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3 - The Rings of Saturn
- from II - Ring Systems by Location
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- By J. N. Cuzzi, NASA Ames Research Center Moffett Field, California, USA, G. Filacchione, INAF-IAPS Institute for Space Astrophysics and Planetology Rome, ITALY, E. A. Marouf, San Jose State University San Jose, California, USA
- Edited by Matthew S. Tiscareno, Carl D. Murray, Queen Mary University of London
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- Book:
- Planetary Ring Systems
- Published online:
- 26 February 2018
- Print publication:
- 22 March 2018, pp 51-92
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Summary
INTRODUCTION
One could become an expert on Saturn's iconic rings pretty easily in the early 1970s, as very little was known about them beyond the distinction between the A, B, and C rings, and the Cassini Division or “gap” between rings A and B (Alexander, 1962; Bobrov, 1970). Water ice was discovered spectroscopically on the ring particle surfaces, and radar and microwave emission observations proved that the particles must be centimeters to meters in size, consisting primarily, not just superficially, of water ice (Pollack, 1975). While a 2:1 orbital resonance with Mimas had long been suspected of having something to do with the Cassini Division, computers of the time were unable to model the subtle dynamical effects that we now know to dominate ring structure.
This innocent state of affairs was exploded by the Voyager 1 and 2 encounters in 1980 and 1981. Spectacular images revealed filigree structure and odd regional color variations, and exquisitely detailed radial profiles of fluctuating particle abundance were obtained from the first stellar and radio occultations, having resolution almost at the scale of single particles. Voyager-era understanding was reviewed by Cuzzi et al. (1984) and Esposito et al. (1984). While the Voyager data kept ring scientists busy for decades, planning which led to the monumentally successful NASA-ESA-ASI Cassini mission, which arrived in 2004, had been under way even before Voyager got to Saturn. A review of pre-Cassini knowledge of Saturn's Rings can be found in Orton et al. (2009).
This chapter will build on recent topical and process-specific reviews that treat the gamut of ring phenomena and its underlying physics in considerable detail (Colwell et al., 2009; Cuzzi et al., 2009; Horányi et al., 2009; Schmidt et al., 2009; Esposito, 2010; Tiscareno, 2013b; Esposito, 2014). We will follow and extend the general organization of Cuzzi et al. (2010), the most recent general discussion of Saturn's rings. For brevity and the benefit of the reader, we will frequently refer to the above review articles instead of directly to the primary literature they discuss. We will focus on new work since 2010, within a general context, and will connect our high-level discussions with more detailed chapters in this volume.
Simulations of light scattering in planetary rings
- Edited by J. A. Sellwood, University of Manchester
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- Book:
- Dynamics of Astrophysical Discs
- Published online:
- 06 July 2010
- Print publication:
- 07 December 1989, pp 25-26
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Summary
Statement of the problem
Most studies of light scattering in planetary rings have assumed layers which are many particles thick, plane parallel, and homogeneous. However, real rings may be thin, vertically warped, and clumpy. We have developed a ray tracing code which calculates the light scattered by an arbitrary distribution of particles. This approach promises to clarify a number of puzzling observations of the Saturnian and Uranian rings.
(1) Many studies have concluded that Saturn's rings are many particles thick (e.g. Lumme et al. 1983), whereas dynamical calculations predict that optically thick rings should be physically thin (Wisdom & Tremaine 1988 and references therein). Lumme et al. argue that the particles in Saturn's B Ring fill only 2% of the volume of the ring, while Wisdom and Tremaine predict a filling factor of 20% or more.
The claim that Saturn's rings are thick is based on their observed opposition surge, a rapid brightening (0.3 mag in the V band) which occurs at phase angles below about 1.5°. The surge is attributed to particles covering their own shadows near opposition. Shadowing can occur either between discrete particles, or within the surface structure of a particle. The range in phase angle over which the brightening takes place is proportional to the volume filling factor of the ring or surface. Thus the very narrow opposition effect of Saturn's rings implies a very porous ring, unless individual particles backscatter extremely strongly.