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NASA NTRS1981

Model exospheres of the ringed planets

The theory of rotating ion exospheres in dipolar magnetic field geometries is used to predict certain effects of the rings on the inner plasmaspheres of the ringed planets Jupiter, Saturn, and Uranus. The analysis here assumes a purely ionospheric source. It is found that if rings affect only the trapped plasma, the cold plasma density at Saturn will be either reduced or increased by a factor of approximately 2 at L = 1.65, depending on whether the ring populates or empties the trapped trajectories. It is found that if all cold plasma particles on trapped orbits are absorbed and the rings are not a source of plasma, this simple exospheric theory can explain the ionospheric density profile inferred from the Pioneer 11 Saturn radio occultation experiment, in which a localized peak in the profile was observed on field lines threading the Guerin division between the C and D rings.

Luhmann, J. G.↗
NASA NTRS1980

Planetary rings

Observations of the Rings of Saturn from the Pioneer spacecraft, discovery of the Ring of Jupiter, ground based polarimetry of the Rings of Saturn and some theoretical studies may be combined to markedly advance our understanding of the Rings of Jupiter, Saturn and Uranus. In particular, narrow rings can be self-gravitatingly stable inside Roche's limit and outside another closer limit. They can be created from a satellite which evolves across its Roche limit either by inward tidal drift or by growth of the planet by accretion. These considerations suggest that Neptune may well be surrounded by one or more narrow rings like those of Uranus.

Cook, A. F.↗
NASA NTRS1980

Physical processes in Jupiter's ring - Clues to its origin by Jove

It is shown that many of the observed properties of the Jovian ring can be explained by the presence of numerious small and unseen parent bodies, or 'mooms', residing within the ring; whose radii are less than 1 km. The small visible ring grains, which are destroyed in short times by sputtering and meteoroid erosion, are derived from these parent bodies largely through meteoroid impacts, and partly from Io's dust. Substantial orbit modification results from plasma drag, and the charge carried by the grains will influence their dynamics and may modify their shapes. It is concluded that the processes discussed, though present in other planetary ring systems, may be highlighted in Jupiter's ring because of its low optical depth and the small size of some of its particles. It is suggested that hidden reservoirs similar to the Jovian 'mooms' proposed may be present in the rings of Saturn and Uranus.

Burns, J. A.↗
NASA NTRS1981

Pioneer fly-by of Saturn and its rings

Results acquired by the imaging photopolarimeter on board Pioneer 11 during the spacecraft fly-by of Saturn and its rings on September 1, 1979 are reviewed. Analysis of the broadband photometry and polarimetry obtained of the Saturn atmosphere has been used to determine a cloud top height of 300 mb and a scale height of the aerosol distribution about 1/4 that of the ambient gas, and to point out differences between the forward scattering and belt and zone characteristics of the Saturn and Jupiter atmospheres. Images of Saturn's rings have been used to derive a profile of ring optical depth between 1.22 and 2.35 Saturn radii, and reveal new divisions and thin rings and azimuthal variations in the brightness of the A ring not observable from earth. Linear polarization observations of Titan in red and blue light reveal that the aerosols near the top of the atmosphere have radii less than about 0.09 micron and that the optical thickness of the small aerosol layer is about 0.6 above an effectively depolarizing surface, and indicate radii of 2845 + or - 25 km and 2880 + or - 22 km in red and blue light, respectively. Earth-based and spacecraft data are consistent with the formation of rings structures as a result of Poynting-Robertson drag and gravitational satellite resonances with the original ice and rock particles.

Gehrels, T.↗
NASA NTRS1981

The F-ring of Saturn

The existence of a split and twisted F-ring, located outside of Saturn's A-ring, as observed during the Voyager Saturn fly-by, is reported. The peculiarities in the appearance of the F-ring are described, and include: (1) the presence of one or more loose strands of matter reaching up to 100 km away from it; (2) a fairly sharp kink or step in the main ring; and (3) the presence of local bright clumps and streaks in the otherwise rather dark ring. It is proposed that, the loose strands are probably the result of the action of the local magnetic field on sufficiently small charged particles of the ring; and that the sharp kinks and steps are a result of sudden changes in the solar magnetic field, carried by the solar wind, which compress temporarily the planetary magnetosphere.

