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Primordial heat is the internal heat energy accumulated by dissipation in a planet during its first few million years of evolution. The main contributions to the primordial heat are accretional energy – the energy deposited by infalling planetesimals – and differentiation energy. The latter is mainly released by core formation and is basically potential energy that is dissipated upon formation of a gravitationally stable layering of the planet. In addition to the primordial heat, the planet’s internal heat source is mainly provided by the radioactive decay of long-lived unstable isotopes such as 238U, 235U, 232Th, and 40K and by latent heat.