... Jupiter's icy moon Europa has a metallic core and layered internal structure similar to the Earth's , while the heavily cratered moon Callisto is a mixture of metallic rock and ice with no identifiable central core, according to new results from NASA's Galileo mission . In addition, recent plasma wave observations from Galileo show no evidence of a magnetic field or magnetosphere around Callisto, but do hint at the prospect of a tenuous atmosphere. ... Jupiter Galileo . ...