The Iron Invariance: Implications for Thermal Convection in Earth's Core

Convection of the liquid iron (Fe) outer core and electrical properties of Fe are responsible for the geodynamo that generates the geomagnetic field. Recent results showed the thermal conductivity of the core and related conductive heat flux may be much larger than previously accepted, suggesting that thermal convection would not be an energy source to power the geodynamo. Here we report experimental measurements of the electrical resistivity of solid and liquid Fe which show invariant values along the melting boundary at pressures up to 24 GPa. The observed resistivity invariance was extrapolated to Earth's predominantly Fe solid inner core and liquid outer core conditions and, using the Wiedemann-Franz law, the thermal conductivity was calculated. We calculate a conductive core heat flow of 8-9 TW at the core-mantle boundary. These results provide strong support for thermal convection as a geodynamo energy source. Plain Language Summary Earth's magnetic field is produced by a dynamo in the core that requires motion of the fluid Fe alloy. Both thermal convection, arising from the transport of heat in excess of conducted heat, and compositional convection, arising from light element exsolution at the freezing inner core boundary, are suggested as energy sources. The contribution of thermal convection (possibly ranging from nothing to significant) depends on thermal conductivity of the outer core. Our experimental measurements of electrical resistivity of solid and liquid Fe at high pressures show that resistivity is constant along the pressure-dependent melting boundary of Fe. Using our derived thermal conductivity value at the inner core (freezing) boundary, we calculate the heat conducted in the liquid outer core and find that thermal convection is needed to carry additional heat through the outer core to match the heat extracted through the core-mantle boundary.