Chapter 13 The Structure of Saturn’s Rings J.E. Colwell, P.D. Nicholson, M.S. Tiscareno, C.D. Murray, R.G. French, and E.A. Marouf Abstract Our understanding of the structure of Saturn’s not massive enough to clear a gap, produce localized pro- rings has evolved steadily since their discovery by Galileo peller-shaped disturbances hundreds of meters long. Parti- Galilei in 1610. With each advance in observations of the cles throughout the A and B rings cluster into strands or rings over the last four centuries, new structure has been re- self-gravity wakes tens of meters across that are in equilib- vealed, starting with the recognition that the rings are a disk rium between gravitational accretion and Keplerian shear. In by Huygens in 1656 through discoveries of the broad or- the peaks of strong density waves particles pile together in a ganization of the main rings and their constituent gaps and cosmic traffic jam that results in kilometer-long strands that ringlets to Cassini observations that indirectly reveal indi- may be larger versions of self-gravity wakes. The F ring is a vidual clumps of particles tens of meters in size. The va- showcase of accretion and disruption at the edges of Saturn’s riety of structure is as broad as the range in scales. The Roche zone. Clumps and strands form and are disrupted as main rings have distinct characteristics on a spatial scale of they encounter each other and are perturbed by close encoun- 104 km that suggest dramatically different evolution and per- ters with nearby Prometheus. The menagerie of structures in haps even different origins. On smaller scales, the A and the rings reveals a system that is dynamic and evolving on C ring and Cassini Division are punctuated by gaps from timescales ranging from days to tens or hundreds of millions tens to hundreds of kilometer across, while the B ring is lit- of years. The architecture of the rings thus provides insight tered with unexplained variations in optical depth on sim- to the origin as well as the long and short-term evolution of ilar scales. Moons are intimately involved with much of the rings. the structure in the rings. The outer edges of the A and B rings are shepherded and sculpted by resonances with the Janus–Epimetheus coorbitals and Mimas, respectively. Den- 13.1 Grand Structure of the Rings sity waves at the locations of orbital resonances with nearby and embedded moons make up the majority of large-scale features in the A ring. Moons orbiting within the Encke The number of distinct rings and ring features has long sur- and Keeler gaps in the A ring create those gaps and pro- passed the alphabetical nomenclature assigned to the rings duce wakes in the nearby ring. Other gaps and wave-like prior to spacecraft investigations of the Saturn system. In features await explanation. The largest ring particles, while many cases these lettered rings contain multiple ringlets within them. Nevertheless, they can be broadly grouped into two categories: dense rings (A, B, C) and tenuous rings (D, J.E. Colwell () E, G). The Cassini Division, a ring region in its own right Department of Physics, University of Central Florida, Orlando, that separates the A and B rings (Fig. 13.1), resembles the C FL 32816-2385, USA ring in many ways. The subjects of this chapter are the dense P.D. Nicholson and M.S. Tiscareno rings as well as the Cassini Division and F ring. The Roche Department of Astronomy, Cornell University, Ithaca, NY 14853, USA Division, separating the A and F rings, contains tenuous ma- C.D. Murray terial more like the D and G rings (see Chapter 16). While Astronomy Unit, Queen Mary, University of London, Miles End Road, the dense rings covered in this chapter contain various tenu- London E1 4NS, UK ous (and likely dusty1) rings within them, these are covered R.G. French Department of Astronomy, Wellesley College, Wellesley, MA 02481, USA E.A. Marouf 1 We use the term “dusty” to refer to particle sizes less than 100 m, Department of Electrical Engineering, San Jose State University, and not composition. All of Saturn’s rings are composed primarily of San Jose, CA 95192, USA water ice (Chapter 15). M.K. Dougherty et al. (eds.), Saturn from Cassini-Huygens, 375 DOI 10.1007/978-1-4020-9217-6_13, c Springer Science+Business Media B.V. 2009 376 J.E. Colwell et al. 5 4 C Ring A Ring 3 Cassini Division 2 1 UVIS Normal Optical Depth B Ring 0 75000 80000 85000 90000 95000 100000 105000 110000 115000 120000 125000 130000 135000 140000 Ring Plane Radius (km) Fig. 13.1 ISS mosaic (NASA/JPL/Space Science Institute) and UVIS taken from an elevation of 4ı above the illuminated (southern) face of stellar