P13A-1293
Beam: A Ray-Tracing Monte Carlo Code for Photometric Studies of Saturn's Rings
The albedos of Saturn's ring particles are not well constrained. This directly relates to the composition of the rings and has implications for their age. One way to estimate ring particle albedos is to use a numerical ray- tracing code to model the brightness of the rings and compare the results with observations. Previous models (classical doubling codes) have not accounted for a variable volume filling factor (or vertical puffiness) for the rings. Such models have not been able to correctly reproduce observed ring brightness, or I/F, profiles of the rings in all regions. This makes it difficult to determine the true albedo of the ring particles. We have developed a new radiative transfer code for modeling photon interactions with ring particles. The Beam code is based on the ray-tracing Monte Carlo technique of Salo and Karjalainen (Icarus, 2003). The code includes the volume filling factor and Saturnshine, as well as a variety of phase functions for both the rings and Saturn. Secondary effects, such as self-gravity wakes and ring-shadowing, are also included. Comparisons between Cassini observations and Beam code simulations will be presented at the meeting.
P13A-1294
O2 and O2+ density from the Rings through Inner Magnetosphere
The main rings and the ice grains in the tenuous F and G rings are a source of O2+ ions for the inner magnetosphere (Tokar et. al. 2005). These ions are formed from neutral O2 through the decomposition of ice by incident radiation (Johnson et. al. 2006). As the principal source of O2+ ions is from the ionization of the neutral O2 molecules through photo and electron impact ionization, O2+ becomes a marker for the radiation induced decomposition of ice and the presence of O2 neutrals. Recently, Martens et al (2008) described O2+ beyond the orbit of Enceladus, noting the possibility that Rhea is a source. Here we focus on O2+ inside the orbit of Enceladus. Through simulations of the neutral cloud created by photo- induced decomposition of the ice in the main rings and the tenuous F and G rings (Johnson et. al. 2006, Tseng et. al. 2008) it is possible to calculate the column density of the neutrals and the O2+ source rate in the inner magnetosphere. Using the Cassini Plasma Spectrometer (CAPS) data we describe the density of the O2+ ions from the rings out to the orbit of Enceladus. The largest source of O2 neutrals is expected to be the main rings. However, here we examine whether or not the energetic ion irradiation of grains in the F and G rings are significant sources of O2 and if ion-neutral reactions in the Enceladus plume are a possible source. References: Johnson, R.E., et. al., "Production, Ionization and Redistribution of O2 Saturn's Ring Atmosphere" Icarus 180, 393-402 (2006). Martens, H. R., et. al., 'Observations of molecular oxygen ions in Saturn's inner magnetosphere'. Geophy. Res. Lett. In press(2008). Tokar, R.L., et. al., 2005. Cassini Observations of the Thermal Plasma in the Vicinity of Saturn's Main Rings and the F and G Rings. Geophys. Res. Lett. 32, doi:10.1029/2005GL022690. L14S04. Tseng et. al. 'Saturn's Ring Atmosphere' to be submitted (2008).
P13A-1295
Regolith Growth and Darkening of Saturn's Ring Particles
Markov chain simulations compare the regolith growth and darkening on 1 m and 10m particles in Saturn's rings. Our results show that pollution of the larger ring particles is ten times slower, allowing the rings to be ancient and still meet strict upper limits on fractional pollution by meteoroid infall. Example UV spectra are shown. Our results indicate that regolith stirring by higher velocity collisions can mix the ring particle regolith, creating brighter haloes around strong density waves, as observed by Cassini VIMS and UVIS. Unfortunately, our incomplete knowledge of meteoritic bombardment rates, particle adhesion and size/velocity distributions do not allow an age estimate.
P13A-1296
Saturn's Titan and Maxwell Ringlets from Cassini RSS Observations
Occultation observations of Saturn's rings during the Cassini mission provide exquisite radial profiles of the rings at sub-km scale, making it possible to measure the orbital properties and internal structure of narrow ringlets and gap edges. Two of the most prominent among these are the Titan and Maxwell ringlets, first measured from Voyager images and occultation profiles more than a quarter of a century ago. By combining Cassini RSS occultation measurements with Earthbased stellar occultation and Voyager data, we have determined highly accurate fits for orbits of the ringlets, including their precession rates. Our current fits include Cassini RSS data from Revs 7 to 12, and are limited in accuracy primarily by km-scale systematic errors in the reconstructed spacecraft trajectory. For the Titan ringlet, we find a=77878.34 ± 0.46 km, e=0.000272 ± 0.000007, and ·ϖ=22.57482± 0.00078°/day (rms=1.85 km). For the Maxwell ringlet, we find a=87509.84 ± 0.31 km, e=0.000440 ± 0.000006, and ·ϖ=14.69289± 0.00020°/day (rms=1.34 km). (All errors are formal errors.) When combined with additional occultation measurements and corrections for spacecraft trajectory errors, such orbital fits will ultimately provide among the strongest constraints on the determination of Saturn's gravitational harmonic coefficients J2, J4, and J6. We have also obtained width-radius fits, and we find that the Titan ringlet varies in width between 16.8 and 32.4 km, while the Maxwell ringlet varies in width between 17.5 and 97.5 km. We have identified a host of additional weakly eccentric and/or inclined narrow features (ae or ai of order a few km or less). We will present a catalog and preliminary fits for these ringlets as well.
