Energetics of the Core

Abstract

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 and secular cooling playing a subsidiary role. Prior to the onset of inner-core solidification, the dynamo was driven purely by secular cooling and required a cooling rate roughly 3 times the present-day rate to maintain the same dissipation. 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 melting behavior of the core material, the thermal conductivity of the core, and the amount of Ohmic dissipation generated by the dynamo. The present-day CMB heat flow is estimated at 6–14 × 1012 W (6–14 TW), sufficient to sustain a dynamo dissipating a few terawatts of heat. Based on four independent studies, this CMB heat flow implies an inner-core age range of 0.37–1.90 Gy, significantly less than both the age of the Earth and measurements of Re–Os isotopes, which may imply an inner-core age of ∼3.5 Gy. Adding potassium to the core results in lower initial core temperatures, and also allows inner-core ages compatible with the geochemical estimates, but only if the CMB heat flow has remained low (e.g., <9 TW for 300 ppm potassium) for the whole of Earth history. Whether such a relatively constant heat flow is a likely outcome of the mantle’s thermal evolution remains to be seen.