Bradford A. Smith

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.

A Circumstellar Disk Around β Pictoris

A circumstellar disk has been observed optically around the fourth-magnitude star β Pictoris. First detected in the infrared by the Infrared Astronomy Satellite last year, the disk is seen to extend to more than 400 astronomical units from the star, or more than twice the distance measured in the infrared by the Infrared Astronomy Satellite. The β Pictoris disk is presented to Earth almost edgeon and is composed of solid particles in nearly coplanar orbits. The observed change in surface brightness with distance from the star implies that the mass density of the disk falls off with approximately the third power of the radius. Because the circumstellar material is in the form of a highly flattened disk rather than a spherical shell, it is presumed to be associated with planet formation. It seems likely that the system is relatively young and that planet formation either is occurring now around β Pictoris or has recently been completed.

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.

NICMOS imaging of the HR 4796A circumstellar disk

We report the first near-infrared (NIR) imaging of a circumstellar annular disk around the young (~8 Myr), Vega-like star HR 4796A. NICMOS coronagraph observations at 1.1 and 1.6 μm reveal a ringlike symmetrical structure that peaks in reflected intensity 105±002 (~70 AU) from the central A0 V star. The ring geometry, with an inclination of 731±12 and a major axis position angle of 268±06, is in good agreement with recent 12.5 and 20.8 μm observations of a truncated disk. The ring is resolved with a characteristic width of less than 026 (17 AU) and appears abruptly truncated at both the inner and outer edges. The region of the disk-plane inward of ~60 AU appears to be relatively free of scattering material. The integrated flux density of the part of the disk that is visible (greater than 065 from the star) is found to be 7.6±0.5 and 7.4±1.2 mJy at 1.1 and 1.6 μm, respectively. Correcting for the unseen area of the ring yields total flux densities of 12.8±1.0 and 12.5±2.0 mJy, respectively (Vega magnitudes equal to 12.92±0.08 and 12.35±0.18). The NIR luminosity ratio is evaluated from these results and ground-based photometry of the star. At these wavelengths, L(λ)/L(λ) is equal to 1.4±0.2×10 and 2.4±0.5×10, giving reasonable agreement between the stellar flux scattered in the NIR and that which is absorbed in the visible and reradiated in the thermal infrared. The somewhat red reflectance of the disk at these wavelengths implies a mean particle size in excess of several microns, which is larger than typical interstellar grains. The confinement of material to a relatively narrow annular zone implies dynamical constraints on the disk particles by one or more as yet unseen bodies.

Variable features on Mars: Preliminary mariner 9 television results

Abstract Systematic Mariner 9 photography of a range of Martian surface features, observed with all three photometric angles approximately invariant, reveals three general categories of albedo variations: (1) an essentially uniform contrast enhancement due to the dissipation of the dust storm; (2) the appearance of splotches, irregular dark markings at least partially related to topography; and (3) the development of both bright and dark linear streaks, generally emanating from craters. Some splotches and streaks vary on characteristic timescales ∼2 weeks; they have characteristic dimensions of kilometers to tens of kilometers. The loci of these features appear in some cases to correspond well to the ground-based albedo markings, and the integrated time variation of splotches and streaks is suggested to produce the classical “seasonal” and secular albedo changes on Mars. The morphology and variability of streaks and splotches, and the resolution of at least one splotch into an extensive dune system, implicates windblown dust as the principal agent of Martian albedo differences and variability.

The Circumstellar Disk of HD 141569 Imaged with NICMOS.

Coronagraphic imaging with the Near-Infrared Camera and Multiobject Spectrometer on the Hubble Space Telescope reveals a large, approximately 400 AU (4&arcsec;) radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected light image at 1.1 µm shows the disk oriented at a position angle of 356&j0;+/-5&j0; and inclined to our line of sight by 51&j0;+/-3&j0;; the intrinsic scattering function of the dust in the disk makes the side inclined toward us, the eastern side, brighter. The disk flux density peaks 185 AU (1&farcs;85) from the star and falls off to both larger and smaller radii. A region of depleted material, or a gap, in the disk is centered 250 AU from the star. The dynamical effect of one or more planets may be necessary to explain this morphology.

The Martian atmosphere - Mariner 9 television experiment progress report.

Abstract Atmospheric phenomena appearing in the Mariner 9 television pictures are discussed in detail. The surface of the planet was heavily obscured by a global dust storm during the first month in orbit. Brightness data during this period can be fitted by a semi-infinite scattering and absorbing atmosphere model with a single-scattering albedo in the range 0.70–0.85. This low value suggests that the mean radius of the particles responsible for the obscuration was at least 10 μm. By the end of the second month, this dust storm had largely dissipated, leaving a residual optical depth ∼0.1. Much of the region north of 45°N was covered by variable clouds comprising the north polar hood. The cloud structures revealed extensive systems of lee waves generated by west-to-east flow over irregular terrain. Extensive cloud systems in this region resembled baroclinic wave cyclones. Clouds were also observed over several of the large calderas; these clouds are believed to contain water ice. Several localized dust storms were seen after the global dust storm cleared. These dust clouds appeared to be intensely convective. The convective nature of these storms and the stirring of large dust particles to great heights can be explained by vertical velocities generated by the absorption of solar radiation by the dusty atmosphere.

Infrared Views of the TW Hydra Disk

The face-on disk around TW Hya is imaged in scattered light at 1.1 and 1.6 km using the coronagraph in the Near Infrared Camera and Multi Object Spectrometer aboard the Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20 to 230 AU. The surface brightness declines as a power law of r~2.6B0.1 between 45 and 150 AU. The scattering pro—le indicates that the disk is —ared, not geometrically —at. The disk, while spatially unresolved in thermal radiation at 12 and 18 km in observations from the W. M. Keck Observatory, shows amorphous and crystalline silicate emission in its spectrum. A disk with silicate grains of a radius D1 km in size in its surface layers can explain the color of the scattered light and the shape of the mid-infrared spectrum. Much larger grains in the disk interior are necessary to —t the millimeter-wave spectral energy distribution, and hence grain growth from an original interstellar size population may have occurred.