P43B-1289
Galileo In-Situ Dust Measurements and the Physics of Jupiter's Gossamer Rings
During its late orbital mission about Jupiter, the Galileo spacecraft flew twice through the giant planet's gossamer ring system. The dusty ring material is produced when interplanetary impactors collide with embedded moonlets. Optical images imply that the rings are constrained both horizontally and vertically by the orbits of the moons Amalthea and Thebe with the exception of a faint outward protrusion called the Thebe Extension. During the ring passages, the Galileo impact-ionization dust detector counted a few thousand impacts but only about 100 complete data sets of dust impacts (i.e. impact time, impact speed, mass, impact direction, etc.) were successfully transmitted to Earth. The instrument verified the outward extension of the gossamer ring beyond Thebe's orbit and measured a major reduction in particle ring material interior to Thebe's orbit. The existence of this partially evacuated gap in ring material is also indirectly confirmed by Galileo in-situ energetic particle measurements (Norbert Krupp, priv. comm.). Detected particle sizes range from about 0.2 to 4 micron, extending the size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging (Showalter et al., Icarus 2007). The grain size distribution increases towards smaller grains, showing a much higher proportion of small particles in the Amalthea gossamer ring than in the Thebe ring and the Thebe Extension. Our analysis shows that particles contributing most to the optical cross-section are about 4 micron in radius, in agreement with imaging results. Finally, Galileo also detected some micron and sub-micron grains on highly inclined orbits with inclinations up to 20 degrees. Recent modelling (Hamilton & Krueger, Nature, submitted) shows that time variable electromagnetic effects can account for all of these surprising results. In particular, when the ring particles travel through Jupiter's shadow, dust grain electric charges vary systematically, driving grains out into the Thebe Extension and matching the Galileo in-situ dust measurements. The model also puts strong dynamical constraints on the local plasma density.
P43B-1290
The Large-scale Structure of Saturn's E ring
Geysers on the recently discovered, geologically active south-polar region of the moon Enceladus are now recognized as the dominant source of material in Saturn's E ring. The ring was traditionally thought to span the region between 3 to 8 RS, where RS is the radius of Saturn. However, new in situ dust measurements indicate that the density of small grains might continuously extend far beyond these boundaries, and the E ring could reach even beyond the orbit of Titan (20.3 RS). We report on the modeling results of the long-term evolution of dust particles comprising the E ring to show that grains from Enceladus could indeed reach the outskirts of Saturn's magnetosphere.
P43B-1291
Cassini UVIS Observations: Structure and History of Saturn's Rings
Cassini observations show unexpected ring variability in time and space. Time variations are seen in ring edges, in the thinner D and F rings, and in short-lived agglomerations . The rings are inhomogeneous, with structures on all scales, sharp gradients and edges. Compositional gradients are sharper than expected, but nonetheless cross structural boundaries. This is evidence for ballistic transport that has not gone to completion. The rings are so pure ice that little evidence exists for pollution by meteoritic material. The autocovariance maximizes in the middle of the A ring, with smaller structure near the main rings' outer edge. Density wave locations have a fresher ice composition. The processes of collisions, diffusion and transport should have homogenized the rings over the age of the solar system. Instead, these differences persist. The mass density in the Cassini division inferred from density waves is so low, that the material there would be ground to dust in 30,000 years. The observed moons that cause such interesting structure in the rings have short lifetimes against disruption by cometary bombardment and against the angular momentum transfers that push them away from the rings. Self-gravity wakes and overstability are evident in the main rings.The rapid processes evident in the Cassini data have been taken as evidence that the rings were recently created, perhaps from a comet that passed too close to Saturn. Instead, an alternative is that primordial material may have been re-used and recycled. In the zone near the Roche limit where rings are found, limited accretion is possible, with the larger bodies able to recapture smaller fragments. The ‘propeller' structures, the self-gravity wakes, and the size distribution of clumps in Saturn's F ring are all indications of the on-going accretion process. Recycling could extend the ring lifetime almost indefinitely. The range of ages, the variety evident in the latest observations and the low mass density inferred for the largest bodies are consistent with extensive recycling of ring material as the explanation of the apparent youth of Saturn's rings. Similar processes are likely occurring in the other ring systems and in the formation of planets around other stars. Particularly, such processes could be important where more rapid growth is frustrated, as in the asteroid belt and the Kuiper belt.
P43B-1292
Spokes in Saturn's B Ring: Could Lightning be the Cause?
