Jupiter's four major moons, discovered by Galileo 401 years ago now, are fascinating studies in contrast. Heat from tidal forces have shaped the inner two moons, giving Io active volcanoes and probably providing Europa with ice-covered oceans. The furthest of the four, Callisto, appears to have frozen as it formed. The remaining moon, Ganymede, was a bit of a mystery. Like Callisto, it appears to be a mixture of rock and water, with a big difference: on Ganymede, the water appears to have melted, allowing the rock to sink to the core, and then froze up again. Two scientists from the Southwest Research Institute now think they know why: Ganymede was hit so hard by asteroids and comets during the solar system's early history that it underwent runaway melting.