Essam A. Marouf

Ring Particle Composition and Size Distribution

We review recent progress concerning the composition and size distribution of the particles in Saturns 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.

An Evolving View of Saturn’s Dynamic Rings

Saturns Secrets Probed The Cassini spacecraft was launched on 15 October 1997. It took it almost 7 years to reach Saturn, the second-largest planet in the solar system. After almost 6 years of observations of the series of interacting moons, rings, and magnetospheric plasmas, known as the Kronian system, Cuzzi et al. (p. 1470) review our current understanding of Saturns rings—the most extensive and complex in the solar system—and draw parallels with circumstellar disks. Gombosi and Ingersoll (p. 1476; see the cover) review what is known about Saturns atmosphere, ionosphere, and magnetosphere. We review our understanding of Saturn’s rings after nearly 6 years of observations by the Cassini spacecraft. Saturn’s rings are composed mostly of water ice but also contain an undetermined reddish contaminant. The rings exhibit a range of structure across many spatial scales; some of this involves the interplay of the fluid nature and the self-gravity of innumerable orbiting centimeter- to meter-sized particles, and the effects of several peripheral and embedded moonlets, but much remains unexplained. A few aspects of ring structure change on time scales as short as days. It remains unclear whether the vigorous evolutionary processes to which the rings are subject imply a much younger age than that of the solar system. Processes on view at Saturn have parallels in circumstellar disks.

The Structure of Saturn's Rings

Our understanding of the structure of Saturns rings has evolved steadily since their discovery by Galileo Galilei in 1610. With each advance in observations of the rings over the last four centuries, new structure has been revealed, starting with the recognition that the rings are a disk by Huygens in 1656 through discoveries of the broad organization of the main rings and their constituent gaps and ringlets to Cassini observations that indirectly reveal individual clumps of particles tens of meters in size. The variety of structure is as broad as the range in scales. The main rings have distinct characteristics on a spatial scale of 104 km that suggest dramatically different evolution and perhaps even different origins. On smaller scales, the A and C ring and Cassini Division are punctuated by gaps from tens to hundreds of kilometer across, while the B ring is littered with unexplained variations in optical depth on similar scales. Moons are intimately involved with much of 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. Density 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 and Keeler gaps in the A ring create those gaps and produce wakes in the nearby ring. Other gaps and wave-like features await explanation. The largest ring particles, while not massive enough to clear a gap, produce localized propeller-shaped disturbances hundreds of meters long. Particles throughout the A and B rings cluster into strands or self-gravity wakes tens of meters across that are in equilibrium between gravitational accretion and Keplerian shear. In the peaks of strong density waves particles pile together in a cosmic traffic jam that results in kilometer-long strands that may be larger versions of self-gravity wakes. The F ring is a showcase of accretion and disruption at the edges of Saturns Roche zone. Clumps and strands form and are disrupted as they encounter each other and are perturbed by close encounters with nearby Prometheus. The menagerie of structures in the rings reveals a system that is dynamic and evolving on timescales ranging from days to tens or hundreds of millions of years. The architecture of the rings thus provides insight to the origin as well as the long and short-term evolution of the rings.

SATURN'S RINGS: PRE-CASSINI STATUS AND MISSION GOALS

Theoretical and observational progress in studies of Saturns ring system since the mid-1980s is reviewed, focussing on advances in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are summarized.

THE ARCHITECTURE OF THE CASSINI DIVISION

The Cassini Division in Saturn’s rings contains a series of eight named gaps, three of which contain dense ringlets. Observations of stellar occultations by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft have yielded ∼40 accurate and precise measurements of the radial position of the edges of all of these gaps and ringlets. These data reveal suggestive patterns in the shapes of many of the gap edges: the outer edges of the five gaps without ringlets are circular to within 1 km, while the inner edges of six of the gaps are eccentric, with apsidal precession rates consistent with those expected for eccentric orbits near each edge. Intriguingly, the pattern speeds of these eccentric inner gap edges, together with that of the eccentric Huygens Ringlet, form a series with a characteristic spacing of 0. ◦ 06 day −1 . The two gaps with non-eccentric inner edges lie near first-order inner Lindblad resonances (ILRs) with moons. One such edge is close to the 5:4 ILR with Prometheus, and the radial excursions of this edge do appear to have an m = 5 component aligned with that moon. The other resonantly confined edge is the outer edge of the B ring, which lies near the 2:1 Mimas ILR. Detailed investigation of the B-ring-edge data confirm the presence of an m = 2 perturbation on the B-ring edge, but also show that during the course of the Cassini Mission, this pattern has drifted backward relative to Mimas. Comparisons with earlier occultation measurements going back to Voyager suggest the possibility that the m = 2 pattern is actually librating relative to Mimas with a libration frequency L ∼ 0. 06 day −1 (or possibly 0. 12 day −1 ). In addition to the m = 2 pattern, the B-ring edge also has an m = 1 component that rotates around the planet at a rate close to the expected apsidal precession rate ( ˙ � B ∼ 5. ◦ 06 day −1 ). Thus, the pattern speeds of the eccentric edges in the Cassini Division can be generated from various combinations of the pattern speeds of structures observed on the edge of the B ring: Ωp =˙ � B − jL for j = 1, 2, 3 ,..., 7.We therefore suggest that most of the gaps in the Cassini Division are produced by resonances involving perturbations from the massive edge of the B ring. We find that a combination of gravitational perturbations generated by the radial excursions in the B-ring edge and the gravitational perturbations from the Mimas 2:1 ILR yields terms in the equations of motion that should act to constrain the pericenter location of particle orbits in the vicinity of each of the eccentric inner gap edges in the Cassini Division. This alignment of pericenters could be responsible for forming the Cassini-Division Gaps and thus explain why these gaps are located where they are.

