Allan F. Cook

The Jupiter System Through the Eyes of Voyager 1

The cameras aboard Voyager 1 have provided a closeup view of the Jupiter system, revealing heretofore unknown characteristics and phenomena associated with the planets atmosphere and the surfaces of its five major satellites. On Jupiter itself, atmospheric motions—the interaction of cloud systems—display complex vorticity. On its dark side, lightning and auroras are observed. A ring was discovered surrounding Jupiter. The satellite surfaces display dramatic differences including extensive active volcanismn on Io, complex tectonism on Ganymnede and possibly Europa, and flattened remnants of enormous impact features on Callisto.

Voyager 2 at Neptune: Imaging Science Results

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptunes atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptunes zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.

The Galilean Satellites and Jupiter: Voyager 2 Imaging Science Results

Voyager 2, during its encounter with the Jupiter system, provided images that both complement and supplement in important ways the Voyager 1 images. While many changes have been observed in Jupiters visual appearance, few, yet significant, changes have been detected in the principal atmospheric currents. Jupiters ring system is strongly forward scattering at visual wavelengths and consists of a narrow annulus of highest particle density, within which is a broader region in which the density is lower. On Io, changes are observed in eruptive activity, plume structure, and surface albedo patterns. Europas surface retains little or no record of intense meteorite bombardment, but does reveal a complex and, as yet, little-understood system of overlapping bright and dark linear features. Ganymede is found to have at least one unit of heavily cratered terrain on a surface that otherwise suggests widespread tectonism. Except for two large ringed basins, Callistos entire surface is heavily cratered.

Voyager 2 in the Uranian system: imaging science results

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70� to -27�), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20�. Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 102 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earths moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Mirandas surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

Saturn's rings - A survey.

Abstract In this review paper we first discuss the dimensions of major ring features and of the disk of the planet. We then summarize the observed photometric parameters, and because frozen H 2 O appears to be a major ring constituent, we compare the appropriate photometric properties of various forms of snow with those of the ring. We examine several ring models, noting certain characteristics that any model should supply. In our view, a physical means of accounting for the observed ring thickness of ∼2 km is a prime requirement. There appears to be one model that presents no clear observational or theoretical inconsistency. Finally, we list certain problems whose solutions should broaden our knowledge of the ring system.

Voyager imaging experiment

The overall objective of this experiment is exploratory reconnaissance of Jupiter, Saturn, their satellites, and Saturns rings. Such reconnaissance, at resolutions and phase angles unobtainable from Earth, can be expected to provide much new data relevant to the atmospheric and/or surface properties of these bodies. The experiment also has the following specific objectives:Observe and characterize the global circulation of the atmospheres of Jupiter and Saturn;Determine the horizontal and vertical structure of the visible clouds and establish their relationship to the belted appearance and dynamical properties of the planetary atmospheres;Determine the vertical structure of high, optically-thin, scattering layers on Jupiter and Saturn;Determine the nature of anomalous features such as the Great Red Spot, South Equatorial Belt disturbances, etc.;Characterize the nature of the colored material in the clouds of Jupiter and Saturn, and identify the nature and sources of chromophores on Io and Titan;Perform comparative geologic studies of many satellites at less than 15-km resolution;Map and characterize the geologic structure of several satellites at high resolution (∼1 km);Investigate the existence and nature of atmospheres on the satellites;Determine the mass, size, and shape of many of the satellites by direct measurement;Determine the direction of the spin axes and periods of rotation of several satellites, and establish coordinate systems for the larger satellites;Map the radial distribution of material in Saturns rings at high resolution;Determine the optical scattering properties of the primaries, rings, and satellites at several wavelengths and phase angles;Search for novel physical phenomena, e.g., phenomena associated with the Io flux tube, meteors, aurorae, lightning, or satellite shadows.

An explanation of the light curve of Iapetus

Abstract This paper seeks a simple explanation for the large-amplitude (∼ 2 mag) periodic light curve of Saturns satellite Iapetus, which, as the light curve shows, apparently rotates synchronously. We point out that the mean densities of Saturns satellites increase from ∼ 1 gm cm −3 for the innermost to ∼ 3 gm cm −3 for Iapetus, a fact suggesting that frozen H 2 O, now quite certainly established as a major constituent of Saturns rings, forms a much larger fraction of the inner satellites than of the outer. We thus assume that Iapetus was once covered by a thin ice or snow layer, which, we show, has been more subject to erosion by meteoroidal bombardment on the “leading” hemisphere of the satellite in its orbit than on the “trailing” one. If the “initial” ice layer was ∼ 1 m thick and the interplanetary meteoroidal density has not changed much during most of the age of the solar system, then erosion by meteoroidal bombardment would produce the currently observed light curve and also account for the similar colors of the bright and faint hemispheres if the bare surface has sufficiently low reflectivity.

A dynamical model for the radial structure of Saturn's rings

Abstract In this paper we inquire whether the major positional features of Saturns ring can be explained by perturbations associated with the planets satellites. We assume a simple but observationally consistent model in which the ring is composed of a single layer of particles, and we further suppose that the system has evoted to a state in which collisions between particles may be ignored. Under these conditions, we find that only gravitational forces and the resulting perturbations by the two satellites Mimas and Titan suffice for a rather complete description of the ring system.

Photometry of Saturn's satellites: The opposition effect of Iapetus at maximum light and the variability of Titan

Abstract We present photometry, V and ( B - V ), of Iapetus at six western elongations (the phase of maximum brightness) that span a range in solar phase, α, from ≈6° to 0.12 mag ( V ), is present. We make a few cautious remarks about the possible relevance of the use of this result to interpret the phase curve of Saturns ring. We also give a few measures of Rhea and Hyperion, at α ≊ 0.3° , and more of Titan, which indicate that the latter has been anomalously bright during much of 1973 and 1974, sometimes by nearly 0.1 mag ( V ).

On a suspected ring external to the visible rings of Saturn

Abstract Reexamination of a photograph of Saturn taken on 15 November 1966, when the earth was nearly in the ring plane, indicates that ring material may exist outside the visible rings, extending to more than 6 Saturnian radii. Although the suspected feature on the photograph appears to be real, the possibility of its being a developed pressure mark or a chance alignment of grains cannot be ruled out. The observed brightness in blue light was estimated to be mB = 19.5 ± 0.5 per linear arcsecond, implying a normal optical thickness, τ ⋍ 10 −7 , for ice-covered particles. For spacecraft passing through this region, the hazards are found to be minimal.