How do moons help reveal whether a planet has oceans under its surface?

Large moons around planets and dwarf planets can raise tides inside a body, repeatedly squeezing and stretching it. That motion creates heat through friction, a process called tidal heating. In the Jupiter system, Europa and Ganymede are important examples, and in the Saturn system, Enceladus is famous for its active icy geysers. Scientists infer subsurface oceans from gravity data, magnetic fields, surface cracks, and plume activity.