Spokes are narrow markings across Saturn's B ring, most likely caused by a cloud of micron sized particles intermittently lofted from the larger boulders comprising the ring. They were observed by the Voyagers, the Hubble Space Telescope, and more recently the Cassini spacecraft in orbit around Saturn since 2004. There are a number of suggestions to explain the triggering and subsequent evolution of spokes. In general, it is expected that spokes form due to sudden, short-lived changes in the plasma environment of the rings. Most recently, lightning in Saturn's atmosphere was suggested to trigger spokes, based on Cassini observations of field-aligned high-energy electron beams. In this talk we discuss the physic of the expected interactions of the rings with high energy electron beams, and show that, based on terrestrial examples, the expected amount of charge released in a lightning event could increase the surface charge density of the rings to sufficiently high values to trigger spokes.
P43B-1293
Investigating the Composition of Saturn's Rings Using Cassini CIRS Data
Although it has long been known that Saturn's rings are primarily composed of water ice, the identities of the trace constituents-- which provide subtle color to the rings and could shed light on their origin and formation-- remain poorly understood. These trace constituents may affect the thermal infrared part of the rings' spectral signature. Previous observations have not observed any such effects. However, they have been hindered by one or more major problems: 1) the inability to observe the entire wavelength interval through the earth's atmosphere, 2) low spectral resolution, and 3) limited range of geometries. Data obtained with Cassini's Composite Infrared Spectrometer (CIRS) instrument circumvents these difficulties. We report results from a preliminary investigation of ring composition using CIRS data over the wavelength range of 10 to 600 cm-1. The rings spectra observed by CIRS are a complicated combination of thermal emissions, the effects of scattering from ring particles (broad size distribution) and noise. Before the signal can be examined for the faint absorption features from contaminants, the contributions of scattering and thermal emission must be accounted for and removed using Mie scattering and standard radiation transfer techniques. We present the initial results of this investigation. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by the Space Grant program.
P43B-1294
Possible Dust Extended From Saturnian Ring Plane or Zodiacal Light as Seen by Cassini ISS.
The night side limb of Saturn's north pole appears silhouetted against a brighter background when observed at a phase angle (sun-target-spacecraft) of 161 degrees. The spacecraft was close to the equatorial plane when the observations were taken, and the bright background originates behind the planet, so the scatterers are more than one Saturn radius (1.0 Rs) above the ring plane. The half-maximum contour of the E ring is only ~0.04 Rs above the ring plane at the orbit of Enceladus, but the wings of the distribution are broad, especially beyond the orbit of Enceladus. Most likely, these observations provide information about the extremely inclined tail of the population of dust particles in the Saturn system. It is also possible that we are seeing zodiacal light particles between the sun and Saturn. We will compare the observed brightness with that from these possible sources.
P43B-1295
Infrared Observations Of Saturn's Rings : Azimuthal Variations And Thermal Modeling
Saturn's rings represent a collection of icy centimeter to meter size particles with their local dynamic dictated by self gravity, mutual collisions, surface roughness and thickness of the rings themselves. The infrared observations obtained by the CIRS infrared spectrometer on board Cassini over the last 3.5 year contain informations on the local dynamic, as the thermal signature of planetary rings is influenced both by the ring structure and the particle properties. The ring temperature is very dependent on the solar phase angle (Spilker et al., this issue), and on the local hour angle around Saturn, depending on whether or not particles' visible hemispheres are heated by the Sun. The geometric filling factor, which can be estimated from CIRS spectra, is less dependent on the local hour angle, suggesting that the non isothermal behavior of particles' surfaces have low impact, but it is very dependent on the spacecraft elevation for the A and C rings. The ring small scale structure can be explored using CIRS data. Variations of the filling factor with the local hour angle relative to the spacecraft azimuth reveals self-gravity wakes. We derive morphological parameters of such wakes in both A and B rings assuming that wakes can be modeled either by regularly spaced bars with infinite or finite optical depth. Our results indicates that wakes in the A ring are almost flat, with a ratio height/width ≈ 0.44 ± 0.16 and with a pitch angle relative to the orbital motion direction of ≈ 27deg. This is consistent with UVIS (Colwell et al., 2006) and VIMS data (Hedman et al., 2007). Such models are more difficult to constrain in the B ring, but small variations of the filling factor indicate that the pitch angle decreases drastically in this ring. We also present a new thermal bar model to explain azimuthal variations of temperatures in the A ring. We compare results with previous ring thermal models of spherical particles. The Cassini/CIRS azimuthal scans data set is crucial input for any future modeling of Saturn's rings' thermal and dynamical properties.
