Abstract
Electrical and thermal transport properties of liquid Fe under high pressure have important implications for the dynamics and thermal evolution of planetary cores and the geodynamo. However, electrical resistivity (ρ) and thermal conductivity (k) of liquid Fe at high pressure still remain contentious properties. To date, only two experimental investigations of ρ of liquid Fe in the pressure region below 7 GPa are reported in literature. Here we report the results of measurements of ρ for solid and liquid Fe (inversely proportional to k through the Wiedemann-Franz law) at pressures from 3 to 12 GPa, using a large multi-anvil press. We show that ρ of liquid Fe decreases as a function of pressure up to the δ-γ-liquid triple point at ~5.2 GPa, and subsequently remains invariant from 6 to 12 GPa, which is consistent with an earlier study on liquid Ni. Our results demonstrate an important effect of solid phase on the structure and properties of liquid Fe. Our values of ρ for solid and liquid Fe are used to calculate k in Mercury’s solid inner core and along the adiabat in the liquid outer cores of Moon, Ganymede, Mercury and Mars. Our robust values of thermal conductivity place the focus on uncertainties in thermal expansion as the cause of variation in values of core conducted heat. Except for Mercury, our adiabatic heat flux values in these terrestrial cores validate the use of similar values used in several previous studies. Our high values of core adiabatic heat flux in Mercury would provide a stabilizing effect on, and lead to an increase in thickness of, the thermally stratified layer at the top of the core.
Highlights
Electrical and thermal transport properties of liquid Fe under high pressure have important implications for the dynamics and thermal evolution of planetary cores and the geodynamo
We have not observed the effects of the ε-phase on ρ at 12 GPa, nor did we see any deviation from the standard α-phase scattering before the Curie T (Tc), suggesting that the sample did not enter the ε-phase
The electrical resistivity decreases almost linearly above Tc as a function of P in the γ-phase, primarily because the long-range order of spin magnetic moments is lost by the effects of temperature, and the electron scattering by magnons becomes reduced compared to temperature-induced phonon scattering, which itself is suppressed by the effects of pressure
Summary
Electrical and thermal transport properties of liquid Fe under high pressure have important implications for the dynamics and thermal evolution of planetary cores and the geodynamo. New diamond anvil cell (DAC) experimental data[6,13,14,15] place constraints on the k of lower mantle minerals, which subsequently limits the magnitude of heat flux through the CMB These experimental results are complemented by numerical studies[16,17] of Fe at core conditions which demonstrate the important role of electron-electron scattering and spin disorder. The absence of a consensus on k and uncertainty in ohmic losses in the core necessitate a consideration of alternative sources of energy required to generate the Earth’s magnetic field throughout history[18] It was recently postulated on the basis of theoretical reasoning that ρ may be invariant for pure simple liquid metals, but not for liquid transition metals, along their respective melting curves[1]. This study is motivated by the possibility that liquid Fe may exhibit the same melting boundary behavior of ρ as Ni and Co, and enable better insight into the thermal state and dynamics of experimentally reachable conditions of cores, at the boundary between the solid inner and liquid outer cores, of the small terrestrial bodies Mercury, Mars, Moon and Ganymede
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