P32A-01 INVITED
Mass Density and Ring Thickness from Cassini UVIS Stellar Occultations
More than 80 stellar occultations by Saturn's rings have been observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during Cassini's four-year prime mission which ended June 30 2008. These occultations occurred over a wide range of geometries defined by the relative orientations of the line-of-sight to the star, the ring plane, and the ring particle orbit velocity. While each occultation thus provides a unique one- dimensional measurement of ring transparency, the combination of multiple occultations allows the three- dimensional ring structure to be reconstructed. In addition, the signal from weak density waves can be identified by combining multiple occultations. Analysis of weak density waves in the Cassini Division show a lower surface mass density than would be anticipated by extrapolation from waves seen in the A ring. The B ring has fewer strong resonances than the A ring, and few waves have been analyzed there. A number of waves have been observed in the C ring, but the resonant forcing for most of these waves has not been identified. In addition to local mass densities, waves provide information on the vertical extent of the rings. We report on the analysis of the dispersion and damping of waves observed in UVIS data and inferred values of surface mass density and thickness. The combination of multiple occultations provides another technique for measuring ring thickness. The Saturn ring system has several abrupt edges where optically thick rings truncate at gaps over a radial range of <100 m. The high spatial resolution of the UVIS stellar occultations allows us to measure the vertical extent of the ring at the edge. Our results for ring edges and wave damping in the Cassini Division give vertical thicknesses of ~5 m. By combining multiple occultations of the same edge, we measure the radial optical depth profile at the edge, as well as the thickness, for comparison with numerical simulations to better understand ring confinement models.
P32A-02 INVITED
The Composition of Saturn's Rings
The Cassini spacecraft has obtained a unique collection of data about Saturn's rings, as it has observed the rings from 0 to 180 degrees in phase angle, and on both lit and unlit sides. Identification of trace contaminants, especially organic compounds, requires that spectra of the rings be uncontaminated by light from Saturn. The Cassini Visual and Infrared Mapping Spectrometer (VIMS) has acquired 0.35 to 5.1 micron, high spatial resolution spectroscopic data near the shadow of Saturn on the rings where scattered light is at a minimum. At low phase angles, the ring spectra show classic crystalline-ice spectral features except for a contaminant causing a UV absorption. VIMS spectra at 180-degree phase angle are generally flat, with only a weak positive feature at 2.86 microns in spectra of the F-ring. The general transmission decrease is due to large ring particles completely blocking light. The 2.86-micron feature indicates the presence of fine ice dust, where the ice's index of refraction is near 1.0, and light is not refracted or diffracted. There are no indications of interparticle scattering in the VIMS data at any phase angle. The lack of interparticle scattering indicates that the dense A and B rings must be very thin, approaching a monolayer, but rigorous constraints have yet to be modeled. Previous studies used tholins and amorphous carbon for the contaminant causing the UV absorption, but these models display additional absorptions and spectral structure in the near infrared not seen in VIMS data. Clark et al. (Icarus, v193, p372, 2008) modeled the changing blue peak and UV absorber observed on Phoebe, Iapetus, Hyperion, and Dione with amorphous carbon and nano-sized hematite. Nanohematite has muted spectral features compared to larger grained hematite, due to crystal field effects at the surfaces of small grains. Nanohematite has a strong UV absorber that matches the steep UV slope observed in spectra of Saturn's rings and has no strong IR absorptions. If the UV absorber in Saturn's rings is due to nanophase hematite then less than 1% hematite would be required, if it is uniformly mixed within the ice grains of the ring particle regoliths.
