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Session 8 - Planets and Comets.
Display session, Monday, June 08
Atlas Ballroom,
NASA's Galileo Orbiter Mission has provided encounters with all four Galilean satellites at distances close enough to measure the quadrupole moments in their gravitational fields. There is a theoretical connection between a satellite's equilibrium figure, as determined by the quadrupole moments, and its interior structure, and the Galileo radio Doppler data show that the satellites are essentially in hydrostatic equilibrium. Therefore, the theory applies. The inner three satellites, Io, Europa, and Ganymede, are strongly differentiated, with three--zone models favored for Io and Ganymede. The zones consist of a metallic core, a rocky envelope, and an outer shell of ice. If Europa is found to have a magnetic field, a metallic core is favored for that satellite as well. Based on Doppler data from a September 1997 close encounter, we conclude that Callisto is most likely a partially differentiated ice--rock mixture, including iron and iron sulfide, with a rock fraction that increases modestly with depth. The data require that the ice and rock in Callisto not be completely separated, nor can Callisto be totally undifferentiated.
