Planetary Geoscience

We explain how heat is produced by radioactive decay, segregation and exothermic crystallization of metallic cores, impacts, and tidal forces. Planetesimals in the early Solar System were most affected by the decay of short-lived radionuclides. Larger, rocky planets were heated primarily by large impacts and core segregation. Because rocks are poor conductors, heat retention in rocky bodies is a function of planet size. Large-scale melting to produce magma oceans was likely a common process facilitating differentiation to form cores, mantles, and crusts. Metallic liquids are probably necessary for core segregation. Primary crusts, formed during planetary differentiation, are rarely preserved. Mantles are residues from the extraction of silicate crustal melts and core materials. Differentiation of the giant planets was driven by density variations in high-pressure forms of gases, ices, and rock more than by heating and melting. The importance of the various planetary heat sources changes over time; in modern planets the effective heat sources are decay of long-lived radioisotopes and, for the Earth, exothermal crystallization of the liquid outer core.