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 10 3 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. The inner A ring exhibits some unexplained Pan (Encke gap) and Daphnis (Keeler gap). The outer edge structure similar to that in the B ring. Sharp gap edges in of the A ring is located at the 7:6 inner Lindblad resonance the A ring, C ring, and Cassini Division exhibit unexpect- (ILR) with the coorbital satellites Janus and Epimetheus. edly complex structure. A clumpy microstructure dubbed The outer region of the A ring, from the Encke gap to the “self-gravity wakes” is beginning to be understood in the A outer edge, is particularly crowded with density waves as and B rings. The A ring contains unseen embedded moon- resonances with nearby Prometheus and Pandora are tightly lets that reveal their presence through the “propeller”-shaped spaced there. The central A ring, between the Encke gap and structures that form around them (Tiscareno et al. 2006a), as the strong Janus/Epimetheus 4:3 density wave at 125,270 km, well as several narrow radial bands of structure that appears is relatively featureless, punctuated by two strong Mimas “ropy” and “straw”-like in images (Porco et al. 2005). Here and Janus/Epimetheus density waves, the Mimas 5:3 bend- we identify a moonlet as an individual object that opens an ing wave, and several weaker waves. It is in this region that azimuthally limited gap but, unlike the embedded moons Pan Cassini cameras have observed the telltale propeller-shaped and Daphnis, does not clear a continuous gap in the ring. It is signatures of perturbations from 100-m sized moonlets that not yet clear whether or not these moonlets simply represent are too small to open a full gap in the rings (Tiscareno the largest members of the general particle size distribution et al. 2006a, 2008; Sremceviˇ c´ et al. 2007). The inner A ring in the rings. The outer edges of the B and A rings can be exhibits higher optical depths than the rest of the ring as understood in the context of resonant gravitational perturba- well as some fluctuations in density reminiscent of the un- tions from the moons Mimas and the coorbitals Janus and explained structure that permeates the B ring (Section 13.3). Epimetheus, respectively. The strong Pandora 5:4 density wave sits at the inner edge We will examine the structure of the rings on all scales, or- and has the longest wavetrain of any density wave except the ganized by ring region. Although certain structural features Janus 2:1 wave in the inner B ring. Just inside the abrupt in- are common to many or all of the ring regions, each main ner edge of the A ring is a step in optical depth and a ramp ring has distinct characteristics that showcase particular fea- that transitions to the Cassini Division. tures. We begin with the A ring and its retinue of waves and Particles throughout the A ring are caught between the wakes, followed by the opaque, massive B ring which is char- competing tendencies for gravitational clumping and Keple- acterized by large-scale structure of unknown origin. We de- rian shear. As a result, at any given time most of the mass scribe the C ring and Cassini Division jointly. Situated on of the ring is in particles that are part of an ephemeral ag- either side of the B ring, these more tenuous regions of the glomeration of particles rather than individual isolated ring main rings exhibit many structural features in common and particles. This has important consequences for the interpreta- are also similar in composition and particle size distribution tion of images and occultation data. These self-gravity wakes (Chapter 15). The F ring, though perhaps the least massive are canted 20–25ı from the azimuthal direction due to Ke- of the main ring regions, has a variety of unique features plerian shear. This alignment of particles introduces an az- due to its location in the outer reaches of Saturn’s Roche imuthal dependence to the apparent optical depth measured zone and strong gravitational perturbations from the moons in radio and stellar occultations (Colwell et al. 2006, 2007; Prometheus and Pandora. Observations from Cassini have Hedman et al. 2007), as well as the well-known azimuthal shown us Saturn’s rings in unprecedented detail, and signif- asymmetry in light reflected from the rings. As a result, the icant progress has been made in understanding some aspects concept of “normal optical depth” usually used to describe of the large-scale and small-scale structure of the rings. Many the rings breaks down. The optical depth observed in an oc- features remain a puzzle, however. Further observations from cultation depends not just on the amount of material in the 378 J.E. Colwell et al.
