John Wilfred Weiss

Saturn's small inner satellites: clues to their origins.

Cassini images of Saturns small inner satellites (radii of less than ∼100 kilometers) have yielded their sizes, shapes, and in some cases, topographies and mean densities. This information and numerical N-body simulations of accretionary growth have provided clues to their internal structures and origins. The innermost ring-region satellites have likely grown to the maximum sizes possible by accreting material around a dense core about one-third to one-half the present size of the moon. The other small satellites outside the ring region either may be close to monolithic collisional shards, modified to varying degrees by accretion, or may have grown by accretion without the aid of a core. We derived viscosity values of 87 and 20 square centimeters per second, respectively, for the ring material surrounding ring-embedded Pan and Daphnis. These moons almost certainly opened their respective gaps and then grew to their present size early on, when the local ring environment was thicker than it is today.

100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures

Saturns main rings are composed predominantly of water-ice particles ranging between about 1 centimetre and 10 metres in radius. Above this size range, the number of particles drops sharply, according to the interpretation of spacecraft and stellar occultations. Other than the gap moons Pan and Daphnis (the provisional name of S/2005 S1), which have sizes of several kilometres, no individual bodies in the rings have been directly observed, and the population of ring particles larger than ten metres has been essentially unknown. Here we report the observation of four longitudinal double-streaks in an otherwise bland part of the mid-A ring. We infer that these ‘propeller’-shaped perturbations arise from the effects of embedded moonlets approximately 40 to 120 m in diameter. Direct observation of this phenomenon validates models of proto-planetary disks in which similar processes are posited. A population of moonlets, as implied by the size distribution that we find, could help explain gaps in the more tenuous regions of the Cassini division and the C ring. The existence of such large embedded moonlets is most naturally compatible with a ring originating in the break-up of a larger body, but accretion from a circumplanetary disk is also plausible if subsequent growth onto large particles occurs after the primary accretion phase has concluded.

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.

SIMULATIONS OF THE DYNAMICAL AND LIGHT-SCATTERING BEHAVIOR OF SATURN'S RINGS AND THE DERIVATION OF RING PARTICLE AND DISK PROPERTIES

We study the light-scattering behavior of Saturns rings for the purpose of deducing the nature and distribution of the particles comprising them and the collisional and dynamical environments in which they reside. To this end, we have developed two complex numerical codes to apply to this objective. One is a geometric ray-tracing code that scatters rays from a light source at an arbitrary illumination angle into a computer-generated patch of ring particles of predetermined photometric properties and size distribution, and counts the rays that emerge into arbitrary viewing directions. The code accounts for singly and multiply scattered light as well as the illumination of the rings by the planet Saturn. We examine the light-scattering behavior of various realizations of particle distribution, ring thickness, and optical depth—assuming macroscopic, backscattering particles with radii in the centimeter-to-meter range—and have compared our experimental results with classical analytical single-scattering and numerical multiple-scattering calculations, and with Cassini images of Saturns A and C rings. We can reproduce the classical photometric results when vertically thick particle distributions are used, and we find good agreement with the observations when physically thin particle distributions are used, that is, in regimes where classical theory fails. This work has allowed us to demonstrate that the particles in the low optical depth portion of the C ring reflect about 32% of the incident sunlight in a manner similar to that of the jovian moon, Callisto. Those orbiting beyond the Encke gap in Saturns A ring are nearly twice as reflective, and are slightly more forward scattering than those in the C ring. The A ring vertical full thickness beyond the Encke gap is likely to be very thin, ~10 m. The thickness of the C ring is not discernible from this work. The optically thicker A and B rings are darker at high phase than classical calculations predict because they are so thin. We have also incorporated the capability to realistically simulate a patch of colliding, self-gravitating particles in Saturns gravity field into a sophisticated N-body parallel tree code. This code can model dissipative collisions among several million particles with optional sliding friction. We have applied our light-scattering code to simulations of Saturns A ring produced by this patch code in which gravitational wakes have been observed to form. We have demonstrated, as have others, that such wakes are the likely cause of the well-known azimuthal brightness asymmetry in Saturns A ring. We match the asymmetry amplitude and shape, as observed primarily in low-solar-phase Voyager images, by assuming a velocity-dependent restitution law that yields a coefficient of restitution ~3.5 times lower at the velocity dispersions appropriate for the smallest particles in Saturns rings than previously assumed; Cassini data are consistent with these results. We simultaneously find a particle albedo and phase function consistent with those deduced from photometric analyses of Cassini images taken on approach to Saturn. These results suggest that the ring particle collisions in Saturns A ring are more lossy than previously expected, a result possibly due to particle surface roughness, a regolith, and/or a large degree of porosity, all of which would lower the coefficient of restitution.

Ring Edge Waves and the Masses of Nearby Satellites

Moons embedded in gaps within Saturn’s main rings generate waves on the gap edges due to their gravitational disturbances. These edge waves can serve as diagnostics for the masses and, in some cases, orbital characteristics of the embedded moons. Although N-body simulations of the edges are far better in inferring masses from edge morphology, the long run-times of this technique often make it impractical. In this paper, we describe a faster approach to narrow the range of masses to explore with N-body simulations, to explore the multidimensional parameter space of edge/moon interactions, and to guide the planning of spacecraft observations. Using numerical, test-particle models and neglecting particle–particle interactions, we demonstrate that the simple analytic theory of the edge waves applies well to Pan in the Encke Gap but breaks down for smaller moons/ gaps like Daphnis in the Keeler Gap. Fitting an analytic model to our simulation results allows us to suggest an improved relationship between moon-mass and edge wave amplitude. Numerical methods also grant freedom to explore a wider range of moon and ring orbits than the circular, coplanar case considered by analytic theory. We examine how pre-encounter inclinations and eccentricities affect the properties of the edge waves. In the case where the moon or ring-edge particle orbits initially have eccentric radial variations that are large compared to the gap width, there is considerable variation in edge wave amplitude depending on the orbital phase of the encounter. Inclined moons also affect the edge wave amplitude, potentially significantly, as well as generate vertical waves on the gap-edges. Recent Cassini images acquired as Saturn approaches equinox and the Sun’s elevation on the ringplane is extremely low have revealed long shadows associated with the Keeler gap edge waves created by the embedded moon Daphnis. We interpret these as being cast by ∼1 km high vertical structure in the waves created by Daphnis’ out-of-plane perturbations on the ring particles.