P13A-1297
The Stream Particle Observation During Cassini's Saturn Tour
Stream particles are nano-meter size dust grains accelerated by Saturn's corotational E field to high velocity (~ 100 km/s). By correlating stream particle impact rates registered by the Cassini dust detector with magnetic field data obtained by the Cassini magnetometer, we found that the stream particle impacts cluster during periods when the interplanetary field is characterised by interplanetary magnetic field (IMF) direction. Furthermore, the directionality and the impact charge of the streams are highly correlated with the corotating interaction region (CIR). The strong correlation between the stream partciles activity and IMF suggests that the dynamics of the stream particles is dominated by the electromagnetic force. Another consequence is that the typical size of a Saturnian stream particle is below 20 nm. Using dust and solar wind measurements from Cassini spacecraft, we perform backward simulations using a method employed first by Zook et al. (1996) to investigate the propagation of Jovian stream particles. The result shows that the most likely source region of Saturnian streams locates between 7 to 9 RS (Saturn radii). The velocity of the stream particles ranges from 40 to 200 km/s. Their charge-to-mass ratio ranges from 1000 to 10,000 C/kg, which is equivalent to 10 to 3 nm if assuming +5 volt surface potential.
P13A-1298
Models of the Jovian Ring and Comparisions With Observations
A number of in situ and remote sensing observations of the Jovian ring system exist so we can now combine observations from Voyager, Pioneer, Galileo and Cassini, as well as ground based and HST measurements. In this presentation we will compare this large body of observations to available theoretical models of the dust dynamics in the Jovian ring. Common to all models (Burns et al., 1985, 2001 ; Horanyi et al.,1996, 2004) is the basic idea that dust is being continuously produced due to micro-meteoroid bombardment of the moons in this region. Also, the spatial distribution of dust in the halo region inward of the main ring is generally accepted to be a consequence of electrodynamic perturbations acting on small charged dust particles. However, in the suggested theoretical models the time scale for orbital evolution is drastically differ. Burns et al. argues, that in the main ring, dust particles evolve inward very slowly due to Poynting-Robertson drag. A typical micron sized grain is predicted to orbit Jupiter for 104 years before crashing into the atmosphere of Jupiter. Horanyi et al. argues that the radial transport is due to resonant charge variations, dictated by the plasma density distribution. In this model grains are transported on a time scale that is orders of magnitude shorter than predicted by PR drag. Here we use both of these models to generate brightness distributions and predict optical depth distributions for same geometries and wavelengths as that of the observations. Quantitative comparisons of the modeled and the real observations lead us to the conclusion that the dust transport in ring/halo region at Jupiter is mainly due to resonant charge variation.
P13A-1299
Estimation of CIRS Sensitivity to Trace Constituents in Saturn's Main Rings
Several instruments on the Cassini spacecraft have been employed to analyze the composition of Saturn's main rings. Despite this, the identities of the compounds that color the rings remain elusive. Cassini's Composite Infrared Spectrometer has returned tens of thousands of spectra from the main rings. These far infrared spectra (10 cm-1 to 600 cm-1) are comprised of radiation that has been highly scattered by ring particles' regolith. This scattering reduces the magnitude of absorption features, increasing the difficulty of distinguishing these features from the instrument's noise. To assist the search for these features, we use a simple model of Mie scattering spheres to estimate what concentrations of trace materials might be detected by CIRS. This allows us to determine the extent to which the current data can be used and whether new observations are necessary.
P13A-1300
Prediction of Saturn's rings temperatures during the 2009 Equinox
The thermal emission of Saturn's rings has been intensively observed over the past 4 years with the Cassini Composite Infrared Spectrometer instrument CIRS. The Sun elevation above the ring plane has progressively decreased from -26 deg in July 2004 to -4 deg in September 2008, which implied a seasonal variation of the ring temperature, as the mutual shadowing between icy particles in rings became more important. The equinox, which will occur in August 2009, will provide a unique opportunity to study the thermal contrast between the North and South sides of the rings at this particular geometry, and the heating rate of the newly lit North side. Ring particles will reach their minimum temperature as the solar heating will reach its minimum. Several observations will be dedicated to measuring the temperature on both lit and unlit side of the rings with the CIRS instrument around the equinox, at different geometries (phase angle, spacecraft elevation, spacecraft azimuth). Modeling the ring temperature during the equinox will provide some insights on the vertical structure of the rings and the dynamic of particles. We first present the predicted temperature of rings obtained with a simple model assuming that the ring temperature is mainly driven by the albedo, the distance from Saturn and the structure of rings (isolated particles or slab). This will give us an approximation of what we could see in 2009. We then present a new model that couples N-body dynamical simulations and thermal modeling in order to study the effect of the vertical structure of the rings. 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.
