La rotation, la cartographie et la photometrie des satellites de Jupiter

Abstract

The four Galilean satellites of Jupiter have been observed frequently during the past 20 years with the high resolution telescopes of Pic-du-Midi. Reliable features are seen and photographed at their surface. They have been mapped on planispheres. Their identification proves that the rotation period is synchronous with the revolution for each satellite, with an axis not significantly departing from the normal to the orbital plane. The transits of satellites in front of the limb-darkened Jupiter disk provide a technique for accurate contrast and albedo determinations of the features observed. For Callisto, the albedos at 5° phase angle range from 0.19 to 0.09 with a contrast 0.5, indicating a lunar type surface, despite the low density involving a different internal structure. For Europa, the albedos are very high, between 0.73 and 0.52, with low contrasts of 0.3, readily explainable by a snow or frost deposit, the spectroscopic evidence indicating water. For Ganymedes, the very high contrasts reach 0.7, and albedos span from 0.50 to 0.15; patches of snow on Callisto-type terrains are suggested. Io has very high albedos of 0.83 for the equatorial zone to 0.46 for the two darkened poles; the yellow color and absence of H2O bands in the spectrum indicate a special surface composition.