R. F. Beebe

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.

Galileo's First Images of Jupiter and the Galilean Satellites

The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiters Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on Io. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.

A Wave Dynamical Interpretation of Saturn's Polar Hexagon

The hexagonal, pole-centered cloud feature in Saturns northern atmosphere, as revealed in Voyager close-encounter imaging mosaics, may be interpreted as a stationary Rossby wave. The wave is embedded within a sharply peaked eastward jet (of 100 meters per second) and appears to be perturbed by at least one anticyclonic oval vortex immediately to the south. The effectively exact observational determination of the horizontal wave number and phase speed, applied to a simple model dispersion relation, suggests that the wave is vertically trapped and provides a diagnostic template for further modeling of the deep atmospheric stratification.

Hubble space telescope observations of the 1990 equatorial disturbance on Saturn: Zonal winds and central Meridian albedos

Abstract Two sets of Hubble Space Telescope (HST) data, obtained in August 1990 and November 1990, are compared with Voyager 1 and 2 data acquired in 1980 and 1981. A rare disturbance developed in the equatorial region of Saturn in late September 1990. Due to the large equatorial wind shear, this disturbance generated inhomogeneous bright cloud structures in the equatorial region that extended to 25° north latitude in November 1990. Meridional albedo scans and wind measurements from the Voyager northern spring equinox and the HST northern summer observations are compared. Differences in average zonal winds in the images obtained with the green F547M and the methane F889N filters are interpreted as evidence of vertical shear in the zonal winds. Longitudinal variations in the equatorial zonal winds are interpreted as evidence of interaction between the storm nucleus, generated during the disturbance, and the local wind field.

The onset and growth of the 1990 equatorial disturbance on Saturn

Abstract The development of cloud structures at low Saturnian latitudes in September 1990 can be divided into three phases: the onset and expansion of the initial disturbance (popularly known as the Great White Spot), the eastward and westward expansion of the bright cloud, and the eventual formation of wave-like structures encircling the equator. The three phases of this storm are consistent with a single convective event. This event is evaluated within the historical context of previous occurrences and general seasonal mechanisms are considered.

Summary of historical data - Interpretation of the Pioneer and Voyager cloud configurations in a time-dependent framework

Ground-based imaging of Jupiter at visible and infrared wavelengths has been used to build up a time sequence of cloud feature variations. The global cloud configuration seen by Voyager 1 appears markedly different than that seen by Pioneer 10 and 11. In the context of historical data, these two different cloud distributions are not unique but part of a continuous spectrum of global variations. The most recent global changes occurred in a pattern which has been a characteristic trend observed many times before.

A seasonal radiative-dynamic model of Saturn's troposphere

Abstract In this study the thermal response of Saturns atmosphere to seasonal insolation variations is investigated using a steady-state, linearized, zonally averaged model. The model is compared to existing data to extract diagnostic information about the troposphere and stratosphere of Saturn. The radiative effect of the rings is included in the model to study the vertical heating and the meridional heat balance at the time of the Voyager spacecraft encounters. The rings are shown to be a strong modulator of the seasonal insolation function. Since the major seasonal effect of the rings is to cool the atmosphere by shadowing, the model is used to calculate radiative stability near the cloud tops. Comparisons of historical “storms,” which have the appearance of convective eruptions, do not correlate with the calculated minima in stability. This suggests that these storms are not caused by simple seasonal processes; however, seasonally decreasing insolation may provide a trigger for a dynamically unstable condition. A detailed comparison of the model with existing temperature measurements is made. Saturns large thermal inertia and uncertainties in wind and thermal measurements overwhelm the thermal effects of the rings. The comparison of the model with existing data suggests that a meridional (i.e., north/south) asymmetry in heating exists, which is consistent with an enhancement of aerosols and photochemically produced ethane and acetylene in the southern hemisphere during its summer season. Measurement uncertainties and approximations used in the “aerosol” model make this a weak conclusion. Comparison of the model with measured meridional temperature variations suggests that frictional damping in Saturns atmosphere is weaker in the equatorial region than in midlatitudes. Differences between the model and the measured thermal structure deduced by the Voyager IRIS experiment suggests that the zonal (i.e., east/west) winds measured during the Voyager encounters may include an unresolved jet structure in the southern latitudes.