Direct measurement of thermal conductivity in solid iron at planetary core conditions

Abstract

The conduction of heat through minerals and melts at extreme pressures and temperatures is of central importance to the evolution and dynamics of planets. In the cooling Earth's core, the thermal conductivity of iron alloys defines the adiabatic heat flux and therefore the thermal and compositional energy available to support the production of Earth's magnetic field via dynamo action. Attempts to describe thermal transport in Earth's core have been problematic, with predictions of high thermal conductivity at odds with traditional geophysical models and direct evidence for a primordial magnetic field in the rock record. Measurements of core heat transport are needed to resolve this difference. Here we present direct measurements of the thermal conductivity of solid iron at pressure and temperature conditions relevant to the cores of Mercury-sized to Earth-sized planets, using a dynamically laser-heated diamond-anvil cell. Our measurements place the thermal conductivity of Earth's core near the low end of previous estimates, at 18-44 watts per metre per kelvin. The result is in agreement with palaeomagnetic measurements indicating that Earth's geodynamo has persisted since the beginning of Earth's history, and allows for a solid inner core as old as the dynamo.

Earth's core problem

“…If, as these controversial studies suggest, the core is losing heat at such a high rate, it means that the magnetic field must work in previously unimagined ways 3 , and that the solid inner core must be less than a billion years old 4 -a mere babe in planetary terms. In this issue, Ohta et al 5 (page 95) and Konôpková et al 6 (page 99) report studies that experimentally tested the simulations' results using complementary, but distinct, approaches and come to different conclusions.…”

The cross-over to magnetostrophic convection in planetary dynamo systems

“…It is presently unclear as to which buoyancy source, thermal or compositional, is the dominant convective driver [10,11]. This depends on the thermal conductivity of the core fluid [12,13], which determines how efficiently heat is conducted across the core and how much of this heat is available over to drive thermal convection in the fluid [14].…”

Deep magnetic field stretching in numerical dynamos

“…Some seismic studies (e.g., Helffrich and Kaneshima 2010) and mineral physics models (e.g., de Koker et al 2012;Pozzo et al 2012) suggest that the top of the core is indeed stably stratified (Gubbins and Davies 2013). In contrast, other studies claimed that the thermal conductivity of the core is as low as previously estimated (Konôpková et al 2016;Ohta et al 2016) and thus that the whole of the outer core convects. Even if the thermal conductivity is high, exsolution of mantle material (Badro et al 2016;O'Rourke and Stevenson 2016) may destabilize the top of the core.…”