Smoluchowski, R.↗
NASA NTRS1981

Saturn's E ring

Observations of the tenuous E ring of Saturn made by an earth-based CCD system at the time of the ring-plane crossing of March 1980 are presented. The observations were made with the CCD system attached to the 1.8-m Perkins reflector at Lowell Observatory using a pupil mask behind a focal plane mask to suppress telescopic diffraction. Photometric analysis of the CCD images reveal the edge-on brightness profile of the ring, beginning at a distance of 3 Saturn radii, to peak sharply in the vicinity of the orbit of Enceladus at about 4 Saturn radii, then decrease to a distance of over 8 Saturn radii. In addition, beyond Enceladus, the edge-on width of the ring is observed to increase with radial distance, reaching nearly 5 arcsec at 7 Saturn radii. Observations suggest, on the one hand, that the E ring is associated with Enceladus and possibly represents material ejected from the satellite, and on the other, that the ring is at an early stage in its evolution.

Baum, W. A.↗
NASA NTRS1981

No evidence of rings around Neptune

The results of two observations of stellar occultations of Neptune to determine if the planet has a ring system are reported. The sightings were made from Mt. Stromlo, Mauna Kea, and Cerro Tololo, noting that an equatorial ring would subtend only two arcsec of view. An upper accretion limit was defined to set the region around Neptune where rings, rather than satellites, could form. The intensities of the starlight from the two selected stars were recorded by photometers on magnetic tape during the occultation period. One of the stars did not occult, but passed through the entire region where a ring system might be present. No definitive evidence for rings was found, although an optical depth for a Neptunian ring was calculated at 0.07, with a width of more than 5 km and a radius of 31,400 km.

Elliot, J. L.↗
NASA NTRS1982

Radial widths, optical depths, and eccentricities of the Uranian rings

Observations of the stellar occultation by the Uranian rings of 15/16 August 1980 are used to estimate radial widths and normal optical depths for segments of rings 6, 5, 4, alpha, beta, eta, gamma, and delta. Synthetic occultation profiles are generated to match the observed light curves. A review of published data confirms the existence of width-radius relations for rings alpha and beta, and indicates that the optical depths of these two rings vary inversely with their radial widths. Masses are obtained for rings alpha and beta, on the assumption that differential precession is prevented by their self-gravity. A quantitative comparison of seven epsilon-ring occultation profiles obtained over a period of 3.4 yr reveals a consistent structure, which may reflect the presence of unresolved gaps and subrings.

Nicholson, P. D.↗
NASA NTRS1981

Image processing and analysis of Saturn's rings

Processing of Voyager image data of Saturn's rings at JPL's Image Processing Laboratory is described. A software system to navigate the flight images, facilitate feature tracking, and to project the rings has been developed. This system has been used to make measurements of ring radii and to measure the velocities of the spoke features in the B-Ring. A projected ring movie to study the development of these spoke features has been generated. Finally, processing to facilitate comparison of the photometric properties of Saturn's rings at various phase angles is described.

Yagi, G. M.↗
NASA NTRS1983

Weak dynamical effects in the Uranian ring system

Published data on the orbits of the rings and satellites of Uranus are analyzed in terms of their dynamic interaction, in an attempt to explain discrepancies between current kinematic models of the ring system and recent, very accurate occultation observations. Apse precession, short-term perturbations, satellite resonance, and the movement of the planet and rings with respect to the barycenter of the planet-satellite system are investigated. It is found that the five known satellites could cause the observed ring residuals by shifting the barycenter, but not by direct or resonance effects. Hypothetical interring shepherd satellites of limited mass (such as about 10 to the 19th g for a shepherd between rings 4 and 5), possibly affected by known-satellite resonances, could influence the rings directly.

Freedman, A. P.↗
NASA NTRS1983

Trapped particle absorption by the Ring of Jupiter

The interaction of trapped radiation with the ring of Jupiter is investigated. Because it is an identical problem, the rings of Saturn and Uranus are also examined. Data from the Pioneer II encounter, deductions for some of the properties of the rings of Jupiter and Saturn. Over a dozen Jupiter magnetic field models are available in a program that integrates the adiabatic invariants to compute B and L. This program is to label our UCSD Pioneer II encounter data with the most satisfactory of these models. The expected effects of absorbing material on the trapped radiation are studied to obtain the loss rate as a function of ring properties. Analysis of the particle diffusion problem rounds out the theoretical end of the ring absorption problem. Other projects include identification of decay products for energetic particle albedo off the rings and moons of Saturn and a search for flux transfer events at the Jovian magnetopause.

