D. P. Simonelli

Composition and radiative properties of grains in molecular clouds and accretion disks

We define a model of the compositon and abundances of grains and gases in molecular cloud cores and accretion disks around young stars by employing a wide range of astronomical data and theory, the composition of primitive bodies in the solar system, and solar elemental abundances. In the coldest portions of these objects, we propose that the major grain species include olivine (Fe, Mg, 2SiO4), orthopyroxene (Fe, Mg, SiO3), volatile and refractory organics, water ice, troilite (FeS), and metallic iron. This compositional model differs from almost all previous models of the interstellar medium (ISM) by having organics as the major condensed C species, rather than graphite; by including troilite as a major grain species; and by specifying the mineralogical composition of the condensed silicates. Using a combination of laboratory measurements of optical constants and asymptotic theory, we derive values of the real and imaginary indices of refraction of these grain species over a wavelength range that runs from the vacuum ultraviolet (UV) to the radio domain. The above information on grain properties is used to estimate the Rosseland mean opacity of the grains and their monochromatic opacity.

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.

Galileo encounter with 951 gaspra: first pictures of an asteroid.

Galileo images of Gaspra reveal it to be an irregularly shaped object (19 by 12 by 11 kilometers) that appears to have been created by a catastrophic collisional disruption of a precursor parent body. The cratering age of the surface is about 200 million years. Subtle albedo and color variations appear to correlate with morphological features: Brighter materials are associated with craters especially along the crests of ridges, have a stronger 1-micrometer absorption, and may represent freshly excavated mafic materials; darker materials exhibiting a significantly weaker 1-micrometer absorption appear concentrated in interridge areas. One explanation of these patterns is that Gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas.

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.

Cassini Visual and Infrared Mapping Spectrometer Observations of Iapetus: Detection of CO2

The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard the Cassini spacecraft obtained its first spectral map of the satellite Iapetus in which new absorption bands are seen in the spectra of both the low-albedo hemisphere and the H2O ice-rich hemisphere. Carbon dioxide is identified in the low-albedo material, probably as a photochemically produced molecule that is trapped in H2O ice or in some mineral or complex organic solid. Other absorption bands are unidentified. The spectrum of the low-albedo hemisphere is satisfactorily modeled with a combination of organic tholin, poly-HCN, and small amounts of H2O ice and Fe2O3. The high-albedo hemisphere is modeled with H2O ice slightly darkened with tholin. The detection of CO2 in the low-albedo material on the leading hemisphere supports the contention that it is carbon-bearing material from an external source that has been swept up by the satellites orbital motion.

High‐temperature hot spots on Io as Seen by the Galileo solid state imaging (SSI) Experiment

High-temperature hot spots on Io have been imaged at ∼50 km spatial resolution by Galileos CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiters shadow) via the SSI clear filter (∼0.4–1.0 µm), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io‧s atmosphere and plumes. A total of 13 hot spots have been detected over ∼70% of Io–s surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed ∼700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (∼1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io.

Phase curves of materials of Io: Interpretation in terms of Hapke's function

Abstract The photometric function developed by B. Hapke (1981, J. Geophys. Res. 86 , 3039–3054; 1984, Icarus 59 , 41–59) has been applied to near-opposition ( α = 2–8°) disk-resolved phase curves for three color classes on Io, and the disk-integrated phase curve ( α = 2–159°) of the satellite as a whole. Derived values of the Hapke compaction parameter h suggest that (1) a large percentage of the material on Ios surface has a porosity significantly greater significantly greater than 60%, supporting the estimate of high porosity made by D.L. Matson and D.B. Nash (1983, J. Geophys. Res. 88 , 4771–4783) and Nelson et al. (1984, Bull. Amer. Astron. Soc. 16 , 683–685; 1984, EOS 65 , 982–983); and (2) Average (“orange”) and Polar (“brown”) materials are significantly more porous than Bright (“white”) materials, a cottrast consistent with the Matson and Nash (1983) SO 2 cold trap model. The best-fit single particle phase function becomes more backscattering on moving from Polar to Average to Bright materials, with the surface of Io on average exhibiting significant backscattering comparable in magnitude to that of the lunar surface. For the color classes, and for Io as a whole, the degree of backscattering tends to increase toward longer wavelengths. The average macroscopic roughness of the Ionian surface, characterized by a mean slope angle of O ≃ 25°, is similar to that of other solid surface in the solar system. Consistency between observed limb darkening and that predicted by the Hapke model requires the presence of significant macroscopic roughness ( O ≥ 20°) for the Average regions, but not necessarily for the Bright and Polar materials.

