Tidal heating is the warming of a moon or planet by the flexing of its interior as gravity stretches and squeezes it along an eccentric orbit. The friction converts orbital energy into heat. It makes Jupiter's moon Io the most volcanically active body in the solar system, and keeps subsurface oceans liquid beneath the icy crusts of moons like Europa, Enceladus, and, surprisingly, Mimas, far from the Sun's warmth.