In a process called tidal heating, gravitational push and pull from Jupiter’s Galilean moons — Europa, Ganymede, Io and Callisto — and the gas giant itself stretch and squish the moons enough to warm them. As a result, some of the icy moons contain interiors warm enough to host oceans of liquid water, and in the case of the rocky moon Io, tidal heating melts rock into magma. Planetary researchers previously believed that Jupiter was...
