P52A-01 10:20h
Cassini Imaging Science: Orbits of Moons and Rings
We report on the orbits of several small Saturnian satellites, either recovered or newly-discovered in recent Cassini imaging observations. The mean motions of Pan and Atlas have been corrected based on recent Cassini imaging combined with Voyager observations. Two small satellites, S/2004 S 1 and S/2004 S 2, have been discovered between the orbits of Mimas and Enceladus on orbits that are nearly circular and uninclined. Both bodies were observed for a fraction of one orbit on June 1, 2004 and S/2004 S 1 was subsequently detected in images shuttered three weeks earlier. Those bodies may be recovered in late October in imaging sequences designed for that purpose. A third new object, S/2004 S3, was detected in images from June 21, 2004, orbiting just outside the F ring. A search for additional detections revealed a fourth object, S/2004 S4, orbiting interior to the F ring near the longitude at which the new object would be expected 5 hours later. A low-residual orbit that crosses the F ring explains all of the observations, but it is not yet clear whether the two sequences imaged the same object or two different objects coincidentally found orbiting at the same longitude but at different orbital semimajor axes. These issues make the nature of these objects -- solid satellites or F ring clumps -- unclear. In addition, Cassini images have been used to develop a kinematical model for the F-ring model. The data, fitting procedures, and results of these orbital analyses will be discussed.
http://ciclops.org
P52A-02 10:35h
Initial Cassini Ultraviolet Observations of Saturn's Rings
The first ring occultation to be observed by Cassini is of the star Xi Ceti on October 6-7, 2004, from a distance of 6.22 million km. The Ultraviolet Imaging Spectrograph (UVIS) includes a High Speed Photometer with a bandpass of $\sim$110-190 nm that has an integration period of 8 msec for this observation. The Fresnel zone at this distance is $\sim$40 m, and the speed of the occultation gives a radial sampling interval of 5-8 m. The occultation covers the C ring, Cassini Division, A ring, and F ring. The occultation is particularly well-suited for the low to intermediate optical depth regions of the C and F rings and the Cassini Division, and will have lower effective resolution in the higher optical depth regions of the A ring. The Xi Ceti occultation is by far the most distant that Cassini will observe. Ultraviolet spectral reflectance observations were also made at Saturn Orbit Insertion on July 1, 2004, providing the highest spatial resolution UV reflectance of the rings, with a 150 km effective resolution. Spectral variations are seen between the C, B, and A rings and the Cassini Division indicating varying amounts of dark material mixed in with water ice in the rings. The A ring is significantly brighter than the B and C rings, and its brightness increases with increasing radial distance. Meteoroid impacts into the rings darken the rings through the addition of silicates and carbonaceous material. Observed fluctuations in the UV brightness at length scales of less than 3000 km can be explained by impact fragmentation of moonlets or large ring particles in the rings. These fragmentation events expose relatively pure ice from the interior of the moonlet, leading to a local brightening of the ring which is gradually reduced and mixed with the surrounding ring material as the meteoroid bombardment continues. We will present our initial findings on ring structure, UV brightness, and particle sizes.
