Radioactive heating refers to the energy dissipated in the interiors of planets, satellites, or asteroids as a consequence of the radioactive decay of radioactive isotopes (see radiochemistry). Radioactive isotopes are characterized by their decay energies and their half-lives. The radioactive isotopes of importance on the timescale of a billion years are the so-called long-lived isotopes uranium 238U, 235U, thorium 232Th, and potassium 40K. Short-lived isotopes may have heated the planets in their early evolutions. The most effective short-lived isotope is 26Al, because of its abundance and decay energy. The concentrations of these elements differ between planets depending on their formation and chemical evolution. Partial melting and differentiation is a mechanism for redistributing radioactive isotopes between crust and mantle reservoirs.