How do scientists know Europa may have a global ocean under its ice?

Europa, one of Jupiter’s largest moons, has a bright icy surface crisscrossed by fractures and streaks. Measurements from the Galileo spacecraft in the 1990s suggested that a conductive layer lies beneath the ice, most likely a salty liquid ocean. Jupiter’s magnetic field also appears to induce a response consistent with a global ocean of electrically conducting water. The moon’s smooth, broken surface is hard to explain without a subsurface layer that can move and deform. Europa is now one of the most promising places in the Solar System to search for habitability outside Earth.