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

• Published in: Nature (London) Vol. 534; no. 7605; pp. 99 - 101
• Main Authors: Konôpková, Zuzana, McWilliams, R. Stewart, Gómez-Pérez, Natalia, Goncharov, Alexander F.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 02-06-2016; Nature Publishing Group
• Subjects: 639/766/119; 704/2151/2809; 704/445/123; Astrogeology; Composition; Core (Geology); Earth; Earth core; Electric properties; Geology; Heat conductivity; Heat transport; Humanities and Social Sciences; Iron; Iron (Metal); Iron alloys; letter; Magnetic fields; Measurement; Mercury; Methods; multidisciplinary; Paleomagnetism; Science; Temperature; Thermal conductivity; Thermal properties; Colombia; United Kingdom; United States; China; Germany
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature18009

The thermal conductivity of solid iron at the pressure and temperature conditions that prevail in the cores of planets is measured directly using a dynamically laser-heated diamond-anvil cell, yielding values that support findings from ancient magnetized rocks that suggest Earth’s magnetic field has persisted since the Earth’s earliest history.
Earth's inner core, ancient or modern
The thermal conductivity of iron and its alloys at high pressure and temperature is a critical factor in the evolution and dynamics of Earth-like planets. Recently, increasing uncertainty in these values has produced dramatically variable predictions for Earth's history that challenge traditional geophysical theories. Two groups reporting in this issue of
Nature
use laser-heated diamond-anvil cells to study the properties of iron at the extreme temperatures and pressures relevant to Earth's core, but using different methodologies, and they arrive at contrasting results. Kenji Ohta and co-authors measured the electrical resistivity of iron at up to 4,500 kelvin and obtained an estimate that is even lower than the low values predicted from recent
ab initio
studies. They conclude that this suggests a high thermal conductivity for Earth's core, which would imply rapid core cooling by conduction and a relatively young inner core. Zuzana Konôpková and co-authors measured heat pulses propagating through solid iron after heating with a laser pulse at pressures and temperatures relevant to the cores of planets ranging in size from Mercury to Earth. Their measurements place the thermal conductivity of Earth's core near the low end of previous estimates, implying that thermal convection in Earth's core could have driven the geodynamo for billions of years, and allowing for an ancient inner core. In a linked News & Views, David Dobson discusses the interpretation of these two tours de force of experimental geophysics.
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
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,
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,
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. Attempts to describe thermal transport in Earth’s core have been problematic, with predictions of high thermal conductivity
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at odds with traditional geophysical models and direct evidence for a primordial magnetic field in the rock record
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. 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
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. 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
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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.