P43B-1296
Characterization of Enigmatic Saturn's Ring B by Cassini Radio Occultations
Cassini radio occultation observations of the extinction and near-forward scattering of 0.94, 3.6, and 13 cm- wavelengths sinusoidal signals have shed much new light on the structure and physical properties of Saturn's main ring system, especially enigmatic Ring B. As of June 2007, the occultations covered 15 distinct ring longitudes and roughly two distinct ranges of ring opening angle B; the first 12 covered B = 19.5-23.5 deg, and the last three B =14-15 deg. Four sub-regions of Ring B, identified as B1 to B4 (bounded by rough ring radius = 92, 99, 104.5, 110, 117.5 thousand km) exhibit clearly distinct structure (Marouf et al., 38th DPS Meeting, 38.05, 2006). Region B2, in particular, is characterized by remarkable 'bi-stable' states, where the optical depth abruptly flip-flops between optical depth of about 2 and more than 5. We consider observational evidence that bear on physical characterization of the observed structure (particle sizes, particle-cluster-sizes and orientation, spatial cluster density, vertical ring profile and physical thickness, ...). On the signal extinction side, this includes differential extinction of the three radio signals, and apparent variation of optical depth with observation longitude and ring opening angle (azimuthal asymmetry). On the forward scattered signal side, it includes strength, bandwidth, spectral shape and Doppler drift-rate of observed spectrogram features. We present representative results for selected features in regions B1, B2, and B4. Of particular interest is the detection of quasi-periodic ring structure of period roughly 100 meters in region B2 (and perhaps B4). In contrast with the prevalent gravitational wakes, the periodic structure is not azimuthally inclined and appears to be an independent structure superposed on the background wake structure (Thomson et al., submitted 2007).
P43B-1297
The Vertical Temperature Distribution Across Saturn's Rings as Observed by Cassini CIRS
We have analyzed observations from Cassini's Composite Infrared Spectrometer (CIRS) from 10 to 600 cm-1 to identify those regions where ring particle vertical motion may be responsible for modifying the overall thermal budget. Dynamical interactions such as mutual collisions between ring particles and resonances with Saturn's moons conspire to insure that the particles comprising Saturn's rings not only have non-zero eccentricities, but finite inclinations as well. As a result, ring particles can generally be expected to cross above and below the ring plane as they orbit Saturn. Thermal models published to date do not consider this vertical transport across the ring plane, but constrain them to lie in either a static monolayer (e.g.\ Froidevaux 1981, Ferrari and Leyrat 2006) or a multilayer (e.g.\ Kawata, 1983). We compare the thermal flux emitted from the lit side of the rings to that coming from the unilluminated side using pairs of CIRS radial scans with otherwise similar geometric parameters (i.e.\ similar phase angle, local hour angle and solar elevation angle). The Sun was at elevation angles, B' , between 20.5° and 23.5° when these observations were made. Practically no discernible temperature difference is observed between the lit and unlit sides of the optically thin C ring. This is expected, as the radiation field should be relatively constant across such a thin layer of particles. The optically thickest portions of the B ring, where large filling factors and high collision rates impede ring particles from crossing the ring plane, display temperature differences of 18 - 20 K at high phase angles ( 125 - 150° ). At low phase angles ( < 60° ), the observed difference in temperature across the core of the B ring is 25 K . We will present a simple, empricial model to construct a framework within which to interpret these observations. However, a full understanding of the model results await more observations and a more complex model that takes the behavior of Saturn's ring particles more fully into account.
P43B-1298
Cassini CIRS Measurements of Thermal Phase Curves in Saturn's Main Rings
After more than three years in orbit around Saturn, the Cassini Composite Infrared Spectrometer (CIRS) has acquired a broad set of thermal measurements of Saturn's main rings (A, B, C and Cassini Division) for a number of different viewing geometries that include solar phase angle, spacecraft elevation, solar elevation and local hour angle. The thermal characteristics of each main ring vary with changing viewing geometry. Ring temperatures decrease with increasing solar phase angle on both the lit and unlit sides of the rings. To first order, the largest temperature changes on the lit face of the rings are driven by variations in phase angle while differences in temperature with changing spacecraft elevation are a secondary effect. We find that the characteristic shapes of the phase curves vary with changing solar elevation angle, We present a preliminary report on the thermal phase curves obtained thus far. For a solar elevation of 14 degrees the outer B ring exhibits a thermal surge in temperature greater than 10 K at low phase angles. For a solar elevation of 21 degrees the thermal profile with phase angle is more nearly linear. Our first attempt at modeling the thermal phase curves is presented, using parameterized shadowing functions as described in the standard Hapke model. We discuss the significance of the retrieved parameters as potential input for forward- modeling of Saturn's ring thermal properties. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA and at CEA Saclay supported by the "Programme National de Planetologie".