JOVIAN SYSTEM DATA ANALYSIS PROGRAM PROPOSAL SUMMARY

The Galilean Moons of Jupiter: Inferring Internal Properties of New Worlds from Their Interactions with the Plasma and Magnetic Field Environment In size, structure, and thermal properties, the Galilean moons of Jupiter fall into the class of the terrestrial planets. Theories of the origin and evolution of the terrestrial planets, appropriately modified, should describe the Jovian satellites. However, currently accepted theories predicted neither Ganymede's internal magnetic field nor Callisto's induced field. These failures require us to reconsider the underlying theories. Improved theories will require the most complete possible knowledge of internal properties. For Europa and Callisto, better estimates of the induced fields, upper limits to the permanent dipole moments and identification of crustal anomalies would be of great value. For Ganymede, estimates of the quadrupole moments would provide critical constraints to theory. We propose to determine these quantities in two steps. Initially, we would characterize the "cometary" environment of the moons. J. A. Linker's suite of magnetohydrodynamic simulations (differing in assumptions regarding the neutral clouds and the internal conductivity) will be used to interpret the magnetic field and energetic particle fluxes. By tracing energetic particle orbits through the realistic magnetic field of the computer simulation and then comparing with fluxes measured by Galileo's energetic particle detector (EPD available from the PDS data archives [Williams et al., 1992]), one can test the consistency of the simulation. Once we have characterized the perturbations arising from plasma currents, at least to a first approximation, we would iterate to identify the modifications produced by internal fields, giving first estimates of higher order multipoles and of possible crustal anomalies for the three outer satellites. With data from only one pass by Io, it will not be feasible to determine its internal field beyond lowest order, but it may be possible to establish more securely whether or not a dipole moment is present.