ALBERT

Description: The satellites of Uranus, with densities between 1.3 and 1.7 g cm(-3) (from Voyager 2 observations) and the Pluto-Charon system, with a mean density of just above 1.8 g cm(-3) (from terrestrial observations of mutual eclipse events), are too dense to have a significant amount of methane ice in their interiors. However, the observed densities do not preclude contributions from such organic materials as the acid-insoluble residue in carbonaceous chondrites and laboratory-produced tholins, which have densities on the order of approximately 1.5 g cm(-3). These and other considerations have led researchers to investigate the carbon mass budget in the outer solar system, with an emphasis on understanding the contribution of organic materials. Modeling of the interiors of Pluto and Charon (being carried out by R. Reynolds and A. Summers of NASA/Ames), assuming rock and water ice as the only constituents, suggests a silicate mass fraction for this system on the order of 0.65 to 0.70. The present work includes the most recent estimates of the C/H enhancements and high z/low z ratios of the giant planets (Pollack and Bodenheimer, 1987), and involves a more careful estimation of the high z/low z mass ratio expected from solar abundances than was used in Pollack et al. (1986), including the influence of the fraction of C in CO on the amount of condensed water ice. These calculations indicate that for a particular fraction of C in CO and a given fraction of C-bearing planetesimals that dissolve in the envelope (most likely in the range 0.50 to 0.75), (1) Jupiter and Saturn require a larger fraction of C in condensed materials than Uranus and Neptune, but (2) the Jupiter and Saturn results are much less strongly constrained by the error bars on the observed C/H enhancements and high z/low z ratios than is the case for Uranus and Neptune. The clearest result is that in the region of the solar nebula near Uranus and Neptune, the minority of carbon that is not in gaseous CO (1) must include a nonzero amount of condensed material, but (2) is most likely not condensed material alone, i.e., there must be a third carbon-bearing component besides condensed material and gaseous CO. Given the implied dearth of methane ice, the condensed carbon is likely dominated by organic material, and the third component present in addition to CO and organics is assumed to be CH4 gas.

Description: The temperature profile of Amalthea's surface layer is modeled as a function of location and time of day, assuming a triaxial ellipsoid shape and thermal properties similar to those of the lunar soil. Although the major heat source is direct insolation, temperatures are slightly increased by thermal radiation from Jupiter, sunlight reflected from the planet, and charged particle bombardment. Possible sources of error in the model are discussed in detail, including satellite shape effects, unusually low emissivity, uncommonly rough surface, abnormal thermal inertia, charged particle flux variability, and Joule heating. Voyager 1 IRIS observations suggest that the Amalthean surface has an emissivity near unity, but cannot put any useful limits on the thermal inertia of the Amalthean surface layer.

Description: We present rotational light-curve data for Saturn's satellite Phoebe taken over the observing period prior to the Cassini mission's encounter with that moon.

Description: In order to derive more accurate normal reflectances of regions on Io, the near-opposition (alpha 10 deg) photometry of a set of regions on the 20 W face of the satellite, the face with the most complete near-opposition Voyager imaging coverage was analyzed. Sixty-eight regions were chosen for study, each approximately 120 km on a side, and divided into three coherent color classes: white, orange, and polar brown regions. Derived Minneart parameters, while generally consistent with previous limb darkening determinations, show that: (1) for the white regions, k rises linearly with increasing B(0), which is the trend to be expected for normal materials; (2) the white regions are slightly limb brightened (k 0.5) at low wavelengths (low values of B(0)); and (3) k appears to drop with rising B(0) for the orange regions, contrary to the trend expected for most materials. Despite similar reflectances, there is one group of orange regions with large opposition surges and a second group showing significantly lower surges, possibly indicating differences in surface texture (porosity and/or particle size).

Description: The brightest, coldest areas on Io, the white regions, may act as cold traps for SO2 gas, and thus have an important role in governing the pressure, diurnal variation, and flow of the satellite's tenuous SO2 atmosphere. Therefore, it is essential to derive accurate albedos for the brightest regions, where the necessary albedos are those in the energy balance equation of the surface used to compute temperatures. Forty-one of the brightest of the white areas, each 60 to 120 km on a side were studied. The simplest way to estimate the required energy balance albedo for each region is to determine the Bond slbedo of a planet covered with that type of material. This process is outlined and resulting albedos are given. with the exception of several darker regions on the poorly-resolved post eclipse face of Io, typical albedos are 0.6 to 0.7. The brightest areas studied are located in the cluster of white regions east of Prometheus (longitudes 90 to 40 deg W). It is possible using Voyager data and fits to Hapke's equation to derive albedos for the bright regions without making any assumptions about the phase integrals.

Description: The results of a comprehensive analysis of disk-integrated photometry of Io derived from Voyager images are presented. Phase curves, rotation curves, and geometric albedos consistent with earth-based observations of Io are obtained, except for a previously noted tendency for the Voyager data to yield somewhat redder colors. It is demonstrated that the phase coefficient decreases with increasing wavelength. The phase integral ranges from about 0.7 in the ultraviolet to about 0.8 in the orange. The Bond albedo is estimated at 0.50 + or -0.10, a value consistent with, though slightly below, previous earth-based estimates.

Description: For conducting a comparison of Voyager photometric measurements of individual areas on the Jovian satellite Io to laboratory data, it is necessary to reduce the two sets of information to a comparable format. In this connection, more accurate normal reflectance spectra for regions on Io are derived by means of a two-step process. The near-opposition limb-darkening characteristics are obtained for areas on the surface of Io. The new limb-darkening results are employed to determine the near-opposition phase behavior of selected regions on the satellite, taking into account also the effects of the opposition surge. A combination of the phase and limb-darkening data makes it possible to obtain improved normal reflectance spectra for regions on Io. These spectra can be readily compared with laboratory spectra of candidate materials.