INDUCED MAGNETIC-FIELDS AS EVIDENCE FOR SUBSURFACE OCEANS IN EUROPA AND CALLISTO

The Galileo spacecraft has been orbiting Jupiter since 7 December 1995 , and encounters one of the four galilean satellites-Io, Europa, Ganym ede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer(1,2) have indicated that neither Europe nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede(3) an d possibly Io(4). Here we report perturbations of the external magneti c fields (associated with Jupiter's inner magnetosphere) in the vicini ty of both Europe and Callisto. We interpret these perturbations as ar ising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induc tion requires eddy currents to now within the moons, and our calculati ons show that the most probable explanation is that there are layers o f significant electrical conductivity just beneath the surfaces of bot h moons. We argue that these conducting layers may best be explained b y the presence of salty liquid-water oceans, for which there is alread y indirect geological evidence(5,6) in the case of Europa.