Both the Voyager and Galileo space missions have told us a great deal about Io, Europa, Ganymede and Callisto, the moons of Jupiter. There has been previous evidence for water on these moons. But the best yet, indicating the existence of subsurface oceans on Europa and Callisto, comes from the Galileo magnetometer experiment. The interiors of these two bodies are electrically conducting, and the most obvious explanation is that water with electrolytic conductivity is present.
