03 February, 1998. The Galileo spacecraft has made great strides in understanding the large satellites of Jupiter. New violent eruptions are seen coming from Io, which appears from its density and magnetic actions to have an iron core. Europa indeed seems to have a deep water ocean topped by ice. Close-up imaging shows cracks and rafting -- ice movement -- effects that look very much like those seen on Earth. Ganymede has an active, grooved surface perhaps produced by relatively recent water volcanoes and has a magnetic field as well. Io, Europa, and Ganymede all seem to have iron cores.