OCCULTATION OBSERVATIONS OF SATURN'S B RING AND CASSINI DIVISION

The outer edge of Saturns B ring is strongly affected by the nearby 2:1 inner Lindblad resonance of Mimas and is distorted approximately into a centered elliptical shape, which at the time of the Voyager 1 and 2 encounters was oriented with its periapse toward Mimas. Subsequent observations have shown that the actual situation is considerably more complex. We present a complete set of historical occultation measurements of the B-ring edge, including the 1980 Voyager 1 and 1981 Voyager 2 radio and stellar occultations, the 1989 occultation of 28 Sgr, two independently analyzed occultations observed with the Hubble Space Telescope in 1991 and 1995, and a series of ring profiles from 12 diametric (ansa-to-ansa) occultations observed in 2005, using the Cassini Radio Science Subsystem (RSS). After making an approximate correction for systematic errors in the reconstructed spacecraft trajectories, we obtain orbit fits to features in the rings with rms residuals well under 1 km, in most cases. Fits to the B-ring edge in the RSS data reveal a systematic variation in the maximum optical depth at the very edge of the ring as a function of its orbital radius. We compare the B-ring measurements to an m = 2 distortion aligned with Mimas, and show that there have been substantial phase shifts over the past 25 years. Finally, we present freely precessing equatorial elliptical models for 16 features in the Cassini Division. The inner edges of the gaps are generally eccentric, whereas the outer edges are nearly circular, with ae < 0.5 km.

A numerical technique for two‐way radio occultations by oblate axisymmetric atmospheres with zonal winds

The Ultra Stable Oscillator aboard the Cassini spacecraft failed in late 2011, which means that all radio occultations after that date have to be done in two-way mode, using a ground-based signal transmitted to the spacecraft as the frequency reference. Here we present the numerical technique we use to analyze the data from the two-way atmospheric radio occultations of both Saturn and Titan that have occurred since the Ultra Stable Oscillator (USO) failure, along with the theoretical reasons behind this technique. Since our two-way technique is based upon our earlier one-way technique which used the USO as the frequency reference, we also present our one-way technique which we used for Saturn occultations prior to the loss of the USO.

A procedure to analyze nonlinear density waves in Saturn's rings using several occultation profiles

Abstract Cassini radio science experiments have provided multiple occultation optical depth profiles of Saturns rings that can be used in combination to analyze density waves. This paper establishes an accurate procedure of inversion of the wave profiles to reconstruct the wave kinematic parameters as a function of semi-major axis, in the nonlinear regime. This procedure is established using simulated data in the presence of realistic noise perturbations, to control the reconstruction error. It is then applied to the Mimas 5:3 density wave. There are two important concepts at the basis of this procedure. The first one is that it uses the nonlinear representation of density waves, and the second one is that it relies on a combination of optical depth profiles instead of just one profile. A related method to analyze density waves was devised by Longaretti and Borderies [Longaretti, P.-Y., Borderies, N., 1986. Icarus 67, 211–223] to study the nonlinear density wave associated with the Mimas 5:3 resonance, but the single photopolarimetric profile provided limited constraints. Other studies of density waves analyzing Cassini data [ Colwell, J.E., Esposito, L.W., 2007. Bull. Am. Astron. Soc. 39, 461 ; Tiscareno, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14–34] are based on the linear theory and find inconsistent results from profile to profile. Multiple cuts of the rings are helpful in a fundamental way to ensure the accuracy of the procedure by forcing consistency among the various optical depth profiles. By way of illustration we have applied our procedure to the Mimas 5:3 density wave. We were able to recover precisely the kinematic parameters from the radio experiment occultation data in most of the propagation region; a preliminary analysis of the pressure-corrected dispersion allowed us to determine new but still uncertain values for the opacity ( K ≃ 0.02 cm 2 / g ) and velocity dispersion of ( c 0 ≃ 0.6 cm/s ) in the wave region. Our procedure constitutes the first step in our planned analysis of the density waves of Saturns rings. It is very accurate and efficient in the far-wave region. However, improvements are required within the first wavelength. The ways in which this method can be used to establish diagnostics of ring physics are outlined.