P32A-03 INVITED
Saturn's E ring as seen by the Cassini dust detector
The data returned by the Cassini spacecraft drastically changed our picture of Saturn's diffuse E ring - the largest known ring in the Solar system. Since Cassini is equipped with a dust detector it became possible for the first time to investigate the evolution cycle of the Saturnian dust. There are two processes feeding the ring with fresh dust: collisions of micrometeoroids with the surfaces of icy moons and dust injection by the recently discovered ice volcanoes on the moon Enceladus. After injection into the ring the particles spend most of their lifespan as ring particles. Finally, the grains get lost by collisions with the main rings or with the moons. More interesting, some of the ring particles interact strongly with Saturn's magnetic field and will finally form fast dust streams, which were discovered by Cassini during her approach to Saturn. We are still at the beginning of our understanding of the physical processes relevant for the dust life cycle. However, Cassini already provided us with some of the major pieces to accomplish a comprehensive picture. Here, on numerical simulations of the long term evolution of ring particles, which are based on most recent Cassini data. We show that most of the ring particles slowly migrate outwards until they get locked in the vicinity of the Rhea orbit.
P32A-04 INVITED
A debris disk surrounding Saturn's moon Rhea
During its November 2005 flyby of Saturn's second-largest moon Rhea, the LEMMS portion of Cassini spacecraft's Magnetospheric Imaging Instrument, MIMI, detected an unexpected decrease in the fluxes of high-energy (>20 keV) magnetospheric electrons. This depletion extended to around 8 Rhea radii on either side of the moon. When combined with data from several other Cassini instruments, including those from the Cassini Plasma Spectrometer, CAPS, that also detected the electron depletion, it was proposed that the MIMI data indicated the presence of a disk of electron-absorbing debris orbiting Rhea. A set of brief, deeper electron depletions on either side of the moon may indicate the presence of discrete rings or arcs within the debris disk. We summarize the case for this interpretation, including supporting studies of the orbital stability of such a disk, hybrid simulations of the magnetosphere-Rhea interaction, and the second detection of the electron depletion by MIMI and CAPS during Cassini's more distant Rhea flyby in August 2007. The Cassini Imaging Science Subsystem (ISS) has conducted several searches for material in Rhea orbit, at both high and low phases, and has found nothing to date. We discuss the implications of this non- detection. Discrete regions of unusual surface colouring have recently been identified close to Rhea's equator in ISS images; we investigate the potential for a relationship between these features and the proposed debris disk.
P32A-05 INVITED
Cassini CIRS: Lessons Learned from the Prime Mission and Plans for Rings Observations in the Extended Mission
During its four-year mission to Saturn Cassini has produced a great wealth of data with its twelve instruments, providing many new insights into Saturn's rings. The four-year measurement baseline of ring observations has allowed ring scientists to observe the rings during a significant portion of the southern hemisphere's summer. The Cassini Extended Mission promises the exciting possibility of extending this observation baseline through the upcoming equinox period in August 2009 and beyond. In this talk, we will discuss the lessons that we have learned from the CIRS observations of Saturn's rings during the Cassini prime mission and the scientific results that have been obtained. Ring temperatures derived from spectra taken with CIRS' FP1 detector (sensitive to wavenumbers from 10 to 600 cm-1 ) vary with the phase angle, inclination angle and local hour angle of the measurement. CIRS has also successfully recorded the occultation of an infrared star, CW Leo, while it passed behind the rings as seen from Cassini. To take advantage of this information and the discoveries of the other Cassini instruments, such as the wake-like structure in the A and B rings, as well as the new geometries offered by the Extended Mission, we have modified our observing strategy during the Extended Mission. Observations that exploit the high-inclination orbits at the beginning of the Extended Mission and the Sun's crossing of the ring plane have been planned. These new strategies will be discussed, as well as some preliminary results from the beginning of the Cassini Extended Mission. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Copyright 2008 California Institute of Technology. Government sponsorship acknowledged.
P32A-06
Weak localization of electromagnetic waves and radar polarimetry of Saturn's rings
We use a state-of-the-art physics-based model of electromagnetic scattering to analyze average circular polarization ratios measured for the A and B rings of Saturn at a wavelength of 12.6 cm. This model is directly based on the Maxwell equations and accounts for the effects of polarization, multiple scattering, weak localization of electromagnetic waves, and ring particle nonsphericity. Our analysis is based on the assumption that the observed polarization ratios are accurate, mutually consistent, and show a quasi-linear dependence on the opening angle. Also, we assume that the ring system is not strongly stratified in the vertical direction. Our numerical simulations rule out the model of spherical ring particles, favor the model of ring bodies in the form of nearly spherical particles with small-scale surface roughness, and rule out nonspherical particles with aspect ratios significantly exceeding 1.2. They also favor particles with effective radii in the range 4–10 cm and definitely rule out effective radii significantly smaller than 4 cm. Furthermore, they seem to rule out effective radii significantly greater than 10 cm. The retrieved ring optical thickness values are in the range 2–3 or even larger. If the rings do have a wake-like horizontal structure, as has been recently suggested, then these optical thickness values should be attributed to an average wake rather than to the optical thickness averaged over the entire horizontal extent of the rings.
