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Tidal heating dissipates orbital energy within moon interiors through friction, generating internal heat that sustains volcanism and melts subsurface oceans. Heating intensity depends on orbital eccentricity, satellite mass, and orbital period; moons like Io, Europa, and Enceladus demonstrate extreme tidal activity.
From your study of tides and orbital mechanics, you know that gravitational interactions between two bodies raise tidal bulges — elongations of the body toward and away from the source of the gravitational gradient. For a moon in a perfectly circular orbit, these bulges would remain fixed in orientation relative to the planet, and no energy would be dissipated. But real moons have eccentric orbits, meaning the planet-moon distance varies throughout each orbit. As the moon moves closer to and farther from the planet, the tidal bulge continuously grows, shrinks, and shifts position. The interior of the moon must physically deform to follow this changing tidal force, and that repeated flexing — like bending a paperclip back and forth — converts orbital energy into frictional heat within the moon's interior.
The amount of heat generated depends on several factors you can reason through from your prerequisites. From Kepler's laws, you know that a moon on an eccentric orbit moves faster at periapse (closest approach) and slower at apoapse. The tidal force varies as the inverse cube of distance, so even modest eccentricity produces large swings in tidal stress over each orbit. The tidal heating rate scales as the square of eccentricity, the fifth power of the moon's radius (bigger moons deform more), and inversely with the orbital period and the sixth power of the semi-major axis. This steep distance dependence explains why inner moons are heated far more than outer ones. The moon's interior rheology — how "lossy" its material is when flexed — also matters enormously; a partially molten interior dissipates far more energy than a rigid one.
The most dramatic example is Io, Jupiter's innermost large moon. Io's orbit is kept eccentric by a gravitational resonance with Europa and Ganymede (the Laplace resonance), which prevents Io's orbit from circularizing despite enormous tidal dissipation. The result is staggering: Io generates roughly 100 trillion watts of internal heat, making it the most volcanically active body in the solar system, with hundreds of active volcanoes resurfacing it continuously. Without the resonance maintaining eccentricity, tidal friction would have long since circularized Io's orbit and shut off the heating — the resonance acts as an orbital engine that perpetually pumps energy into Io's interior.
Europa and Enceladus demonstrate a subtler but perhaps more consequential effect. Both moons experience enough tidal heating to maintain liquid water oceans beneath their icy shells — Europa's ocean may contain twice the water of all Earth's oceans combined. The heat is not enough to produce Io-like volcanism, but it is sufficient to prevent the ocean from freezing solid, creating environments where liquid water, chemical energy, and mineral nutrients coexist. This makes tidally heated moons the leading candidates for extraterrestrial life in our solar system, extending the concept of the habitable zone far beyond the traditional distance from the Sun where surface liquid water can exist. Tidal heating shows that a moon need not be close to a star to be geologically and potentially biologically active — it only needs the right orbital architecture.