2.0 Cassini Division Ramp UVIS: Alpha Arae Rev 32 Encke Gap Keeler Gap RSS: Rev 7E X Band (X 0.5) 1.5 Mimas 5:3 Janus 6:5 Janus 5:4 Janus 4:3 BW DW 1.0
0.5 Normal Optical Depth
0.0 120000 125000 130000 135000 Ring Plane Radius (km)
Fig. 13.2 The A ring and outer Cassini Division seen in UVIS and tically thin regions (inner A Ring Ramp) appear dark in the ISS image RSS occultation profiles and an ISS image mosaic (NASA/JPL/Space in this viewing geometry. Differences between the UVIS and RSS op- Science Institute) from above the unilluminated face of the rings with tical depths are due to different viewing geometries with respect to the a radial resolution of 6 km/pixel. The occultation data are shown at ubiquitous self-gravity wakes 10 km resolution. Both optically thick regions (inner A Ring) and op-
ring and the path length of the occulted beam through the 13.2.1 Satellite Resonance Features ring, but also on the orientation of that beam relative to the self-gravity wakes. Nevertheless, here we report normal op- 13.2.1.1 Density and Bending Waves tical depths computed in the usual way from the slant angle, B, of the observed beam relative to the ring plane, Spiral density waves are densely packed throughout the A ring because of its proximity to moons and the resulting I0 n D ln (13.1) close-spacing of resonances. Spiral waves are generated in I b a ring at the locations of resonances with perturbing moons, and they propagate away from the resonance location in a sin- where I is the measured intensity of the occulted beam, I0 gle radial direction. Density waves are radially-propagating is the unocculted intensity, b is any background signal, and compressional waves that arise at locations where a ring par- Djsin .B/ j. While this neglects the effect of self-gravity ticle’s radial frequency › is in resonance with the perturbing wakes, it provides a consistent means for presenting occul- moon, while bending waves are transverse waves that arise at tation profiles. Differences in the absolute value of optical locations where a ring particle’s vertical frequency is in res- depths between profiles can then be interpreted as due to onance with the perturbing moon. The spiral structure arises the different aspect of the wakes with respect to the viewing because the phase of the wave depends on longitude within geometry2. the ring. The multiplicity of resonance types (and thus wave types) is due to Saturn’s oblateness, which causes and › to differ slightly from the orbital frequency . Goldreich and Tremaine (1978a, b, 1980) were the first to point out that structure could arise in Saturn’s rings due 2 There can be additional differences in optical depth at different wave- to the same physics that causes galaxies to have spiral arms lengths depending on the size distribution of the ring particles. Also, (Lin and Shu 1964). This insight was proved correct by the radio occultation optical depths are typically a factor of two higher than Voyager flybys, as a plethora of waves was revealed in high- stellar occultation optical depths because the diffracted signal can be separated from the directly transmitted signal in radio occultations. See resolution stellar occultations (Lane et al. 1982; Holberg also Cuzzi (1985) and Section 13.3 for more details. et al. 1982; Esposito et al. 1983a) and radio occultations 13 The Structure of Saturn’s Rings 379
(Marouf et al. 1986). The general theory of resonant interac- satellite, the phase '0 depends on the longitude of observa- tions within planetary rings was reviewed by Goldreich and tion and that of the satellite, while the damping constant D Tremaine (1982) and by Shu (1984), and many aspects of describes the ring’s viscous response. the Voyager results were summarized by Cuzzi et al. (1984). The integral in Eq. 13.2 is a Fresnel integral, which sig- Follow-up studies of spiral waves in Saturn’s rings were done nificantly modulates the result near the wave’s generation by Nicholson et al. (1990), who used both Voyager PPS and point, but oscillates about unity for higher values of Ÿ.Down- RSS data to pinpoint the locations of resonant wave gener- stream, then, the dominant component of Eq. 13.2 has the ation and thus refine the direction of Saturn’s pole, and by form of a sinusoid with constantly decreasing wavelength (as Rosen et al. (1991a, b), who wrote extensively on methods well as modulating amplitude), such that the wavenumber, of density wave analysis and were the first to fit wave am- k D 2 =œ, increases linearly with distance from rL: plitudes in order to derive masses for the perturbing moons.
