concept

Tidal heating

Heat generated inside a body when the gravity of the object it orbits repeatedly kneads it. A moon on a slightly off-circular orbit is squeezed harder when it swings close and less when it swings far; that rhythmic flexing warms its interior by friction. It is why Jupiter's moon Io erupts so violently and Europa keeps an ocean under its ice - and, in canon, what drives Pandora's geological heat.