occultation data showing the A, B, C rings and Cassini Division the rings, so optically thick regions appear brighter than optically thin (CD) at approximately 10 km radial resolution. The F ring is visible in regions the ISS mosaic beyond the outer edge of the A ring. The images were in Chapter 16. Overall the dense rings have typical optical or liquid but were instead made up of an indefinite number of depths greater than 0.1 and are predominantly comprised of small particles, each on its own orbit about Saturn (Maxwell particles larger than 1 cm while the dusty rings have optical 1859). In 1866, D. Kirkwood was the first to suggest that the depths of 103 and lower. The F ring, torn quite literally be- Cassini Division is caused by a resonance with one of Sat- tween the regime of moons and rings, has a dense and com- urn’s moons (Kirkwood 1866), an idea that would later be plex core embedded in a broad sheet of dust, and is covered taken up by P. Goldreich and S. Tremaine in applying the the- in this chapter. ory of Lindblad resonances in spiral galaxies to describe spi- The study of structure within Saturn’s rings originated ral waves in the rings (Goldreich and Tremaine 1978b). Our with G. Campani, who observed in 1664 that the inner half knowledge of ring structure was thoroughly revolutionized of the disk was brighter than the outer half. More famously, by the Pioneer (1979) and Voyager (1980, 1981) encoun- G. D. Cassini discovered in 1675 a dark band between the ters with Saturn (Gehrels and Matthews 1984; Greenberg brighter and dimmer halves, which he interpreted as a gap. and Brahic 1984). Three new rings were discovered (D, F Careful observations by W. Herschel confirmed in 1791 that and G) as well as several new ring-shepherding moons, and Cassini’s Division appears identical on both sides of the detailed ring structure was revealed for the first time – by rings, convincing Herschel that it is in fact a gap and not images taken at close range, by stellar occultation (observing just a dark marking (Alexander 1962; Van Helden 1984). In the flickering of a star as it passes behind the rings) and by ra- 1837, J. F. Encke observed that the middle part of the A ring dio occultation (measuring the attenuation of the spacecraft’s appears dimmer than the inner and outer parts, but not until radio signal as it passes behind the rings as seen from Earth). 1888 did J. E. Keeler clearly see what is now called the Encke Esposito et al. (1987) catalogued 216 features based on the Gap, narrow and sharp near the A ring’s outer edge (Oster- Voyager 2 • Sco stellar occulation. Subsequently, ring struc- brook and Cruikshank 1983). By that time astronomers had ture was probed by widespread Earth-based observation of a noted a number of other markings on the rings, though they stellar occultation in 1989 (Hubbard et al. 1993; Nicholson did not agree on their locations or their nature. Meanwhile et al. 2000; French and Nicholson 2000) and finally by the the C ring, also called the “crepe” ring, was discovered in arrival of the Cassini orbiter in 2004. 1850 by W. C. Bond and G. P. Bond, and independently by Various structural features repeat across the ring system. W. R. Dawes. Sharp-edged gaps are present in the C and A rings and The mystery of the rings’ nature was finally resolved in Cassini Division. Waves launched by resonances with satel- 1859, when J. C. Maxwell proved that they could not be solid lites are seen throughout the main ring system (Chapter 14). 13 The Structure of Saturn’s Rings 377 The inner edges of the A and B rings are remarkably sim- Cassini as well as theoretical and numerical studies of parti- ilar morphologically, a fact that has been attributed to re- cle disks should help us understand this complex and beauti- distribution of material at edges through ballistic transport ful system. of meteoroid ejecta (Durisen et al. 1992). Perhaps what the rings have most in common with each other, however, is the abundance of unexplained structure. Large and abrupt fluc- tuations in optical depth are evident in stellar and radio oc- 13.2 A Ring cultations by the rings. The most striking examples of these are in the central B ring where opaque regions .£ > 5/ are The most prominent features of Saturn’s A ring (Fig. 13.2) punctuated by valleys of moderate optical depth .£ < 2/.The are the multitude of density waves launched at Lindblad reso- outer C ring has numerous plateaus of £ D 0:4 embedded in nances with nearby moons (mostly Prometheus and Pandora) a background with optical depth £ 0:1, whose origin is and the two gaps in the outer A ring cleared by the moons still unknown.