Finally, Spilker et al. (2004) undertook a comprehensive sur- DL vey of spiral density waves in the Voyager PPS data set. k.r/ D .r rL/: (13.5) 2 G 0rL By far the most common waves in Saturn’s rings are spi- ral density waves driven at inner Lindblad resonances (ILR). Each of the dozens of spiral waves in Saturn’s rings serves These can be described as variations on the background sur- as an in situ probe, by which local properties of the ring can face density ¢0. Wave dynamics are described in more de- be obtained. The ideal wave profile (Eq. 13.2) contains four tail in Chapter 14; here we will discuss waves in the context tunable parameters that can be constrained by an observed of data analysis. At orbital radius r greater than the reso- wave’s morphology: rL, ¢0, AL, ŸD. Changing the resonance nance location rL, the classical linear theory (ignoring pres- location rL amounts to a radial translation of the entire wave. sure effects) gives The background surface density ¢0 controls the wavelength ( " Z #) dispersion, or the slope with which the wavenumber in- 2 2 3 D i0 i =2 i .=D / .r/ Re iALe 1 ie e d e ; creases with radius (Eq. 13.5). The wave amplitude A is 1 L (13.2) proportional to the perturbing moon’s mass. The damping where the dimensionless radial parameter is length D controls the point at which the wave amplitude be- gins to decay after its initial growth, and is a measurement of 1=2 the ring’s kinematic viscosity (which is dominated by inter- DLrL r rL D ; (13.3) particle collisions). A fifth tunable parameter in Eq. 13.2 is 2 G 0 rL the phase '0, which can be calculated from the perturbing and further terms are defined below. Similar equations exist moon’s position on its orbit, though this is usually so well for density waves generated at outer Lindblad resonances and constrained by direct observation of the moon that it is an for bending waves. input for spiral wave analysis rather than a result. Assuming Saturn’s gravity is well described as a point The easiest parameter to obtain from spiral wave data is the surface density. Values inferred by a number of in- mass plus a J2 harmonic, the factor DL is given by Cuzzi et al. (1984) and Marley and Porco (1993) vestigators are plotted in Fig. 13.3. The surface mass den- sities measured for the three innermost waves in the Cassini 2 R 2 21 9 Division are 1 gcm in regions where £ 0:1 (Porco D D 3.m 1/ 2 C J S .m 1/ 2 ; L L 2 r 2 2 L et al. 2005; Tiscareno et al. 2007; Colwell et al. 2008). In con- L 2 trast, waves in the A ring typically indicate ¢0 40 gcm (13.4) where £ 0:5. The larger value of the opacity › D £=¢ in the Cassini Division suggests a different ring particle size where the second term is a small correction except for m D 1. distribution (see also Chapter 15). In addition, investigators The Lindblad resonance occurs at rL. The local mean have obtained viscosity (Esposito et al. 1983a; Tiscareno motion is L, which must be calculated accounting for the et al. 2007) and amplitudes (Rosen et al. 1991a, b) from higher-order moments of Saturn’s gravity field (Murray and density waves. The kinematic viscosity is derived from the Dermott 1999). For the purposes of calculating spherical damping length using (Tiscareno et al. 2007) harmonics, Saturn’s radius RS D 60;330 km by convention (Kliore et al. 1980). The resonance’s azimuthal parameter is 9 r 1=2 m, a positive integer that gives the number of spiral arms. L 3=2 D 3 .2 G 0/ ; (13.6) 7L DL The amplitude AL is related to the mass of the perturbing D 380 J.E. Colwell et al.
