Stewart A. Collins

A new look at the saturn system: the voyager 2 images.

Voyager 2 photography has complemented that of Voyager I in revealing many additional characteristics of Saturn and its satellites and rings. Saturns atmosphere contains persistent oval cloud features reminiscent of features on Jupiter. Smaller irregular features track out a pattern of zonal winds that is symmetric about Saturns equator and appears to extend to great depth. Winds are predominantly eastward and reach 500 meters per second at the equator. Titan has several haze layers with significantly varying optical properties and a northern polar collar that is dark at short wavelengths. Several satellites have been photographed at substantially improved resolution. Enceladus surface ranges from old, densely cratered terrain to relatively young, uncratered plains crossed by grooves and faults. Tethys has a crater 400 kilometers in diameter whose floor has domed to match Tethys surface curvature and a deep trench that extends at least 270� around Tethys circumference. Hyperion is cratered and irregular in shape. Iapetus bright, trailing hemisphere includes several dark-floored craters, and Phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours. Within Saturns rings, the birth of a spoke has been observed, and surprising azimuthal and time variability is found in the ringlet structure of the outer B ring. These observations lead to speculations about Saturns internal structure and about the collisional and thermal history of the rings and satellites.

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.

Effects of proton damage on charge-coupled devices

Recent analytical and experimental work has provided new insights into the production of damage sites in silicon Charge-Coupled Devices (CCDs) by energetic particles and into the effects of these sites on CCD performance. An approximate correlation is presented between experimental results and a prediction of proton-induced displacement damage, and possible explanations for remaining inconsistencies are discussed. As a consequence of this agreement, it is now possible to predict the effect of complicated space proton environments upon CCD charge transfer efficiency and other CCD performance parameters. This prediction requires evaluation of the damage resulting from only a small number (

CCD Advances For X-Ray Scientific Measurements In 1985

A theoretical model is presented which predicts the output response of a CCD to soft X-ray spectra. The model simulates the four fundamental parameters that ultimately limit CCD performance: Quantum efficiency, charge collection efficiency, charge transfer efficiency, and read noise. Simulated results are presented for a wide variety of CCD structures, and general conclusions are presented about achieving a practical balance of sensitivity, energy, and spatial resolution for an AXAF instrument. We compare the results of the analysis to an existing state-of-the-art CCD and project improvements which will be made in the near future.

Matching A Curved Focal Plane With CCD's: Wide Field Imaging Of Glancing Incidence X-Ray Telescopes

The design of a wide field imaging camera suitable for use with a glancing incidence X-ray telescope is complicated by the sharply concave nature of the optimum focal surface of such a telescope. We are currently designing such a camera made up of a mosaic of Charge Coupled Devices (CCDs) intended for flight aboard the Advanced X-ray Astrophysics Facility (AXAF). We discuss the design rationale and tradeoffs and present our layout for the imaging CCD array. We also discuss the related issue of optimizing performance of transmission objective gratings and present the array of CCD orientations suitable for this problem.

Voyager imaging investigation requirements and results

The Voyager cameras include unique features required by the characteristics of a mission to Jupiter Saturn Uranus and Neptune. These characteristics include low illumination levels long mission duration large communication distances and intense Jovian radiation belts. The presence of several modest programmable processors on the spacecraft has permitted the post-launch refinement of image acquisition and the development of imaging capabilities such as image motion compensation and data compression which were not originally part of the baseline experiment design. The quantity and quality of Neptune imaging data exceeds by a large factor the yield which was foreseen at the time of launch thirteen years ago. The initial expenment design is descnbed as well as its subsequent post-launch evolution. The key conclusion is that flexibility must be a design goal for imaging systems developed for remote sensing of new territory. 1.

Planetary Imaging: Past, Present, and Future

Recent exploration of the planets has been highlighted by the development of visual imaging systems carried on board the spacecraft. This paper describes the evolution of planetary camera systems from the earliest reconnaissance flight to Mars in 1965 (Mariner 4) through the planned mission to Jupiter and Saturn in 1977. Advances in telecommunication performance, mission planning and operations, and digital processing of images are also discussed. Science objectives and changes in the imaging systems required to meet these objectives are discussed for the Mariner Mars 1971 (Mariner 9), Mariner Venus-Mercury (Mariner 10), Viking 1975 (Mars Orbiter), and Mariner Jupiter-Saturn 1977 missions. The last section of the paper describes future plans for imaging experiments based on cameras using solid-state sensors, particularly charge-coupled devices.