Voyager Photometry of IO
Abstract
Detailed disk-integrated and disk-resolved photometric studies of the Jovian moon Io have been carried out using Voyager images. Disk-integrated properties derived from the Voyager data, such as phase curves, rotation curves, geometric albedos, phase integrals, and the Bond albedo, are generally consistent with earth-based estimates. Near -opposition limb-darkening behavior, as parameterized by the Minnaert photometric function, has been accurately measured for regions on the surface of Io in three distinct color classes: Bright ("white"), Average ("orange"), and Polar ("brown"). The limb-darkening results allow derivation of accurate near-opposition disk-resolved phase curves, revealing substantial differences in opposition surge among the color classes. Modelling of the phase curves using the Hapke photometric function supports the contention that the uppermost layer of the Ionian surface is on average extremely porous, and suggests that this layer is substantially more porous in Average and Polar areas than in the Bright regions, a difference consistent with models of Io's surface layer. Combination of limb-darkening and phase information leads to determination of accurate normal reflectance spectra for the color classes; careful comparison with laboratory data supports earlier claims that the spectra of Ionian materials can be explained by mixtures of sulfur and SO(,2) frost, although this is not a unique diagnostic identification. Consolidation of the Bright region photometric data acquired in the above studies allows calculation of accurate energy-balance albedos for, and mapping of the locations of, the brightest areas on Io; the results suggest that the albedos of the brightest areas are only moderately higher than that of the satellite as a whole. Finally, if the Ionian surface were covered with ordinary laboratory sulfur, or experienced temporary SO(,2) frost deposits in response to cold temperatures, temperature-dependent effects should be apparent in the photometric behavior of the Ionian dayside; a detailed search reveals no evidence of such effects, a result that is consistent with arguments that the SO(,2) atmosphere of Io is extremely tenuous, and mechanisms that would diminish changes in the spectrum with temperature for any sulfur present in the environment of the Ionian surface.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1987
- Bibcode:
- 1987PhDT.........1S
- Keywords:
-
- JOVIAN MOON;
- Physics: Astronomy and Astrophysics;
- Io;
- Photometry;
- Voyager Project;
- Albedo;
- Atmospheres;
- Frost;
- Images;
- Limb Darkening;
- Mapping;
- Rotation;
- Spectrum Analysis;
- Surface Layers;
- Lunar and Planetary Exploration