P52A-03 10:50h
Sources for the Radiation Environment of Saturn's Main Rings and the Innermost Radiation Belt
The energetic and plasma charged particle environment around and inwards of Saturn's main rings in the innermost radiation belt has sources from cosmic ray albedo neutron decay (CRAND), charge stripping of neutral atoms from the magnetosphere and gas clouds and tori beyond the main rings, and direct production of secondary charged particles from the same source as CRAND, high-energy cosmic ray interactions with icy ring bodies. Cosmic ray interactions with the planet's upper atmosphere provide an additional but weaker source of neutrons for the ring environment. Charged particle sputtering on ring body surfaces contributes to the neutral atmosphere extending from within the rings outward into the Saturn magnetosphere, and the CRAND source populates the higher energy proton and electron radiation belts in the magnetosphere. Radial distributions for remotely measurable surface composition by Cassini on ring bodies may be due in part to interactions with the radiation belt components and sources. Trapped particle radiation within the main ring region and towards the upper atmosphere of Saturn has been partly explored by the Pioneer 11 spacecraft in-situ in 1979 (e.g., Chenette et al., 1980; Cooper, 1983; Cooper et al., 1985), and more recently by Cassini, in-situ during Saturn orbit insertion and remotely by energetic neutral atom emissions. Gamma ray emissions at Pioneer 11 from the rings were reported by Cooper et al. (1985) and contributed with charged particle data to determination, independent from Voyager ring imaging, of the A and B ring mass column densities. Voyager and earth-based ultraviolet observations measured neutral cloud densities over and beyond the rings, and Voyager plasma instrumentation surveyed the associated pickup ion populations in the magnetosphere. Future missions could increase main ring coverage with spectroscopic x-ray, gamma-ray, and neutron emission measurements and provide new information on ring body composition and mass distribution. Within these rings the loss rates of trapped charged particles are high, but much larger fluxes could accumulate inwards of the B ring towards Saturn, similar to the well-known inner radiation belt and low-altitude equatorial belt of energetic particles at Earth. Diffusion models also predict that radial transport times would be relatively large in this region, probably largest for higher energy trapped particles and resulting in significant fluxes above the planetary atmosphere. Available data and models for particle flux spectra from Pioneer 11, Voyager, and later measurements will be used to estimate time scales for significant radiation chemistry effects on ring particles and in Saturn's upper atmosphere. Improved modeling of surface sputtering and energetic particle interactions for the rings will provide better models for neutral atom and CRAND contributions respectively to the Saturn neutral cloud and magnetospheric hot plasma environments beyond the rings. References: Chenette, D. L., et al., JGR, 85, 5785-5792, 1980 ; Cooper, J. F., JGR, 88, 3945-3954, 1983; Cooper, J. F., et al., JGR, 90, 3415-3427, 1985.
P52A-04 11:05h
From Saturnian Ring Plasma to Energetic Magnetospheric Ions
Because of continuous collisional bombardments by interplanetary meteoroids, interstellar dust and dust particles of satellite origin, a tenuous exosphere could be built up in the Saturnian ring system. Photodissociation of the water molecules will lead to a composition dominated by oxygen molecules. Subsequent photoionization and electron impact ionization will lead to the formation of a thin layer of plasma disc embedded on one side of the ring plane. This storage zone is created by the vertical shift of the center of the axisymmetric magnetic field. With a nominal gas production rate of 2.5E27 O2/s and an equilibrium between impact vapor production and charge exchange loss, the neutral number density in the ring exosphere can be estimated to be about 8.3E5 molecules per c.c. and the ring plasma density to be about 300 per c.c. as constrained by the Voyager measurements in the inner Saturnian magnetosphere. The fast neutral atoms (FNAs) created by charge exchange loss of the trapped ring ions will be injected into the outer magnetosphere forming a thin gas disk. The inward radial diffusion of the pickup heavy ions could produce energetic ions with energy reaching as high as 30 keV at L = 5. It is in this manner that the ring system could have direct influence on the dynamics and composition of the Saturnian magnetosphere.
P52A-05 11:20h
Meteoroid Impacts onto Saturn's Rings
Over the past decade, flash impacts have been observed on the moon and in the laboratory in both the IR and visible portions of the spectrum. These phenomena have been used to constrain impact parameters, such as impact velocity and composition. With the arrival of the Cassini spacecraft at Saturn this past July, we have embarked on a study of impact flashes in Saturn's rings. We begin by modeling high energy, hypervelocity impact events using CTH, a shock physics hydrodynamics code developed at Sandia National Laboratories. The simulated impacts involve two icy bodies (ranging from a centimeter to several meters in diameter) impacting each other at velocities over 30 km/s. The resulting impact-induced vapor plume is post-processed to consider its radiative evolution using NEQAIR, a radiative transfer code developed at NASA Ames. The results of this study will be used as an aid to investigate flash impact in Saturn's rings using Cassini's Ultraviolet Imaging Spectrograph (UVIS). Here we present our latest modeling results. This work is supported by a Graduate Student Researchers Program Fellowship from NASA Headquarters and by the Cassini project.
