Ring Particle Composition and Size Distribution

Abstract

We review recent progress concerning the composition and size distribution of the particles in Saturn's main ring system, and describe how these properties vary from place to place. We discuss how the particle size distribution is measured, and how it varies radially. We note the discovery of unusually large “particles” in restricted radial bands. We discuss the properties of the grainy regoliths of the ring particles. We review advances in understanding of ring particle composition from spectrophotometry at UV, visual and near-IR wavelengths, multicolor photometry at visual wavelengths, and thermal emission. We discuss the observed ring atmosphere and its interpretation and, briefly, models of the evolution of ring composition. We connect the ring composition with what has been learned recently about the composition of other icy objects in the Saturn system and beyond. Because the rings are so thoroughly and rapidly structurally evolved, the composition of the rings may be our best clue as to their origin; however, the evolution of ring particle composition over time must first be understood.

15.1 Appendix 15: The Zero-Phase Opposition Effect

An entirely separate subset of scattering theory must be considered for very small phase angles (less than a degree or so), characterized by very strong brightening with the approach of true opposition. This so-called ‘opposition effect’ was initially interpreted in terms of shadow hiding in the regolith surface, and porosities were derived from the strength and width of the opposition surge. Early measurements of the opposition effect in Saturn's rings were obtained by Franklin and Cook (1965) and Lumme and Irvine (1976). Lumme et al. (1983) concluded that the opposition effect resulted from shadow hiding (SH) amongst different ring particles in a classical many-particle-thick layer (Irvine 1966) with a very low volume filling factor. This was at odds with dynamical studies (Brahic 1977, Goldreich and Tremaine 1978) indicating that the rings should be only a few particles thick, as shown by N-body dynamical simulations (Salo 1987, 1992; Wisdom and Tremaine 1988, Richardson 1994, Salo et al. 2004, Karjalainen and Salo 2004; see Chapters 13 and 14). A partial resolution to the apparent contradiction between the photometric observations and the simulations was work by Salo and Karjalainen (2003), who used Monte Carlo ray tracing studies in dense particle layers. Interparticle shadowing can even produce a narrow, sharp opposition brightening for broad particle size distributions (French et al. 2007, Salo et al. 2008 DPS).

In addition however, SH within the regolith of an individual ring particle can contribute to the opposition brightening (Hapke 1986) and coherent backscattering (CB), or the constructive interference of incoming and outgoing light rays (Muinonen et al. 1991; Mishchenko and Dlugach 1992; Hapke 1990; Mishchenko 1993), can also contribute. Both SH in regoliths and CB are complicated functions of the surface structure of the particles and the optical properties of the grains, and have been the subjects of extensive theoretical and laboratory studies (Nelson et al. 2000, Nelson et al. 2002, Hapke et al. 2005, 2009).

Fig. 15.41

Comparison of the observed A ring phase curves (crosses) to the mutual shadowing opposition effect calculated by photometric Monte Carlo simulations (curves). Dynamical simulations with seven different particle size distributions were conducted, ranging from q = 3 power laws for 0.05–5 m radius, to simulations with identical 5 m particles, (shown by different line types). At left, the two extreme size distribution models are compared to observations at different wavelengths. The single scattering albedos for the models, indicated in the middle panel, are chosen to fit the observed I/F at α ~ 6°. At right, the observations and single-scattering models are normalized to α = 6.35°. Also shown is the contribution from the adopted power-law phase function alone, (lowest dashed line) amounting to about 1.1 for the interval ° = 0° to 6.35°. The color code refers to the wavelength of the observation, as shown in the center panel

Clearly, the narrow core of the opposition surge cannot be explained by interparticle shadowing alone. French et al. (2007) fitted the opposition measurements to the composite model of Hapke (2002), which incorporates a wavelength-dependent CB component based on the theoretical predictions of Akkermans et al. (1988) and an explicit representation of SH by a particulate surface. The fits imply that the porosities of the ring particle regoliths are very high, ranging from 93% to 99%, and that the width of the narrow CB surge actually decreases with wavelength, rather than increasing. However, current CB models are somewhat idealized, and thus far, agreement between theory and experiments has been imperfect (Shepard and Helfenstein 2007, Hapke et al. 2009).

Fig. 15.42

Radial variations in the amplitude, width, and slope of the opposition surge from linear-exponential model fits to HST WFPC2 observations of Saturn's rings at five wavelengths, taken during Cycles 10–13. The colors are the same as in Fig. 15.41 The amplitude of the opposition effect (top) is nearly independent of wavelength except for the F336W filter (violet line), especially in the A and B rings, where the amplitude increases sharply at short wavelengths. (The gap in the F336W profiles between 107,000–118,000 km results from saturation of a unique low phase angle image, making the model fits unreliable in this region for this filter.) The width of the opposition surge varies strongly with ring region at short wavelengths in the A and B rings, and shows strong correlations with optical depth in the inner and outer C ring. The normalized slope (third panel) is most shallow for the optically thick central B ring. A radial profile of ring brightness is shown in the fourth panel, taken near true opposition (α = 0.0043° on January 14, 2005). The bottom panel shows the Voyager PPS optical depth profile, truncated at optical depth = 2 because of limited signal to noise at high optical depths.

It seems likely that most of these variations are attributable to differences in the degree of interparticle shadowing and to the relative widths of particle size distributions, rather than to strong regional variations in the intrinsic particle or regolith scattering properties. In the C ring, the detailed variations correlate strongly with the optical depth variations, which affects the amount of interpar-ticle shadowing. The opposition effect changes markedly at the boundary between the outer C and inner B ring, while (as shown in Section 15.4.5), the particle albedo and color, and thus presumably regolith properties, do not. Over the least opaque (inner) part of the B ring, the amplitude exceeds 0.5, decreasing gradually with increasing radius and optical depth. The Cassini division resembles the C ring in optical depth, composition and color, and possibly in particle size distribution, and these similarities are also seen in the opposition effects of these two separated ring regions. The A ring and the inner B ring have comparable optical depths, and the overall characteristics of the opposition effect are similar, including significant strengthening and broadening at short wavelengths. The particle size distribution in the inner A ring is similar as well. There is a striking contrast between the inner and outer A ring opposition effect. Salo and French (2009) used the wavelength-dependence of the opposition effect, its variation with ring tilt, and numerical modeling, to disentangle the interparticle and intraparticle oppositions effects using HST observations, and concluded that there is a very narrow, wavelength-dependent CB contribution to the opposition effect.

Cassini observations: In June 2005 (B = −21°) and July 2006 (B = −21°), Cassini conducted remote sensing observations of the opposition spot traversing the rings over a range of phase angles restricted by the angular half-width of the VIMS and ISS fields of view. Only preliminary analyses are available at the time of this writing (Nelson et al. 2006, Hapke et al. 2005, 2006, Deau et al. 2006). Based on thermal infrared observations from CIRS, Altobelli et al. (2008) measured temperature phase curves of the rings. For the C ring and Cassini Division, they interpret the opposition effect as caused by regolith on the surface of individual grains, whereas for the more optically thick A and B rings, the opposition surge is attributed to interparticle shadowing.