Introduction

In Chapter 2 we summarized observations of Saturn's rings from the time of Galileo's first telescopic observations in 1610 through the end of the Voyager 2 encounter (1981). In this chapter we will discuss the scientific analysis of the Voyager (and subsequent Earth-based data) up to June 1, 2004, a month before the insertion of the Cassini Orbiter and the attached Huygens probe into orbit around Saturn on July 1. In the next chapter (Chapter 10), we will discuss the early findings of the Cassini Orbiter relative to Saturn's ring system. We purposely separate the two sets of observations, in part to emphasize the incredible rate with which understanding of the Saturn ring system (and indeed ring processes in general) is growing. As is often the case, Nature conspires against our complete understanding of the processes operative within the Saturn ring system. While additional observations improve our data set and answer many of our a priori questions, they also reveal myriads of details that call for new explanations. But then, such is the nature of scientific exploration. As unsettling as the thought might be to some readers of this book, we may never completely understand the processes operating within Saturn's rings, but we are certainly obtaining a more complete description of their physical characteristics and composition, and our understanding, while meager, continues to grow.

In Section 9.2 we summarize the radial dimensions, orbital speeds and orbital periods, and other radial characteristics of the rings. Vertical structure of the rings, including both physical and optical (including microwave) thicknesses is discussed in Section 9.3. Section 9.4 discusses the third dimension of the rings, namely their azimuthal structure. In Section 9.5 we discuss some of the gravitational mechanisms operating within the rings, along with the tangible outcomes of those interactions. Clear gaps within the rings and eccentric (non-circular) ringlets within those gaps are probably related to gravitational interactions, either with known or yet-to-be-discovered moonlets, but this interaction is not well understood; these are discussed in

Section 9.6. In Section 9.7 we discuss the gases that surround the rings and form an extremely tenuous ring atmosphere. The rings, of course, are actually collections of particles of varying size and composition; while there are no data that resolve the individual ring particles, much can be inferred about individual particle characteristics from the scattering properties of the rings, as we discuss in Section 9.8. Section 9.9 covers other optical properties of the rings, including color variations and variations in ring brightness with solar phase angle. Thermal properties of the rings are discussed in Section 9.10. Voyager radio data and its implications are discussed in Section 9.11. Evidence for interactions of the rings with Saturn's magnetic field is reviewed in Section 9.12. In Section 9.13 we discuss Earth-based observations of the rings since the Voyager encounters. We summarize in Section 9.14 some of the major remaining unanswered questions before consideration of Cassini data. Finally, in Section 9.15, we briefly outline some of the major scientific objectives for ring observations by the Cassini Orbiter.

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