Voyager disk-integrated photometry of Io

Abstract Voyager full-disk images of Io, available at solar phase angle of α = 2−29° and 101−159°, allow comparisons of the satellites near-opposition photometric behavior with Earth-based results and the determination of the phase curve out to very high phase angles. The near-opposition data were reduced iteratively for self-consistent phase and rotation curves in each Voyager filter; the resulting phase coefficients, geometric albedos, and rotational lightcurves are consistent with Earth-based findings, except for a previously noted tendency for Voyager to yield somewhat redder spectral information. The derived near-opposition phase coefficients, ranging between 0.016 and 0.024 mag/ deg, decrease with increasing wavelength, a trend weakly noted in some Earth-based observations. The full, α = 2−159° phase curves allow the first direct determination of the phase integral of Io at several wavelengths: q rises from ≈0.7 in the ultraviolet to ≈0.8 in the orange. Combination of the Voyager phase integrals with Earth-based albedo information leads to a best estimate of the bolometric Bond albedo of 0.50 ± 0.10, a value consistent with, but slightly below, previous estimates.

Galileo imaging of SO2 frosts on Io

Ios visible appearance changes dramatically with solar phase angle. The polar regions and some plume deposits near active volcanic centers become comparatively bright with increasing phase angle, while the equatorial band grows relatively dark. We suggest that the areas of Io that appear unusually bright at high phase are covered by thin frosts of SO2 that are transparent under normal illumination. A global disk-resolved photometric analysis indicates that the frosts exhibit more nearly isotropic or forwardscattering behavior and less opposition brightening than average Ionian materials. Comparison with Near-Infrared Mapping Spectrometer (NIMS) results suggests that these frosts have relatively strong 4.1 μm absorptions indicative of fine-grained SO2.

Regolith variations on Io: Implications for bolometric albedos

Global maps of lo produced from two sets of Galileo images (solar phase angles 4°-14° and 71°-86°, respectively) reveal that this satellites color and albedo patterns change dramatically with phase. At low phase the equatorial band is the brightest, whitest part of Io; that is, it is brighter than the polar regions at all wavelengths. At high phase, however, the equatorial band becomes a dark gray, exhibiting little contrast with the polar regions at violet and green wavelengths and appearing darker than the polar regions at red wavelengths. To quantify these phase-related changes, we derive global maps of the Henyey-Greenstein asymmetry factor g that show how the strength of backscattering by Ios regolith varies from region to region. In the green and red, where albedo patterns change most radically with phase, the equatorial band forms a well-defined unit of strongly backscattering material; material that is more weakly backscattering at these wavelengths, mainly in the polar regions, shows a broad continuum of different g values. In the violet the phase-related changes in albedo patterns are more subdued, and the scattering units are poorly defined. Using this information on g, we generate a global map showing variations in the bolometric Bond albedo A B , the true energy balance albedo that governs insolation-based surface temperatures. The mean albedo of this map, A B 0.52, is similar to A B values computed previously for Io, but the distribution of albedos is markedly different. In previous Bond albedo maps the highest A B coincided with the bright equatorial band seen at low phase; the new map, however, more closely resembles Ios high-phase albedo patterns (i.e., the band of high Bond albedos at the equator is absent). This change in A B patterns has a significant effect on how insolation-based temperatures vary with latitude on Io; in particular, it increases the satellites equator-to-pole temperature contrast.