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NASA NTRS1980

Radar observations of Saturn's rings at intermediate tilt angles

The 12.6-cm-wavelength radar observations of Saturn's rings, corresponding to ring-plane tilt angles of 18.2, 11.7, and 5.6 degrees, respectively, are reported. Measurements of geometric albedo and circular polarization ratio are used to investigate acceptable ring models and radar scattering properties of individual particles. Attention is given to the dependence of the rings' polarization properties on wavelength or tilt angle, or both. All large-particle monolayer models of Saturn's rings are ruled out; however, extended-layer models postulating ice or metal particles are compatible with the radar albedo measurements and are less severely restricted by the polarization data.

Ostro, S. J.↗

NASA NTRS1980

The 20-micron brightness temperature of the unilluminated side of Saturn's rings

Observations of the 20-micron brightness temperature of the unilluminated (north) side of Saturn's rings are presented and discussed in terms of models of B ring heating. Observations centered on the B ring ansae were made at 19.8 microns by the 3-m IR telescope facility on Mauna Kea during a period when the unilluminated side of the rings could be observed from earth. Flux measurements indicate a 20-micron brightness temperature of 56 + or - 1 K, in agreement with that measured at 45 microns by Pioneer 11. Under the assumptions that the emission of the Cassini division is negligible and that the ring brightness temperature is close to the actual temperature, it is shown that the brightness temperature can be accounted for by heating by the disk of Saturn, and is proportional to the sine of the Saturnicentric declination of the sun.

Tokunaga, A. T.↗

NASA NTRS1979

Kinematics and dynamics of vortex rings in a tube

Kinematic theory and flow visualization experiments were combined to examine the dynamic processes which control the evolution of vortex rings from very low to very high Reynolds numbers, and to assess the effects of the wall as a vortex ring travels up a tube. The kinematic relationships among the size, shape, speed, and strength of vortex rings in a tube were computed from the theory. Relatively simple flow visualization measurements were used to calculate the total circulation of a vortex rings at a given time. Using this method, the strength was computated and plotted as a function of time for experimentally produced vortex rings. Reynolds number relationships are established and quantitative differences among the three Reynolds number groups are discussed.

Brasseur, J. G.↗

NASA NTRS1981

The origin of the eccentricities of the rings of Uranus

The effect of gravitational perturbations from a nearby satellite on the eccentricity e of a narrow particulate ring is considered. The perturbations near a resonance in an eccentric ring may be divided into corotation and Lindblad terms. For small e, the corotation terms damp e, whereas the Lindblad terms excite e. In the absence of saturation the corotation terms win by a small margin, and e damps. However, if the perturbations open gaps at the strongest resonances, then the Lindblad terms win, and e grows. This result offers an explanation for the existence of both circular and eccentric rings around Uranus. It is also shown that eccentricity changes induced by circular rings on eccentric satellite orbits are similar to those induced by satellites with circular orbits on eccentric rings.

Goldreich, P.↗

NASA NTRS1980

Some possible effects of Jupiter's rings on the Jovian inner plasmasphere

The ionospheric plasma density on magnetic field lines threading the Jovian rings which are located inside approximately 1.8 Jupiter radii on the jovigraphic equatorial plane is calculated by using a rotating ion exosphere model. It is found that the bulk of the ionospheric particles on these field lines are on ballistic trajectories. On field lines approximately symmetric with respect to the jovigraphic equator, the ring, which to a first approximation would absorb the population of trapped particles, consequently has little effect. On field lines which are made asymmetric by the higher-order multipoles of Jupiter's field and the tilt of the dipole axis, the rings may have a significant effect. It is suggested that better definition of the rings' atmospheric and ionospheric properties is required to model these localized effects. If the rings are found to be an important plasma source for the inner magnetosphere, the present exospheric model will have to be revised.

Luhmann, J. G.↗

NASA NTRS1981

The Gemini-Monoceros X-ray enhancement - A giant X-ray ring

A 1.5 deg spatial resolution map of the 1/4 KeV diffuse X-ray background enhancement in the Gemini and Monoceros constellations is found to show a circular ring-shaped emission feature with a diameter of 20 deg. The feature and possible X-ray contributions from the Mon OB1 association and neighboring supernova remnants are discussed. From 300 pc, the region has a radius of 50 pc, with an emitting electron density of 0.01 per cu cm. A shell of expanding neutral hydrogen and nonthermal radio spur is observed outside the ring with the X-ray emitting pulsar PSR 0656 + 14 lying close to the center of the ring. Origins of the ring are discussed, ruling out formation by the association Mon OB1. The ring is considered to be a field supernova remnant formed by the progenitor of the central pulsar, and providing constraints on theories of remnant evolution. This conclusion is found to agree with estimated supernova rates, and the absence of additional examples of this stage of evolution is an observational selection effect.

