The Plasma Environments of Europa, Ganymede and Callisto

The plasma environments of Jupiter’s icy moons differ greatly from one another due to their differing locations in the Jovian magnetosphere, and due to the fact that one of the three, Ganymede, has an intrinsic magnetic field and its own magnetosphere. Each was visited by the Galileo spacecraft on multiple occasions and with relative spacecraft trajectories that allowed for investigation of different parts of the interaction regions. The environments are of fundamental interest as examples of phenomena associated with the sub-alfvenic flow of plasma past bodies that, while nearly airless, possess atmospheres sufficient to affect the magnetospheric plasmas both in the near vicinity of the moons, and at larger distances. Ionization and charge exchange within the atmospheres and the extended exospheres affect the energy distribution and composition of the magnetospheric ions, and electric current systems divert much of the incident magnetospheric plasma that would otherwise be lost at the surfaces of the moons. In this presentation, evidence of current systems and other features of the interaction regions obtained with the Galileo plasma analyzer (PLS) are discussed. Among other things, these observations are a baseline for validation of models, and they provide context for analyses of magnetic signatures associated with the inductive response of subsurface seawater to the changing external magnetic field. The search for water is a compelling objective for future contemplated missions to the Jupiter system, and it is hoped that the present analysis can contribute to planning and promote greater scientific gain.