a
b
Fig. 13.3 (a) Surface density ¢ and (b) viscosity and RMS velocity in Saturn’s A ring and Cassini Division, as inferred from density waves
where, for this purpose, the radial frequency ›L is approx- ring, H, can be estimated from the RMS velocities shown in imately equal to the orbital frequency L. The viscosity is Fig. 13.3 by dividing those values by the value of (which directly related to the ring particles’ rms random velocity c is 1:4 10 4 s 1 at r D 124;500 km). However, such an (Araki and Tremaine 1986; Wisdom and Tremaine 1988) estimate is only an upper limit because it assumes that ran- dom velocities in the ring are isotropic, when in fact self- c2 gravity wakes cause in-plane random velocities to be higher D k C k D2; (13.7) 1 1 C 2 2 than out-of-plane velocities. Upper-limit estimates of the thickness H are 3–5 m in the Cassini Division and 10–15 m where £ is the local optical depth, D is the particle diam- in the inner A ring (Tiscareno et al. 2007; Colwell et al. eter, and k1 and k2 are constants of order unity (see also 2009). Chapter 14). When the ring particle interactions are isolated A fundamental assumption of the classical linear theory two-particle collisions and the ring particle density is not so (Eq.13.2) is that the perturbations in density are small com- high that particle size becomes important, Eq. 13.7 can be ap- pared to the background density. In practice, this assumption proximated by the first term alone with k1 D 2 (e.g. Tiscareno is violated for many of the stronger waves in Saturn’s rings, et al. 2007). This assumption is valid for the Cassini Division. including probably all of the waves observed by Voyager. While self-gravity wakes complicate the issue in the A ring, In non-linear waves, density peaks become strong and nar- the expression is nonetheless useful and provides a reason- row while troughs become wide and flat, and the wavenum- able approximation to the velocity dispersion, c. ber increase with radius is more quadratic than linear (Shu Under the assumption that random velocities are isotropic, et al. 1985). This calls into question the application of clas- the ring’s vertical scale height can be estimated as H c= . sical linear wave theory to strong waves. However, this assumption is violated in much of the A Ring Some investigators have avoided this difficulty by an- due to self-gravity wakes (see below), which cause random alyzing waves other than the prevalent non-linear density velocities to be larger within the equatorial plane than in the waves. Bending waves have been used to infer surface den- vertical direction, thus depressing the vertical scale height sity and viscosity in a few locations (Shu et al. 1983; implied by a given magnitude of rms velocity. An approx- Lissauer et al. 1984, 1985; Chakrabarti 1989), though only imate upper limit on the thickness or scale height of the five bending waves have been clearly identified in Saturn’s 13 The Structure of Saturn’s Rings 381 rings (Lissauer et al. 1985; Rosen and Lissauer 1988). More are deflected with just one impulse. Because random veloci- recently, Tiscareno et al. (2007) used the greater sensitiv- ties in Saturn’s rings are generally quite small, one can rea- ity of Cassini imaging to probe weak density waves, includ- sonably equate a particle’s semimajor axis a with the radial ing waves driven by the small moons Pan and Atlas and at location at which it passes most closely to the moon, and second-order resonances with the larger moons. Yet another thus call the impact parameter a. The outgoing ring par- approach is to directly confront the challenge of constructing ticle (or streamline) now has an eccentricity imparted to it a comprehensive theory of non-linear density waves that has (Dermott 1984), practical applications (Borderies et al. 1986; Longaretti and 4m a 2 Borderies 1986; Rappaport et al. 2008). e D : (13.8) The density waves driven by the co-orbital moons Janus 3M a and Epimetheus, which “swap orbits” every 4 years, have Due to Keplerian