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Experiments relating to Earth's inner core raise questions about its age

2 June 2016
Measurements of iron's thermal conductivity hint that the planet's solid center is relatively young—and not responsible for Earth's dynamo long ago.

Earth's magnetic field (illustrated below) is sustained by liquid iron that is continuously churning in the planet's outer core. Iron that crystallizes onto the solid inner core releases latent heat, which powers convection that drives Earth's dynamo (see the article by Daniel Lathrop and Cary Forest, Physics Today, July 2011, page 40). But lately scientists have questioned whether that set of mechanisms has always provided the energy for Earth's magnetic mojo. Simulations from a 2012 study suggested that iron has surprisingly high thermal conductivity under the extreme conditions that prevail in the core. The finding hinted that the core may export heat to the rocky mantle much faster than previously thought. If the core truly does lose heat so quickly, then it would have taken less than a billion years for the inner core to reach its current size. Some other process, then, would have had to power the dynamo for a significant interval of Earth's 4.5-billion-year history.

Experiments relating to Earth's inner core raise questions about its age - Illustration credit: DESY

Now two research teams have heated diamond-anvil cells with lasers to determine iron's thermal conductivity at core-like temperatures and pressures. Kenji Ohta and his colleagues crushed iron wires and determined their electrical resistance, which is inversely proportional to thermal conductivity. The team estimated a conductivity of 90 W/(m⋅K), a measurement that is roughly in line with the simulation predictions and sets an upper age limit for the inner core of about 700 million years. Zuzana Konôpková and her colleagues measured the propagation of laser-delivered heat through an iron sample. Her collaboration obtained a value of about 30 W/(m⋅K), which supports the more traditional view of a gradually cooling core with an early-forming solid center. David Dobson, who was not affiliated with either study, notes that the Konôpková result is more dependent on modeling than Ohta's, and any unnoticed melting of the iron could have skewed the measurement.

Follow-up experiments, perhaps ones that capture electrical and heat-propagation measurements simultaneously on a single sample, could resolve the discrepancy between the two teams' results. Even in the absence of a solid core, theorists can devise exotic mechanisms, such as the wobble of Earth's axis, to explain how the planet could have maintained a magnetic field a few billion years ago. (K. Ohta et al., Nature 534, 95, 2016; Z. Konôpková et al., Nature 534, 99, 2016. Illustration credit: DESY.)

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