales of years, months, and even days. Cassini has made a number of other new discoveries as well, concerning the ring-particle composition and size distribution, and how they vary from place to place. As this chapter goes to press with less than half of Cassini's planned ring observations being completed, many surprises are yet to come and many puzzles will be resolved.

The interaction between the rings and nearby moons includes the physics of spiral waves driven at orbit resonances with moons (see Section 9.5). Cassini has garnered many new observations of these waves (Section 10.3). Related to the interaction between distant moons and rings that causes spiral waves is the case where a smaller, but closer, moonlet affects ring material. The moonlet may even be embedded within the rings (Section 9.5), in which case it can clear an empty gap. Detailed analysis during the decade after the Voyager encounters did lead to the discovery of one moonlet in the Encke gap, and Cassini observations have now revealed another (Section 10.5). The same physics—by which the gravity of a local moonlet actually pushes ring material away from it—was once thought to explain how narrow ringlets were confined or prevented from spreading out. The term "shepherd moons'' was applied to moons that straddled and confined a ringlet, such as the narrow, stranded and kinky F ring lying just outside Saturn's main rings, because they were thought to act like sheepdogs confining an unruly flock of sheep [1]. However, over the years we have realized that the so-called shepherds' mutual interactions make their orbits "chaotic", undergoing occasional glitches and jumps [2]. It is likely that the entire region between these moonlets, including the F ring itself and any little shards and small objects that populate the region, is also permeated by orbital chaos rather than stable confinement. Do these chaotic orbits lead to occasional collisions and new ringlets [3]? Some tantalizing hints have been provided by analysis of old Voyager data, showing formation of new clumps on weekly or monthly timescales [4], and from Hubble Space Telescope observations of new clumps, observed fleetingly when Saturn's rings turned edge-on to the Earth in 1995 [5]. Some new Cassini observations are already revealing dramatic time evolution in the entire F-ring region (Section 10.5).

Most of the known structure in the rings remains a puzzle; for example, structure having no clear pattern or preferred length-scale fills Saturn's B ring and inner A ring—the most optically thick parts of the rings—and is known as irregular structure. It is only found in the optically thicker parts of the rings; current theories may be able to explain some of it, but certainly not all or even most of it. How opaque is it? What kinds of structures or perhaps buried moonlets lurk in its depths? How does it vary with angle around the rings? Cassini has now conducted nine optimized radio occultations that penetrate nearly all of the densest parts of the rings, revealing new kinds of structure. Moreover, Cassini has observed several stellar occultations that also penetrate this structure along various lines of sight. These results will be discussed in Section 10.3.

A different kind of structure is too minuscule in scale to be directly observed even with Cassini's instruments, but manifests itself indirectly in several ways which are now under study. Section 9.4 describes how the brightness of the A ring varies with orbit longitude; this appearance is thought to result from transient streamers or "gravity wakes'' [6], formed when clumps of particles began to collapse under their own self-gravity but were just as quickly sheared out by their differential rotation [7]. These shearing clumps have about the same scale as the ring thickness—tens of meters. This dynamical clumping into temporary "superparticles" creates small-scale local variances that confuse the issue of what the "typical" packing density, or the size distribution, of the actual ring particles are. However, Cassini observations are now starting to constrain the properties of the wake effect, the vertical thickness and packing density, and the underlying ring-particle size distribution, in detail, as we discuss in Section 10.3.

This gravity-wake effect is connected to the fundamental dynamical question of the collisional or random velocity of the ring particles, which is at the heart of all the dynamics that causes their structure. The collisional velocity is manifested in the ring vertical thickness (Section 9.3) and packing density of the particles, which can, in principle, be constrained in several ways by remote observations. Interparticle collisions give the rings a viscous nature (Section 9.5), which enters into all the physics of spiral waves, shepherding, ring spreading, etc. Indirect determinations of the particle random velocities come from a variety of different Cassini observations (Section 10.3).

Besides deciphering the structure of the rings, we also need to understand the composition of the underlying ring particles. As described in Section 9.8, water ice is the main constituent of the rings [8]. However, the general tawny color of the rings tells us that other materials, in addition to ice, must be widespread, even if only in trace amounts. Also, the brightness (albedo, or reflectivity) of the ring particles, as well as their color, does vary from place to place in the rings, indicating that their composition also varies with location [9]. Cassini carries several new capabilities for measuring composition remotely, and its extended tour presents the variety of geometrical opportunities needed to separate out compositional variations from variations in, for instance, abundance or optical depth (Section 10.4).

How ring-particle composition varies from region to region can tell us whether the rings formed of material with a uniform composition, or in several possibly overlapping bands of different composition, but we need to understand first if there has been blurring and spreading of material from one place to another. Particles can swap material in collisions, and so parcels of matter with different composition can slowly diffuse through the rings. More important, probably, is spreading around of material ejected by high-speed impacts onto the rings by interplanetary meteoroids [10] (Section 10.6). Models suggest this is to be expected and is potentially a diagnostic of the ring age.

These models of the process of meteoroid bombardment find it to be potentially of great importance, but dependent on several poorly known parameters. The importance comes from the fact that the rings have such an enormous area for their relatively puny mass—the area/mass ratio for the rings is about a million times larger than for, say, Earth. We don't really know what the meteoroid mass flux is at Saturn, unfortunately, but estimates have been made which indicate that the rings might have swept up their own mass in meteoroids over the age of the solar system! This would obviously change both their dynamics and their composition dramatically. No icy particle could absorb an equal mass of dark, carbon-rich meteoroid material without becoming nearly as dark as charcoal (such as the Uranus and Neptune rings—Section 11.4). Thus, the fresh, icy surfaces of most of Saturn's ring particles might be telling us that Saturn's rings aren't as old as the solar system after all. In Section 10.6 we review other ways in which Cassini observations will help us understand this process better.

A young age for the rings, implied by both their dynamical interactions with nearby moons (Section 9.5) and their evolution under meteoroid bombardment, remains controversial because of many uncertainties in both lines of argument. The arguments are independent, and they both give about the same result, but perhaps this is a coincidence. There are difficulties with envisioning just how such a massive ring system could have formed so recently, after all the heavy bombardment of the very early solar system had long since died down. Cassini will help to resolve this issue in several ways, helping to bring the age and origin of Saturn's rings into sharper focus (Sections 10.6 and 10.7).

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