P52A-06 11:35h
The Production and Redistribution of Oxygen in Saturn's Magnetosphere: CAPS Cassini Data
The Cassini Plasma Spectrometer (CAPS) has detected O2+, O+ and H2+ in the inner magnetosphere and over the main rings. Here we examine the formation and redistribution of molecular oxygen over the icy rings and in the inner magnetosphere. Thin oxygen atmospheres exist on Europa and Ganymede formed by radiolysis (Johnson et al. 2004). Because the escape of O2 from these satellites is inefficient,Europa has a tenuous molecular oxygen cloud (Burger and Johnson 2004, Hansen et al. 2004; Shematovich et al 2004). However, radiation induced chemistry on icy objects in Saturn's system will directly populate the magnetosphere with molecular oxygen leading to the formation of O2+ and its observed dissociation products O and O+. Therefore, in addition to neutral H (Shemansky et al. 1992), OH (Jurac et al. 2002) and nitrogen clouds(Smith et al. 2004), there is likely a molecular oxygen cloud in Saturn's magnetosphere. Since water vapor can be produced by the incident radiation, by meteoroid impacts, and by collisions between grains and small bodies, the observation of molecular oxygen can be a marker for radiation-induced decomposition of ice (Shi et al. 1995, Johnson et al. 2003). Here we present the CAPS data and calculations of the UV production of O2 and H2 over the main rings and the charged particle production over the tenuous rings. We also describe the redistribution of oxygen in Saturn's magnetosphere by ion molecule reactions and charge exchange (Eviatar et al. 1983, Johnson et al. 1989). References Burger, M.H., R.E. Johnson Icarus 171, in press 2004. Eviatar, A., et al., JGR 88, 823-831, 1983. Hansen, C. J., et al., Icarus, submitted, 2004. Johnson, R.E. et al., Astrobiology , v3, 823-850,2003. Johnson, R.E. et al., In Jupiter-The Planet, Satellites, Magnetosphere, ed. F. Bagenal et al.,Cambridge Press, 2004. Jurac, S. et al.,GRL 29, 2172, 25-1 V 4, 2002. Shematovich, V.I. et al., Icarus, in press, 2004 Shemansky, D. E., D. T. Hall. JGR 97, 4143~V4161,1992. Shi, M. et al. GRL 100 26,387-26,395, 1995. Smith, H.T., R.E. Johnson, V.I. Shematovich, GRL 31, doi:10.1029GL020580, 2004.
P52A-07 11:50h
Dusty Plasma Effects in Saturn's Rings
Planetary rings are our best examples of environments where dusty plasma effects can establish the size and spatial distributions of small grains. Simultaneously, dust often influences the composition, density and temperature of the plasma surrounding it. The dynamics of charged dust particles can be surprisingly complex and fundamentally different from the well understood limits of gravitationally dominated motions of neutral particles or the adiabatic motion of electrons and ions in electromagnetic fields that dominate gravity. In this talk we focus on observations that are best explained by theories concerning dusty plasma effects at Saturn. In addition to presenting our current models we also discuss our expectations for new discoveries based on existing observations at Jupiter, or on purely theoretical considerations. Our intent is to give an up-to-date overview of dusty-plasma effects in Saturn's magnetosphere and to draw attention to several outstanding problems that could be resolved by the {\it Cassini} mission.
P52A-08 12:05h
Keck Near-Infrared Observations of Saturn's E and G Rings during Earth's Ring Plane Crossing in August 1995
We present near-infrared (1.24 -- 2.26 micron) images of Saturn's E and G rings which were taken with the W.M. Keck telescope in 1995 August 9 - 11, during the period that Earth crossed Saturn's ring plane. Our data confirm that the E ring is very blue. Its radial and vertical structure are found to be remarkably similar to that apparent in the HST ringplane crossing data at visible wavelengths, reinforcing models of the ring's peculiar narrow or very steep particle size distribution. Our data show unambiguously that the satellite Tethys is a secondary source of material for the E ring. The G ring is found to be distinctly red, similar in color to Jupiter's main ring, indicative of a (more typical) broad particle size distribution.