How old is the Earth’s inner core? Lab recreation revises previous findings

AUSTIN, Texas — Estimating the age of Earth’s core has proven difficult for decades. For a long time, scientists believed it was somewhere within the age spectrum of 1.3 billion to 4.5 billion years old. More recently, researchers lowered that figure to a mere 565 million years old. Now, a new study that recreated the conditions of Earth’s core in a laboratory setting is adding its own conclusion to the mix. Scientists now believe the planet’s solid inner core is 1 billion to 1.3 billion years old.

Additionally, this new research helps clarify just how much the planet’s core conducts heat, as well as the subsequent energy sources that fuel Earth’s geodynamo. Geodynamo refers to the process or mechanism that is responsible for maintaining Earth’s magnetic field.

Earth's Inner Core
A computer simulation of the Earth’s magnetic field, which is generated by heat transfer in the Earth’s core. (Image credit: NASA/ Gary A.Glatzmaier)

Our planet’s magnetic field is pretty important. Without it, compasses wouldn’t point north and dangerous cosmic rays would hit Earth’s surface.

“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,” says lead researcher Jung-Fu Lin, a professor at The University of Texas at Austin’s Jackson School of Geosciences, in a release.

What is the Earth’s core?

The Earth’s core as a whole is made up mostly of iron, with the inner core being solid and the outer core much more liquid. The iron is so important because it transfers heat through conduction (thermal conductivity), which plays a role in numerous core functions and characteristics — including when the core was formed in the first place.

Core age/conductivity estimates have varied greatly over the years, but many of these estimates only produce more questions than answers. For instance, one paradox in particular has emerged from younger age estimates: the Earth’s core would have had to have attained unrealistically high temperatures to accommodate the geodynamo billions of years before the inner core formed.

These new findings offer an answer to that riddle by keeping the core’s estimated temperatures within a much more realistic realm. Researchers made this breakthrough by examining iron conductivity while under “corelike conditions.”

Corelike conditions don’t sound particularly fun, with temperatures reaching as hot as the sun’s surface and pressure above one million atmospheres.

The innovative lab model

How in the world were scientists able to artificially recreate this type of environment? By squeezing laser-heated iron samples between two diamond anvils.

“We encountered many problems and failed several times, which made us frustrated, and we almost gave up,” recalls article co-author Youjun Zhang, an associate professor at Sichuan University in China. “With the constructive comments and encouragement by professor Jung-Fu Lin, we finally worked it out after several test runs.”

The new conductivity measurements are 30% to 50% less than earlier young core estimates. Also, this new estimate suggests that the geodynamo has been maintained by two separate energy sources and processes: compositional convection and thermal convection.

All of this helped the research team make a more precise estimation of the inner core’s age.

“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 crystalize,” Lin concludes.

The study is published in Physical Review Letters. 

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