Gravitational Flexing
As a moon orbits its planet on an eccentric path, the gravitational pull varies — stronger at closest approach (periapsis), weaker at greatest distance (apoapsis). The moon's solid body deforms in response, with tidal bulges growing and shrinking each orbit. This cyclic flexing dissipates enormous energy as internal friction, heating the moon's interior and driving geological activity that would otherwise be impossible for such small worlds.
The Io Paradigm
Jupiter's innermost Galilean moon Io is the most volcanically active body in the Solar System. Peale, Cassen, and Reynolds predicted Io's volcanism in 1979, just days before Voyager 1 confirmed it — one of planetary science's greatest theoretical triumphs. Io's eccentricity of 0.041, maintained by orbital resonance with Europa and Ganymede, drives approximately 93 terawatts of tidal heating through continuous gravitational flexing.
The Tidal Heating Equation
Tidal dissipation power scales as P proportional to r⁵ e² / (a⁶ μ), where r is moon radius, e is eccentricity, a is orbital distance, and μ is interior rigidity. The devastating sixth-power dependence on distance means inner moons can be volcanically extreme while outer moons are geologically dead. The rigidity parameter captures how easily the interior deforms — partially molten interiors dissipate far more energy than cold, rigid ones.
Ocean Worlds and Habitability
Tidal heating does not always produce volcanism. At lower intensities, it can maintain subsurface liquid water oceans beneath icy shells — as on Europa, Enceladus, and possibly Titan. These ocean worlds are prime targets in the search for extraterrestrial life, since liquid water, energy, and chemical nutrients are the three ingredients for biology. Understanding tidal heating is thus central to astrobiology.