Planetary bodies can be thought of as combinations of heat reservoirs and heat engines. The heat reservoirs store internal energy, E, and the heat engines convert some of this thermal energy into various types of mechanical, electrical and chemical energies. This simple physical picture is true of all active planetary bodies, regardless of their composition (rocks, gases or ices) or size. The details, however, vary widely throughout the Solar System. In this chapter we discuss the storage of thermal energy in planetary bodies.

We begin by distinguishing internal from external heat reservoirs, and we define the latter as those that derive their energy from solar electromagnetic radiation. External heat reservoirs occur in surface and near-surface environments. Examples include the Earth's oceans and atmosphere. Internal heat reservoirs store energy at various depths, from near-surface environments to the planet's core. They are fed by dissipation of various types of non-thermal energy but there is one unifying characteristic, which is that dissipation takes place deep enough that the rate of heating exceeds the rate of heat transfer to the planet's surface (Chapter 3). The relative magnitudes of the energy fluxes from external and internal reservoirs at a planet's surface vary widely among the bodies of the Solar System. In solid planetary bodies (rocky and icy) surface energy flux is typically dominated by solar radiation, despite the fact that internal energy reservoirs in some of them are large enough to make noticeable, perhaps dominant, contributions to the planet's surface features.