shear, ring particles inward of the moon a particularly irregular morphology that changes with time. move faster, and those outward of the moon move slower; Tiscareno et al. (2006b) modeled three weak second-order the completion of one eccentric orbit corresponds to one si- Janus/Epimetheus waves (thus avoiding the problems of non- nusoid in the streamline, and the azimuthal wavelength in linearity) by adding together wave segments from differ- a frame rotating with the satellite is 3 a. The canoni- ent moon configurations, assuming that each segment ceases cal example of a gap of this kind is the Encke Gap in the being generated but continues to propagate with a finite outer A Ring (Fig. 13.4). Its wavy edges were first noticed by group velocity when the perturbing moons change their or- Cuzzi and Scargle (1985), who concluded that a moon must bits. Their results show that density waves can be used as exist within the gap (see Section 13.2.2.2 for the continua- historical records in the case of a perturbation that changes tion of that story). However, more thorough mapping of the with time. Encke Gap edges by Cassini is revealing much more com- plex structure than the expected 3 a sinusoids (Tiscareno et al. 2005; Torrey et al. 2008). 13.2.1.2 Outer Edge of the A Ring Although the structure of the Encke Gap edges is enig- matic, it is relatively subdued, with amplitudes ( 1 km) a A Lindblad resonance will drive a spiral density wave only small fraction of the moon-edge distance (160 km). Not so if sufficient dense ring material is available downstream. The the Keeler Gap, near the A Ring’s outer edge, which is only original application of Goldreich and Tremaine (1978b) was 40 km across with an inner edge that varies by 15 km. These that the angular momentum deposited into the ring by the variations were first catalogued by Cooke (1991), who was resonance will entirely clear the downstream region of mate- unable to find a simple model that would fit the data. Again, rial. This turns out to be true not only for the outer edge of mapping of the gap edges by Cassini has revealed a very the B Ring, but for several ringlets in the C Ring and Cassini complex Keeler inner edge, characterized to first order by Division and for the outer edge of the A Ring. a 32-lobe structure attributable to the Prometheus 32:31 res- The A Ring edge is confined by the 7:6 Lindblad res- onance straddled by the edge, but with a great deal of other onance with the co-orbital moons Janus and Epimetheus, structure superimposed on top of that, some perhaps due to and to first order exhibits a 7-lobed shape consistent with the nearby Pandora 18:17 resonance (Tiscareno et al. 2005; streamlines perturbed by the resonance (Porco et al. 1984b). Torrey et al. 2008). Cassini mapping of the edge is revealing more complex The outer edge of the Keeler Gap, at least away from structure, with a time-variable component that may be linked Daphnis, is close to circular, punctuated periodically by to the changes in Janus’ and Epimetheus’ orbits (Spitale feathery “wisps” (Porco et al. 2005) that have a sharp trail- et al. 2008a). ing edge of 1 km amplitude and a gentle slope on the lead- ing side (Torrey et al. 2008). Only the region within a few degrees longitude of Daphnis shows the canonical 3 a wavy edges. Weiss et al. (2008) have shown that the usual 13.2.2 Satellite Impulse-Driven Features expression for deriving Daphnis’ mass from the edge am- plitude does not apply for the narrow Keeler Gap. Numer- 13.2.2.1 Wavy Edges of the Encke and Keeler Gaps ical models are able to reproduce the irregular shape of the wavy edges surrounding Daphnis which arise in part from the Unlike at resonant locations, where the effects of many dis- much closer proximity of the moon to the ring edge than in tant encounters with a perturbing moon add up construc- the Encke Gap (Lewis and Stewart 2005, 2006; Perrine and tively, the orbits of ring particles that pass close by a moon Richardson 2006, 2007). 382 J.E. Colwell et al.