Nousek, J. A.↗

NASA NTRS1982

Identification of resonance features within the rings of Saturn

The Voyager 2 UV spectrometer observed a stellar occultation by the rings of Saturn, which located ring features with an accuracy of 12 km. A high-resolution (3 km) optical depth atlas of the rings shows at least nine features, including four density wave patterns, identified with satellite resonances. Analysis of these density wave patterns yields the first surface mass densities for the A ring and, together with the optical depth atlas, a total ring mass of 6.4 x 10 to the -8th Saturn masses.

Holberg, J. B.↗

NASA NTRS1981

Discovery of multi-ring basins - Gestalt perception in planetary science

Early selenographers resolved individual structural components of multi-ring basin systems but missed the underlying large-scale multi-ring basin patterns. The recognition of multi-ring basins as a general class of planetary features can be divided into five steps. Gilbert (1893) took a first step in recognizing radial 'sculpture' around the Imbrium basin system. Several writers through the 1940's rediscovered the radial sculpture and extended this concept by describing concentric rings around several circular maria. Some reminiscences are given about the fourth step - discovery of the Orientale basin and other basin systems by rectified lunar photography at the University of Arizona in 1961-62. Multi-ring basins remained a lunar phenomenon until the fifth step - discovery of similar systems of features on other planets, such as Mars (1972), Mercury (1974), and possibly Callisto and Ganymede (1979). This sequence is an example of gestalt recognition whose implications for scientific research are discussed.

Hartmann, W. K.↗

NASA NTRS1982

Stability of negatively charged dust grains in Saturn's ring plane

The boundary between the inner and outer parts of Saturn's B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 Saturn radius. Observations by Pioneer 11 and Voyager 2 in the infared see the boundary as a prominent change in ring brightness at this radius. The occultation of delta-Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of two change in optical depth beginning very close to this radius.

Northrop, T. G.↗

NASA NTRS1982

Mysteries of the ringed planets

An assessment is presented of the recent progress in the theory of planetary rings which was in evidence at the IAU's recent, 75th Colloquium. Observational material was dominated by spacecraft data, and theoretical consideration of the problems posed comes predominantly from gravitational mechanics. An understanding of collective effects, in light of both fluid mechanical and statistical mechanical methodologies, is being approached, and the importance of electromagnetic phenomena studies is noted. Voyager observations of Saturn's rings, and accumulating data from stellar occultations by the rings of Uranus, provided most of the observational material. Jupiter's faint ring was closely examined by the 1979 Voyager flight. These three known ring systems are found to exhibit such family resemblances as their proximity to the parent planet and magnetospheric environment.

Cuzzi, J. N.↗

NASA NTRS1982

Maps of the rings of Uranus at a wavelength of 2.2 microns

Maps of the ring system of Uranus, as seen in reflected sunlight at a wavelength of 2.2 microns, are presented for May 1978 and July 1979. Large azimuthal brightness variations revealed by these maps are consistent with the variable width established for the epsilon ring from occultation studies and support the existing elliptical model for this ring. An upper limit of approximately 0.010 is placed on the normal optical depth of any broad (approximately 5000 km wide) axisymmetric ring component. There exists, however, an unexplained east-west asymmetry in the May 1978 map. In addition, multiaperture broadband infrared photometry of Uranus is reported, from which the geometric albedo of the planet at wavelengths of 1.25, 1.65, and 2.2 microns and the geometric albedo of the rings at 2.2 microns are derived.

Matthews, K.↗

NASA NTRS1983

Bending waves in Saturn's rings

Saturn A ring brightness variations are investigated and noted to be caused by vertical corrugations of the local ring plane due to a spiral bending wave, resonantly excited by Mimas, which propagates inwardly by way of the ring particle collective gravity. Some aspects of the presently developed theory of forced bending waves have been previously treated in the galactic context. It is noted that the theory is generally in good agreement with observations, and in particular may resolve the conflict between ground-based estimates of 1-2 km for the global ring thickness and Voyager stellar occultation measurements of less than 200 m for the local ring thickness.

Shu, F. H.↗

NASA NTRS1983

A model for the formation of spokes in Saturn's rings

Evidence is mounting which implies that the generation, evolution, and motion of spokes require a dense plasma near the Saturn ring plane. It is presently suggested that spokes consist of charged, micron-sized dust particles which are elevated from the rings by radially moving, dense plasma columns due to meteor impacts on the ring. Electrostatic wall sheaths at the ring, and ring charging by electric fields sufficiently strong to overcome the gravitational force on small dust particles, arise from the dense plasma, which also increases the probability of dust particle excess electronic charge. The radial motion of the plasma column is due to an azimuthal polarization electric field due to the relative motion between the corotating plasma and the negatively charged dust particles moving at Keplerian speeds.

