How spin down and radioactive decay drive rocky planet evolution

Abstract

ABSTRACT Most differences in the gross surface morphologies, tectonic styles, overall geologic histories, and atmospheres of the rocky bodies in the solar system can be explained by contributions and dissipation of gravitational and radiogenic energy over geologic time. These two energy sources are large and measurable and can be extrapolated back in time. Accretion was likely cold, and directly converted gravitational potential energy into axial spin, a prominent feature of planets that is otherwise unexplained. Impact heating was mostly limited to planetary surfaces in the final stages of accretion. Frictional dissipation of spin contributed sufficient energy to ignite the primordial Sun and heated Earth and Venus by nearly as much as has the radioactive decay of K, U, and Th over geologic time. Energy inputs have been continuously offset by loss of heat to the surroundings. The magnitudes of most important energy contributions depend on the planet radius R and also on the distance r to the Sun. Quantitative, albeit approximate, relationships show that the net specific energy (kJ/kg) contributed to the rocky bodies over geologic time goes as: Earth ~ Venus >> Mars ~ Mercury ~ Moon >> asteroids. Net energy inputs increased the average internal temperatures of Earth and Venus by ~3000 K but heated asteroids by only a few hundred kelvins.