P53E-01 INVITED
Saturn's Rings Observed with Cassini-VIMS
Since Saturn Orbit Insertion (SOI) in July 2004, the Cassini spacecraft has completed over 50 orbits around Saturn. During this time, the Visual and Infrared Mapping Spectrometer (VIMS) has made numerous observations of the rings, including high-resolution multi-spectral scans covering the wavelength range from 0.35 to 5.1~μm, at various phase angles and illumination geometries; spectral imaging at both very low (<1°) and very high (>175°) phase angles; imaging the edge-on rings; and occultations by the sun and several bright stars. In this review we will concentrate on radial variations in the rings' reflectance spectrum, with implications for ice grain size and purity, and on structural features revealed by the VIMS stellar occultation data. Included in the latter are self-gravity wakes in the A and B rings, evident in azimuthal variations in the average transmission as well as in ring microstructure; several unidentified wave trains in the C ring; evidence for a bimodal optical depth distribution in the inner B ring; and anomalous dispersive effects seen in dusty regions such as the F ring and Encke Gap. This work was supported by NASA under a contract with the Cassini-Huygens Project.
P53E-02 INVITED
Thermal Studies of Saturn's Rings
The observed thermal emission from Saturn's rings has a locational and directional variation that ultimately results from the time dependent response of individual ring particles to the quasi-periodic radiation forcing they experience as they orbit Saturn. The observed thermal emission from any radial region of the rings is representative of a large ensemble of particles or structures which are subject to statistically similar conditions as they orbit. Near any particular radius, the thermal forcing and response are quasi-periodic around an orbit, varying secularly with Saturn's twenty-nine year seasonal cycle. This talk will discuss the coupled thermal and radiative transfer processes within the rings as determined by the interplay between individual particle properties and those of the ensemble (i.e., the ring structure), and the constraints that are placed on those by the most comprehensive thermal observations of the rings to date, taken with the Cassini Infrared Spectrometer (CIRS). Over one hundred thousand thermal-infrared spectra of the rings, between 10 and 500 cm-1, have been taken with the CIRS Focal Plane 1 (FP1) detector since Cassini orbit insertion in July, 2004. They resemble scaled Planck functions with well resolved peaks indicative of temperatures between approximately 60 and 120 K. We investigate the properties of a mapping from a space of physical parameters describing ring particles and their distribution onto predicted time-dependent spectral thermal emission. Ring emission is modeled using a radiative transfer code augmented with ray tracing calculations; a thermal model is embedded within the calculation to model the particles as a thermal source, and statistical averaging is incorporated. The model is specified by a vector of parameters describing a vertically varying particle size distribution, spin distribution, thermal inertia, albedo and optical depth, and when driven by radiation calculated from ephemeris parameters it produces a self-consistent, time- and direction-dependent emission spectrum accounting for mutual shading, interparticle heating, and directionally varying emission. Any parameter vector generates a set of emission spectra that vary with orbital location and direction, and perturbations of the parameters allow investigation of the mapping represented by the model. We present analyses of the dimensionality of some sets of Cassini observations, evaluate the agreement of some simple mono- and multi-layer toy models with these, and present sensitivity studies highlighting the interplay of parameters and limitations of the model. This research was carried out at JPL/Caltech, under contract with NASA.
P53E-03 INVITED
F Ring Objects and Ring History
The peculiar nature of Saturn's narrow F ring has been a puzzle to dynamicists since the ring was first detected by the Pioneer 11 spacecraft in 1979. Most explanations for its unusual structure have involved the gravitational effects of the shepherding satellites Prometheus and Pandora, or perturbations and even collisions with an unseen population of smaller objects in the vicinity of the ring. Images of the F ring acquired by the Imaging Science Subsystem (ISS) on the Cassini spacecraft have shown a multi-stranded ring with extensive azimuthal structure. The mechanism by which Prometheus creates the observed "streamers" and "channels" in the F ring is understood and similar, regular structures due to Pandora have now been detected. However, these satellites are not sufficient in themselves to explain the observed radial and azimuthal structure in the ring. Occultation data from Cassini's Ultraviolet Imaging Spectrograph have revealed the presence of objects with diameters in the range 30m to 600m in the vicinity of the F ring. Several objects have also been detected in sequences of high resolution ISS images but their nature is uncertain. A series of ISS observations of the F ring obtained between September 2006 and May 2007 has provided dramatic evidence of collisional events taking place in the F ring. An object, believed to be S/2004 S 6, appears to have undergone a series of collisions with the F ring core beginning in late 2006. The ISS images show clear evidence for the formation and subsequent shearing of a succession of ~~1000km long radial "jets". The resulting F ring is remarkably similar to its state in April 2005 suggesting that collisions may occur in cycles. The images suggest that there may be other objects undergoing collisions with the F ring. Furthermore, a detailed examination of the evolving nature of small-scale azimuthal structure and numerical modelling suggest the presence of an additional population of small (diameter <1km) objects interacting with ring material. These observations may help to explain the dynamical evolution of the F ring region on a variety of timescales.
P53E-04 INVITED
Propellers: Theory and observation
The question on the origin and evolution of planetary rings is one of the prominent unsolved problems of planetary sciences with direct implications for planet-forming processes in preplanetary disks. The recent detection of four propeller-shaped features in Saturn's A ring (Tiscareno et al., 2006) proved the presence of large boulder-sized moonlets in the rings (Spahn & Sremcevic, 2000). Their very existence favors a ring creation in a catastrophic disruption of an icy satellite (Sremcevic et al., 2007) rather than a co-genetic origin with Saturn, since bodies of this size can hardly have accreted inside the rings. Here we will review the current state of theoretical modeling and Cassini observations.