P13A-1301
Temperature variation of Saturn's Rings with Solar Elevation
In its four-year orbit around Saturn, the spacecraft Cassini has achieved a large number of infrared observations of Saturn's main rings through the CIRS experiment (Cassini Composite Infrared Spectrometer). We analyze the change of temperature in the main rings (A, B and C) as function of the solar elevation with respect to the plane of the rings at very low (<6°) and high (>120°) phase angles. Specific regions of every ring were chosen in every case to rule out other effects that may produce variations in temperature as well and to account only effects due to the solar latitudinal variations. For solar elevations that cover a range from -10.1° to -23.5°, in average, the temperature variations of the A, B and C rings are around ~{8 K}, near ~{12 K} and barely ~{3 K} respectively. Simple analytical functions which depend on the solar elevation angle and the heliocentric distance are used to fit the trends observed in the data assuming either a continuous slab or idependent particle models. 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". Copyright 2008 California Institute of Technology. Government sponsorship acknowledged.
P13A-1302
Modeling Saturn Ring Temperature Variations as Solar Elevation Decreases
After more than four years in orbit around Saturn, the Cassini Composite Infrared Spectrometer (CIRS) has acquired a wide-ranging set of thermal measurements of Saturn's main rings (A, B, C and Cassini Division). Temperatures were retrieved for the lit and unlit rings over a variety of ring geometries that include solar phase angle, spacecraft elevation, solar elevation and local hour angle. 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 and local time are a secondary effect. Decreasing ring temperature with decreasing solar elevation are observed for both the lit and unlit faces of the rings after phase angle and local time effects are taken into account. For the lit rings, decreases of 2- 4 K are observed in the C ring and larger decreases, 7-10 and 10 - 13 K, are observed in the A and B rings respectively. Our thermal data cover a range of solar elevations from -21 to -8 degrees (south side of the rings). We test two simple models and evaluate how well they fit the observed decreases in temperature. The first model assumes that the particles are so widely spaced that they do not cast shadows on one another while the second model assumes that the particles are so close together they essentially form a slab. The optically thinnest and optically thickest regions of the rings show the best fits to these two end member models. We also extrapolate to the expected minimum ring temperatures at equinox. 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". Copyright 2008 California Institute of Technology. Government sponsorship acknowledged.
P13A-1303
Pitch angle and energy diffusions by interaction between energetic electrons and neutral particles in Saturn's inner magnetosphere
Neutral particles in Saturn's inner magnetosphere play the dominant role in a loss of energetic electrons and ions because of abundance of neutral particles (e.g., Paranicas et al., 2007; Sittler et al., 2008). The estimation of interaction between energetic and neutral particles gives a clue to understand particle acceleration and loss processes. These interactions produce the butterfly type pitch angle distribution which is dominant distribution in Saturn's inner magnetosphere (e.g., Roussos et al., 2005). In order to understand loss processes of energetic electrons, we have estimated pitch angle diffusion coefficients by interaction between energetic electrons (> keV) and neutral particles. A result showed that diffusion coefficients by neutral particles are greater than those by electron-ion interactions around Enceladus (L~3.94). We have further estimated pitch angle and energy diffusion coefficients in Saturn's inner magnetosphere by using water molecule distribution model. The neutral distribution follows Saturn's and Enceladus's gravity fields after neutral particles are ejected from Enceladus into the magnetosphere due to sputtering. In this presentation, we will show an importance of energetic electron loss process by interaction between energetic electrons and neutral particles.
P13A-1304
The Structure and Time Variability of the Ring atmosphere and ionosphere
The Saturnian system is subject to constant bombardment by interplanetary meteoroids and irradiation by solar UV photons. Both effects release neutral molecules from the icy ring particles either in the form of impact water vapor or gas emission in the form of H2O, O2 and H2. The SOI observations of the Cassini spacecraft have shown the existence of molecular and atomic oxygen ions. Subsequent modeling efforts have led to the picture that an exospheric population of neutral oxygen molecules is maintained in the vicinity of the rings via photosputering and other means. At the same time, the ring system is embedded in a system of O+ and O2+ ions threading through the Cassini division. Charge exchange and collisional interactions between the exospheric ions and neutrals will create a scattered component of O2 molecules (and O atoms) which can be injected into Saturn¡¦s upper atmosphere or the inner magnetosphere. In other words, the ring atmosphere could serve as a source of O2+ ions in Saturn¡¦s magnetosphere. The structure of the ring atmosphere/ionosphere complex and the injection rate of O2+ ions are, however, subject to modulation by the seasonal variation of Saturn along its orbit. In this work, we will demonstrate how the physical properties of the ring oxygen atmosphere and the scattered component (and the source rate of the magnetospheric O2+ ions) would vary as the ring system going through the cycle of solar insolation.