Thermal Evolution of the Core

Geoffrey F. Davies 1

1 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia

The thermal budget of the core is a key quantity relating the generation of the magnetic field, the growth of the solid inner core and the generation of plumes in the mantle. Recently modellers of the core dynamo responsible for generating the magnetic field have appealed to the possibility of radioactivity in the core to accommodate all of these phenomena, but the results of this study indicate this may not be necessary.

The challenge is to reconcile the estimated rate of heat loss from the core, the energy required to maintain the magnetic field by dynamo action in the core, evidence for the magnetic field having existed through much of Earth history and the present relatively small size of the inner core, formed by solidifcation of the core as it cools. Previous attempts to avoid core radioactivity implied implausibly high core temperatures in the past.

The present work has used a more appropriate parameterisation of the removal of core heat by plumes in the mantle, a lower estimate of core conductivity and a more thorough exploration of how the mantle modulates core heat loss, which reveals another family of solutions in which early core temperatures are not excessively high. The result is that the core dynamo could be maintained without radioactivity so long as no more than about 0.5 Terawatts of heat flow is required to maintain the dynamo.

There are considerable uncertainties in some of the relevant parameters, and the study has identified the most important as being the present rate of heat loss from the core, the conductivity of the core, and the energy or entropy flow required to maintain the dynamo.