Prometheus 15:14
Pan Wakes
Pandora 12:11
Prometheus 17:16
Wavy Edge
Fig. 13.4 The Encke gap (320 km wide) imaged by Cassini at Saturn here by the inner Lindblad resonances that launch them. Streamlines Orbit Insertion showing dusty ringlets (Chapter 16), a wavy inner edge near the edge of a gap are deflected by the embedded moon, creating recently perturbed by the satellite Pan (roughly five image widths up- a wavy edge and satellite wakes, due to the moon Pan, within the ring stream of the inner edge, or up in this view of the south face of the (see also Chapter 14) (image: NASA/JPL/Space Science Institute) rings), and satellite wakes. Density waves are also visible, indicated
13.2.2.2 Satellite Wakes Showalter et al. (1986) were able to constrain the orbit of the yet-undiscovered moon. With this guidance, Pan was discov- Not only at gap edges do ring particles have their orbits de- ered by Showalter (1991) in archival Voyager images. flected by close passes by a moon. Particle streamlines con- Frequency-based analysis of radial scans of Pan’s wakes tinue to be deflected in the interior of the ring, though with (Horn et al. 1996; Tiscareno et al. 2007) shows that damping decreasing amplitude and increasing wavelength as a in- of the wakes is surprisingly inefficient. Signatures can be de- creases. This results in a pattern of alternating regions of rel- tected that imply a relative longitude of more than 1000ı,or atively greater or lesser density which remains stationary in nearly three Pan encounters ago. By contrast, although satel- the moon’s reference frame, thus allowing the satellite to ex- lite wakes are also observed in the vicinity of Daphnis, they ert a torque on the ring. First observed in Voyager data, these do not extend more than a few degrees of longitude or a few satellite wakes were observed in remarkable detail by Cassini tens of km radially from the moon. The proximity of Daph- at Saturn Orbit Insertion on July 1 2004 (Fig. 13.4). This pat- nis to the Keeler Gap edges results in particle streamlines tern does not depend on mutual particle interactions and does (cf. Showalter 1986) crossing immediately after encounter- not propagate; it is purely a kinematic effect as neighboring ing Daphnis, and the wakes are therefore damped over a streamlines bunch together or spread apart.3 much smaller azimuthal extent than the Pan wakes. After the initial suggestion by Cuzzi and Scargle (1985) that the Encke Gap’s wavy edges implied an embed- ded moon, Showalter et al. (1986) found the radial sig- 13.2.3 Self-Gravity Wakes and Propellers nature of satellite wakes in stellar and radio occultation scans. Since the radial scan of a satellite wake changes in frequency with the perturbing moon’s relative longitude, Another type of wake in the rings is the result of the compet- ing processes of gravitational accretion of ring particles and Keplerian shear. Sometimes called gravity wakes or Julian- Toomre wakes for their similarity to wakes produced by a 3 The same applies to wavy gap edges, which should not strictly be massive body in a galaxy (Julian and Toomre 1966; Colombo called “edge waves.” et al. 1976), these features are sheared agglomerates of ring 13 The Structure of Saturn’s Rings 383 particles that form due to the mutual gravitational attrac- wakes do arise in a self-gravitating ring. Dones et al. (1993) tion of ring particles. For a ring that is marginally unstable measured the strength and shape of the azimuthal brightness (Toomre’s parameter Q D 1, Toomre 1964), the most unsta- variations in the A ring from Voyager images, and found ble length scale is a sharp maximum in amplitude near the center of the A ring (R D 128;000 km). In an extensive examination of 2 2 4 G 0= (13.9) Hubble Space Telescope WFPC2 images taken over a full Saturn season, French et al. (2007) systematically explored where 0 is the ring surface mass density and is the orbital the nature of the brightness variations over a broad range frequency. In the central A ring where these wakes are most of ring opening angles, and compared the observations to prominent, this wavelength is about 60 m. In the inner B ring, models based on a suite of N-body dynamical simulations