Goertz, C. K.↗

NASA NTRS1983

Electrostatic discharges in Saturn's B-ring

The Voyager observations of electrical discharges in Saturn's rings strongly support earlier speculations on the role played by electrostatics, magnetic fields, and lightning phenomena in the primitive solar system. They also suggest conditions then by direct analogy rather than by extrapolating backwards through time from conditions now. The observed discharges show a pronounced 10h periodicity, which suggests a source in Keplerian orbit at 1.80 + or - 0.01 Saturn radii. In that region, the B ring is thicker than optical depth 1.8 for about 5,000 km. At 1.805 + or - 0.001 Saturn radii, however, the ring is virtually transparent for a gap of width 200 m. It is concluded that a small satellite orbits Saturn at that radius and clears the gap. The gap edges must prevent diffusive filling of the gap by fine material which is especially abundant at this position in the rings and would otherwise destroy the gap in minutes. The discharges represent the satellite's interaction with the outer edge of the gap. Spoke formation may involve the interaction of ring material in the vicinity of the gap.

Warwick, J. W.↗

NASA NTRS1983

The inner edge of Saturn's B ring

The sharp, 90-km wide transition from an optical depth of 0.2 in the C ring to 1 in the B ring begins at 91,970 km from Saturn's center. This radius is found to be almost exactly at the inward stability limit of charged particles launched in the ring plane at the local Kepler velocity, provided these particles have large charge to mass ratio. The zonal harmonic models of Saturn's magnetic field from the Voyager data and the gravitational field model from Pioneer data are essential to get the very close agreement between theory and observation. The theoretical stability limits are 91,973 + or - 145 km from Voyager 1 magnetic field data and 91,991 + or 145 km from Voyager 2 magnetic data. The zonal harmonic magnetic field lines are not perpendicular to the ring plane. Therefore, in addition to the magnetic mirror, gravitational, and centrifugal forces, an unknown force must be postulated to produce equilibrium in the ring plane and make the stability calculation meaningful.

Northrop, T. G.↗

NASA NTRS1983

Precise thermal control test demonstration on simulated space telescope main ring

A simulated section of the main ring of the Space Telescope, (ST) the part that holds the primary mirror, was manufactured and tested for thermal stability characteristics in a complex structural assembly. The other telescope subassemblies are also mounted on the main ring, which is constructed of a titanium alloy. The entire ring is wrapped in heater elements around the outside diameter to reduce the external temperature gradient effects by two orders of magnitude. A thermal vacuum test was performed using a stainless steel alloy tailored to represent the titanium ring's thermal characteristics. The tests examined the thermal performance and controllability of the test ring, the effects of a 24-hr slew manuever that produced worst-case environment changes, hot and cold simulation of predicted ST orbital thermal environments, and the effects of heater failure. Pointing and thermal control features were verified, but a heater failure on the 'on' side was found to quickly lead to exceeding the thermal design limits.

Bettini, R. G.↗

NASA NTRS1983

The rings of Uranus - Occultation profiles from three observatories

Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15-16 August 1980 are compared. The alpha ring shows a 'double-dip' structure; the eta ring shows a broad and narrow component (similar to Saturn's F ring); and the epsilon ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles.

Elias, J. H.↗

NASA NTRS1984

Saturn's rings - 3-mm low-inclination observations and derived properties

To determine a more precise brightness temperature and more accurate properties for Saturn's rings, 3.3 mm low inclination observations have been made at 90 GHz with the Aerospace 4.6 m radio telescope. A mean brightness temperature of 17 plus or minus 4 K has been determined by comparing the data with the variation of the inclination of the total flux from the planet and rings predicted by a simple model with uniformly bright A and B rings. Variation of the normal optical depth from 0.4 to 1.0 resulted in a total variation of about 1.5 K in A and B brightness. A portion of the brightness attributed to ring particle thermal emission has been determined to be at a temperature of 11 plus or minus 5 K. If the maximum particle radius (approximately 5 m) deduced from Voyager bistatic radar observations is correct, results indicate a particle distribution ranging between 1 cm and several meters radius of the form r exp -s with s = 3.3-3.6, or a material absorption coefficient ranging between 3 and 10 times lower than that of pure water ice at 85 K, or both. An increase in the porosity of the ice particles through a decrease in their density will not produce the observed particle albedo. If the rocky material is uniformly distributed, low ring brightness temperature would allow a silicate upper limit of approximately 10 percent by mass; however, the silicate material could be more abundant if it is separated from the icy material.

Epstein, E. E.↗