Thermal and electrical conductivity of iron at Earth’s core conditions

A research collaboration including Dario Alfè and Dr. Monica Pozzo of the LCN and Thomas Young Centre at UCL have successfully calculated the thermal and electrical conductivity of the Earth's core in a paper published in Nature.

Their results, 2-3 times higher than current best estimates will have a major impact on future studies of mantle evolution.

Our planet acts as a giant heat engine, and this gives rise to plate tectonics, volcanoes and the formation of mountains. Another key feature, the geomagnetic field, is generated by a dynamo in the liquid iron core. The power supplied to the geodynamo is measured by the heat flux across the core mantle boundary (or CMB), and this factor places constraints on Earth's evolution.

Calculating the heat flux across the CMB requires an estimation of the thermal and electrical conductivity of the iron mixtures within the core, something that has traditionally proven unreliable due to the difficulties in experimentation and theory.

The team tackled this problem by using density functional theory to compute these properties from first principles. Their figures, obtained at the U.K. national facility HECToR, are the first direct calculations which do not rely on estimates based on extrapolations. Producing a series of models using existing data on the composition of the core iron mixtures they came to a new estimate of 15-16 TW for the adiabatic heat flux at the core mantle boundary.

This result, significantly higher than has previously been predicted based on mantle convection suggests that the top of the core must be thermally stratified, with any convection in the upper core driven by chemical convection or lateral variations in CMB heat flow. This will have implications for any further research, with power for the geodynamo greatly restricted future models must incorporate a high core mantle boundary heat flux, and explain recent formation of the inner core.

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Figure:  Electrical and thermal conductivity of iron at Earth’s outer core conditions.

ac, Electrical conductivity, σ (a), and electronic component of thermal conductivity, k (b), of pure iron corresponding to the three outer-core adiabatic profiles (adiabats) displayed in c. Black lines, adiabat corresponding to the melting temperature of pure iron at ICB pressure; red lines, that of the mixture containing 10% Si and 8% O; and blue lines, that of the mixture with 8% Si and 13% O. Lines are quadratic fits to the first principles raw data (symbols). Error bars (2 s.d.) are estimated from the scattering of the data obtained from 40 statistical independent configurations. Results are obtained with cells including 157 atoms and the single k-point (1/4,1/4,1/4), which are sufficient to obtain convergence within less than 1%.

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