The inner core of our planet is between 1 and 1.3 billion years old, according to new research led by the University of Texas at Austin and Carnegie Institution for Science.
Earth’s internal structure: dense solid metallic core, viscous metallic outer core, mantle and silicate based crust. Image credit: NASA.
“People are really curious and excited about knowing about the origin of the geodynamo, the strength of the magnetic field, because they all contribute to a planet’s habitability,” said Professor Jung-Fu Lin, a researcher in the Jackson School of Geosciences at the University of Texas at Austin.
The Earth’s core is made mostly of iron, with the inner core being solid and the outer core being liquid.
The effectiveness of the iron in transferring heat through conduction — known as thermal conductivity — is key to determining a number of other attributes about the core, including when the inner core formed.
Over the years, estimates for core age and conductivity have gone from very old and relatively low, to very young and relatively high.
But these younger estimates have also created a paradox, where the core would have had to reach unrealistically high temperatures to maintain the geodynamo for billions of years before the formation of the inner core.
The new research solved that paradox by finding a solution that keeps the temperature of the core within realistic parameters.
Finding the solution depended on directly measuring the conductivity of iron under corelike conditions — where pressure is greater than 1 million atmospheres and temperatures can rival those found on the surface of the Sun.
Professor Lin and colleagues achieved these conditions by squeezing laser-heated samples of iron between two diamond anvils.
“We encountered many problems and failed several times, which made us frustrated, and we almost gave up,” said first author Dr. Youjun Zhang, a researcher in the Institute of Atomic and Molecular Physics at Sichuan University and the Shanghai’s Center for High Pressure Science and Technology Advanced Research.
The newly measured conductivity is 30% to 50% less than the conductivity of the young core estimate, and it suggests that the geodynamo was maintained by two different energy sources and mechanisms: thermal convection and compositional convection.
At first the geodynamo was maintained by thermal convection alone. Now, each mechanism plays about an equally important role.
“With this improved information on conductivity and heat transfer over time, we could make a more precise estimate of the age of the inner core,” Professor Lin said.
“Once you actually know how much of that heat flux from the outer core to the lower mantle, you can actually think about when did the Earth cool sufficiently to the point that the inner core starts to crystallize.”
The findings are published the journal Physical Review Letters.
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Youjun Zhang et al. 2020. Reconciliation of Experiments and Theory on Transport Properties of Iron and the Geodynamo. Phys. Rev. Lett 125 (7): 078501; doi: 10.1103/PhysRevLett.125.078501
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