Fillius, W.↗
NASA NTRS1984

Thickness of Saturn's rings inferred from Voyager 1 observations of microwave scatter

Earth-based telescopic observations indicate that Saturn's rings are about 1 kilometer thick, while spacecraft measurements and theoretical considerations give an upper bound of about 100 meters. Analysis of a shielding effect present in radio occultation provides a sensitive new measure of the ring thickness. On the basis of this effect, Voyager 1 microwave measurements of near-forward scatter imply a thickness ranging from less than 10 meters in ring C to about 20 and 50 meters in the Cassini division and ring A, respectively. Monolayer models do not fit the observations in the latter two regions. The discrepancy between the earth-based and spacecraft measurements may be due to warps in the ring plane or effects of tenuous material outside the primary ring system.

Zebker, H. A.↗
NASA NTRS1984

Dynamics of the Uranian Rings

Some of the problems of the shepherding satellite model of Goldreich ant tremaine are discussed. The following topics are studied: (1) optical depths of the all the observed narrow rings; (2) satellite and ring separation timescales; (3) ring edge sharpness; (4) shock formation in narrow rings; (5) the existence of small satellites near the Uranian rings; and (6) the apse and node alignments of the eccentric and inclined rings.

Dermott, S. F.↗
NASA NTRS1985

Investigations of Planetary Ring Phenomena

The structure of Jupiter's rings was studied in an effort to understand the processes that generate the rings. The faint material in the Saturn system was investigated leading to the discovery of a new ring and a way to interpret the wavy features surrounding Encke's division in terms of perturbing moonlets. The interaction of faint rings and magnetospheres helped identify some of the charged particle absorbers seen by Pioneer 11 in the neighborhood of the F ring. Angular momentum drain might account for the slow rotations of intermediate sized asteroids. Much of the Jovian ring structure can be understood in terms of debris loss from satellites, rapid orbital evolution, resonance, and charged dust dynamics.

Burns, J. A.↗
NASA NTRS1985

Studies of Planetary Rings

Dynamic processes in ring structures (spiral density waves; shepherding processes; and erosional transport and global redistribution of ring materials) and planetary ring observations (Jupiter's ring; the nature of the F ring; and the irregular structure of Saturn's B ring) were examined.

Cuzzi, J. N.↗
NASA NTRS1984

Saturn's rings - Voyager 1 radio occultation experiment results

The Voyager 1 encounter with Saturn in 1980 included the first radio occultation measurements of the ring system. Opacity of and scattering by Saturn's rings were measured at 3.6- and 13-cm wavelengths using transmissions from the spacecraft to the earth. Detailed models of ring structure have been derived from these data at radial resolutions as small as 1 km, depending on the radio opacity of the region probed. Inverse power-law-type distributions of ring particle sizes with indices of about 3 and upper size cutoffs near 5 m are in good agreement with the data, but parameters vary throughout the rings. Occultation measurements have also provided information on ring dynamics and have led to a more precise definition of Saturn's pole vector.

Simpson, R. A.↗
NASA NTRS1984

The E ring of Saturn and satellite Enceladus

Photographic imagery from Pioneer and Voyager flyby satellites has been used to study the microphysical properties of Saturn's outermost E-ring, and to determine the physical relationship between the E-ring and the satellite Enceladus. The optical and infrared characteristics of the E-ring are explained in terms of Mie scattering of ice spheres with an effective diameter of 2 to 2.25 microns and an effective variance of 0.1 to 1.5. It is suggested that the E-ring is continuously replenished by volcanic eruptions on Enceladus, and recent tectonic evidence is cited in support of this hypothesis. A number of similarities in the relationship between the E-ring and Enceladus and Io and its torus are discussed, within the framework of a general model of outer solar system volcanism and planetary ring interaction.

Pang, K. D.↗
NASA NTRS1984

Structure of the Uranian rings. I - Square-well model and particle-size constraints

A least squares fitting is added to the diffraction model for the Uranian rings and the model is then used to determine the relative optical depths of the rings at visible and near-IR wavelengths. Light transmission is assumed constant on a scale of a few radial km in the rings. The amount of starlight passing through the rings is calculated from occultations with account taken of a monolayer ring, diffraction effects, and multiple ring layers. A diffracted occultation profile is generated with consideration given to the angular diameter of the star, the impulse response of the detector, and atmospheric aberrations. Calculation procedures which remove noise are outlined and results are compared with the observational data base. The wavelength dependence of optical depth is formulated. It is shown that occultation data do not support the presence of a large fraction of submicron particles. No optical depth variations are projected for 0.88-2.2 microns wavelengths. Unchopped data are needed to verify the model predictions.

Elliot, J. L.↗