Primordial heat is the internal heat energy accumulated by dissipation in a planet during its first few million years of evolution. It is the heat left over from when the planet formed and accreted, which has not yet been lost. The process of planet formation generates a lot of heat because each time a rock hit the growing planet, it transformed its kinetic energy into thermal energy. The deeper you go into the Earth, the hotter it gets because a lot of primordial heat remains.

There are three main sources of heat in the deep Earth: heat from when the planet formed and accreted, frictional heating caused by denser core material sinking to the center of the planet, and heat from the decay of radioactive elements. A new study reveals that only about half of our planets internal heat stems from natural radioactivity. The rest is primordial heat left over from the planets formation.

Primordial heat is almost non-renewable and comes from gravitational potential energy. It is hard to calculate the loss of primordial heat for a given age and mass of a planet, but it is an important factor in understanding the planets evolution and current state.