P32A-07
The Spectral Classes of the Saturnian System Ices: Rings and Satellites Observations by Cassini-VIMS
The entire population of the Saturnian system ices was investigated by VIMS (Visual and Infrared Mapping Spectrometer) experiment on board Cassini spacecraft. By the end of the nominal mission a very large dataset of hyperspectral data had been collected in the spectral range 0.35-5.0 micron, which includes the regular satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, Phoebe), minor moons (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso) and rings. In this work we present an analysis of spectrophotometric indicators selected to describe the properties of the ices (I/F continuum levels, visible spectral slopes, band depths and positions), and which were retrieved from about 1500 full-disk observations of satellites as well as from mosaics of the main rings (A, B, C, CD, F) sampled with a resolution of 125 km/pixel along the radial axis. This comparative method allows us to highlight the spectral differences in this population of objects orbiting in the Saturnian system. In particular we have retrieved the distribution of the water ice abundance, which varies between the almost pure icy surfaces of Enceladus and Calypso to the carbon dioxide- and organic-rich Hyperion, Iapetus and Phoebe. Noteworthy is that a significant dichotomy is observed between the two co-orbital moons Epimetheus and Janus, possibly indicating a different origin and evolutionary process: while the first shows a very red visible spectrum (similar to Hyperion), the second has more neutral visible colors, making it a very peculiar object in the Saturnian system. Rings have very peculiar spectral differences when compared with the icy satellites: in the visible range their spectra are characterized by a spectral knee at bluer wavelengths (at about 520 nm compared to 550 nm on satellites); in the infrared range the 1.5-2.0 micron water ice band depths are in general deeper across the A and B rings, indicative of a larger fraction of pure water ice in comparison to the CD and C rings. This research was supported by a grant from the Italian Space Agency (ASI).
P32A-08
Cassini INMS Observations of Neutral Molecules in the E-Ring
The Cassini Ion Neutral Mass Spectrometer (INMS) investigation makes in situ measurements of neutral species and ions in the vicinity of the Cassini Orbiter spacecraft. Neutral water products (the hydroxyl radical OH) have been observed in an equatorial-plane torus that spanned most of the E-ring, and peaked at 4 Saturn radii (Rs). These observations indicate a significant source of water to the magnetosphere. Recent INMS measurements made while Cassini traversed Saturn's rotational equatorial plane between 3.9 and 4.5 RS produced the first identification of water in situ. These results compare favorably with models of neutral densities with Enceladus as the source. The INMS data also contain tentative detections of two other neutral species: H2 and CO and/or N2 at mass 28. Although the presence of these additional species is unsurprising, the apparent densities are an order of magnitude greater than predicted by previous simulations of the transport and destruction of Enceladus-source neutrals.
P32A-09
Overstability in Saturn's Rings
Overstability was predicted as a spontaneous instability for Saturn's rings about ten years ago (Schmit and Tscharnuter, 1995, Icarus). If the ring is overstabl e, it develops axisymmetric waves of one hundred meters to kilometers in length. Such waves were indeed found in data obtained by the Cassini Radio Science Subsystem (Thomson et al., GRL, 2007) and the Ultraviolet Imaging Spectrograph (Colwell et al., Icarus, 2007). We review theoretical aspects of overstability using simple hydrodynamic models and simulations. In this approach overstable modes are found to form travelling nonlinear wavetrains. Due to the effect of self-gravity, the wavelength assumes a value of roughly one hundred particle diameters.