where they are also observed (see Section 13.3.6), the scale of self-gravity wakes and a realistic ray-tracing code (Salo 2 may be somewhat smaller (43 m using 0 D 70 gcm from and Karjalainen 2003). They found evidence for asymmetry the Janus 2:1 density wave (Holberg et al. 1982; Esposito not only in the A ring (Fig. 13.5), but in relatively optically et al. 1983a)). No massive seed particle is required for thin regions in the B ring as well (Fig. 13.6; see also the formation of self-gravity wakes so that, unlike satellite Section 13.3.6). Porco et al. (2008) reproduced Voyager and wakes, they are not “wakes” in the usual sense of the word Cassini observations of the azimuthal brightness asymmetry (Colwell et al. 2006). Their cant angle of 20–25ı to the lo- in the A ring, also with a combination of N-body simulations cal orbital flow is a natural consequence of the gradient in and ray-tracing calculations. Their results place constraints orbital velocities in a Keplerian disk (e.g. Salo et al. 2004). on the dissipation in ring particle collisions, with a smaller N-body simulations produce wakes with properties that are coefficient of restitution than previous models (e.g. Bridges consistent with measurements made by Cassini as well as et al. 1984; Supulver et al. 1995) needed to match the phase Voyager imaging observations (e.g. Salo et al. 2004; Porco and amplitude of the asymmetry, and 40% greater energy et al. 2008). loss in collisions than in French et al. (2007). Evidence for Observations of a pronounced azimuthal brightness strong azimuthal asymmetry has also been found at radio asymmetry (Camichel 1958; Ferrin 1975; Reitsema et al. wavelengths, from radar imaging of the rings (Nicholson 1976; Lumme and Irvine 1976; Lumme et al. 1977; Thomp- et al. 2005) and from observations of Saturn’s microwave son et al. 1981; Gehrels and Esposito 1981) first provided thermal radiation transmitted through the rings (Dunn et al. the clue to the existence of these structures in the A ring 2004, 2005, 2007; Molnar et al. 1999). (Colombo et al. 1976; Franklin et al. 1987; Dones and Porco The existence of self-gravity wakes was dramatically 1989). Direct numerical simulations by Salo (1992, 1995) manifested in Cassini occultation measurements of the rings and by Richardson (1994) demonstrated that self-gravity which showed significant variations in the apparent normal
0.10 |Beff| 25.6° 26.6° 26.0° 23.6° 0.05 20.1° 15.4° 10.2° Asymmetry amplitude Fig. 13.5 Radial variations in the 4.5° amplitude of the azimuthal brightness asymmetry in the 0.00 A ring for a range of ring opening angles jBeffj, from Hubble Space Telescope WFPC2 images obtained between 1996–2004 (French et al. 2007). The radial optical depth profile is the Cassini 115 120 125 130 135 140 UVIS ’ Arae Rev 033 occultation Radius (1000 km) 384 J.E. Colwell et al.
0.04
|Beff| 25.6° 26.6° 0.02 26.0° 23.6° 20.1° 15.4° 0.00 10.2° 4.5° Asymmetry amplitude
–0.02
–0.04 80 90 100 110 120 Radius (1000 km)
Fig. 13.6 Radial variations in the amplitude of the azimuthal brightness (French et al. 2007). The radial optical depth profile is the Cassini UVIS asymmetry in the B ring over a range of ring opening angles jBeffj, from ’ Arae Rev 033 occultation Hubble Space Telescope WFPC2 images obtained between 1996–2004 optical depth in the A ring. We use the term “apparent nor- directly-transmitted and diffracted signals – and thus mea- mal optical depth” because we have applied the normal slant sures the total extinction cross-section of the ring particles – path correction to the observed optical depth (Eq. 13.1), but whereas an occultation of a broadband source such as a star the actual normal optical depth that would be observed from inevitably measures a combination of direct and diffracted a vantage point normal to the ring plane would generally flux. (See Cuzzi (1985) and French and Nicholson (2000) be higher than that measured by any occultation at a lower for further discussion of this topic; in their terminology, value of B. The reason for this is that the rings are not a the “effective extinction efficiency”, Qocc D 2 for RSS oc- homogeneous medium of minute absorbers but are rather a cultations, whereas 1