This Insulates Our Earth From the Deep Cold of Space

Measurements of the electrical resistance and thermal conductivity of iron at extreme pressures and temperatures cast fresh lite on controversial numerical simulations of the properties of Earth’s outer core.
See Letters

p.95 & 99

Earth’due south core acts like a storage heater, with heat released during crystallization of the inner core that buffers the deadening cooling of the planet as information technology radiates its heat to infinite. The most obvious expression of this heat transfer is World’south magnetic field, which is generated by convection in the liquid outer core. But the magnitude of the transfer is controlled past thermal conduction across the boundary between the core and mantle.

In 2012, kickoff-principles numerical simulations^ane,2
indicated that the thermal conductivity of liquid iron in the outer core is then loftier that this region might human action as a pump that pushes heat towards the core–mantle boundary faster than convection tin can. If, as these controversial studies suggest, the core is losing oestrus at such a high rate, it ways that the magnetic field must work in previously unimagined means^three, and that the solid inner cadre must exist less than a billion years old⁴
— a mere infant 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, only distinct, approaches and come to different conclusions.

Both groups use laser-heated diamond-anvil cells to generate the extreme temperatures and pressures of the core–curtain boundary, but that is where the similarity ends. Ohta
et al. measured the electrical resistance of iron wires, which is closely related to the wires’ thermal conductivity (Fig. 1a). To convert the resistivity measurements to a measure of the thermal conductivity of liquid iron in the outer core, the authors fitted their data to a model of resistivity that assumes that resistance approaches a limit at loftier temperature (a phenomenon called resistivity saturation). This then allowed them to employ the Wiedemann–Franz human relationship betwixt resistance and thermal conduction in metals to calculate the thermal conductivity. Both of these procedures have practiced theoretical bases and are well established for low-pressure observations. The observed high electrical conductivities resulted in a predicted outer-cadre thermal conductivity of around 90 watts per metre per kelvin, which is in reasonable understanding with the 2012 simulations^ane,2.

In diamond anvil cells, the pressure generated betwixt the tips of diamonds tin exceed millions of atmospheres. Lasers can be fired through the diamonds to direct estrus a sample of a material to four,000 kelvin or more.
a, Ohta
et al.⁵
connected electrodes to a sample of solid iron and measured its electrical resistance (which is inversely proportional to thermal conductivity in metals) at high temperatures and pressures.
b, In separate experiments, Konôpková
et al.⁶
pulsed the laser, and measured the fourth dimension taken for oestrus pulses to diffuse through a solid iron sample on the basis of changes in the brightness and wavelength of the light emitted from the sample. This allowed them to mensurate the thermal rate of improvidence, which is closely related to thermal conductivity.

Full size image

By contrast, Konôpková
et al. directly measured thermal conduction past watching a oestrus pulse propagate through a solid iron sample subsequently heating with a nanosecond laser pulse (Fig. 1b). The fourth dimension taken for the pulse to laissez passer from the heated side of the sample to the other side, and the aamplitude divergence of the pulse between the two sides, are functions of the thermal electrical conductivity of the sample, besides every bit of the surrounding solid medium that transmits pressure level from the diamonds to the sample and thermally insulates the sample from the diamonds. After some careful mathematical modelling of the temperature field in the diamond cell, the authors extracted the thermal electrical conductivity of atomic number 26 from time-resolved changes in the brightness and wavelength of the glow from the white-hot sample. They obtained a thermal conductivity of most xxx W 1000^−one
K⁻¹, like to early on predictions of outer-cadre conductivity^vii.

But this leaves u.s. with a conundrum: how to reconcile the high thermal conductivity reported by Ohta and colleagues on the ground of resistance measurements with the low thermal electrical conductivity measured by Konôpková and co-workers. Peradventure there were unknown complications with the experiments? For case, the extremely short light amplification by stimulated emission of radiation pulses used by Konôpková
et al. might have acquired the sample to partially melt for a brusk period, which could have gone unnoticed during the experiment. If so, then the melting phase transition would have acted as a thermal buffer (much as the crystallization of the inner core buffers Earth’s temperature) and caused an apparent decrease in thermal conductivity. This might explain why the measured thermal conductivities decrease so strongly with temperature, particularly at temperatures approaching the melting temperature.

Or mayhap Ohta
et al. underestimated the oestrus loss through the electrodes in their experiments, which would mean that the average sample temperature was less than the measured value. This could have made it wait every bit though resistivity was saturating, even if it wasn’t. Alternatively, the proportionality constant betwixt electrical resistance and thermal conduction (the Lorenz number) might get strongly temperature dependent at the extreme pressures and temperatures of the experiment — this would betoken to previously unobserved fundamental physics.

Despite the discrepancy, these two studies are experimental feats, measuring complex physical backdrop of samples smaller than a pinhead at pressures greater than 1 one thousand thousand atmospheres, and at temperatures above 4,000 Grand. The fact that the results agree within a factor of iii is a remarkable success, but the devil is in the detail. The discrepancy makes a big difference to estimates of when the inner core formed, and hence when Earth generated a stable magnetic field — the inner core could be every bit lilliputian as 700 meg years old, near the aforementioned age as circuitous life; or as much as 3 billion years onetime, about three-quarters of Earth’s age. More experimental and theoretical piece of work is needed to resolve the discrepancy and hence to constrain the age of the inner core and the workings of Earth’south magnetic field.

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