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.