Energetics of the Core

Synopsis This chapter reviews theoretical approaches to calculating the energy and entropy budgets of the Earth’s core. The energy budget allows the growth of the inner core with time to be calculated; the entropy budget determines whether or not a geodynamo can function. The present-day geodynamo is driven primarily by compositional convection, with latent heat, secular cooling and (possibly) exsolution playing a subsidiary role. Prior to the onset of inner core solidification, the dynamo was likely driven purely by secular cooling and required a cooling rate roughly 3 times the present-day value to maintain the same rate of entropy production. The largest uncertainties in the present-day energy and entropy budgets arise from uncertainties in: the core-mantle boundary (CMB) heat flow; the compositional density contrast between the outer and inner core; the thermal conductivity of the core material; and the amount of Ohmic dissipation generated by the dynamo. The present-day CMB heat flow is estimated at 12± 5 TW; heat flows less than 5 TW are insufficient to drive the current dynamo. Prior to formation of the inner core, sustaining a dynamo required a CMB heat flow of 15 TW or more, implying rapid core cooling and a molten lower mantle. Such high heat flows appear to be compatible with models of mantle convection. These high heat flows also imply an inner core age of less than 0.7 Gyr. Adding potassium to the core results in lower initial core temperatures, but does not significantly alter the inner core age. Thermallyor compositionally-stratified regions may have existed (or still exist) within the core, but there is currently no consensus on their nature or effects on the dynamo.