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SYFY WIRE geology

Earth’s inner core is only a billion years old…way younger than most of us thought

By Elizabeth Rayne
NASA view of Earth from space

A billion years is not the unfathomable stretch of time that we humans—who are really around for the blink of the universe’s proverbial eye—in cosmic terms. It isn’t even much for our planet.

Earth is thought to be about 4.6 billion years old. Until now, its core was often estimated to be closer to that age, but recreating the conditions of the inner core in a lab changed all that. The inner core is made of solid iron encompassed by an outer core of liquid iron. A mind-blowing experiment by an international team of scientists revealed a much younger age for Earth’s inner core than was previously assumed by many (though it’s still old enough for the dinosaurs in Journey to the Center of the Earth to be historically accurate).

The experiment figured out what the age of the inner core must be by measuring its thermal conductivity, or ability to transfer heat through conduction.

“The thermal conductivity of the core determines the cooling rate of the core and the growth rate of the inner core. Therefore, it can tell us the age of the inner core and the energy sources and mechanism to power the geodynamo,” physicist Youjun Zhang, who co-authored a study recently published in Physical Review Letters, told SYFY WIRE.

Collisions and subsequent heat transfer between atoms or molecules that are already close to each other determine the extent to which the core conducts heat, and what powers the geodynamo, or the force that keeps our magnetic field strong. This force is the liquid iron in the outer core. It also is thought to contain lighter elements like carbon, hydrogen, oxygen, silicon, and sulfur. Finding out the power of the magnetic field and how the geodynamo came into being can also tell us about our planet’s habitability, since the magnetic field wards off harmful cosmic rays and prevents Earth from turning into a radiation-blasted desert like Mars.

But wait. The geodynamo paradox, which questioned how viable the thermal convection going on was depending on the level of the core’s thermal conductivity, was in the way. There was no way the geodynamo could have been powered up at an early stage in Earth’s evolution if its thermal conductivity was too high, since a young and hot outer core would have formed later without enough time to cool. Estimated core age would go up depending on how low the thermal conductivity of the geodynamo was found to be. The earlier the geodynamo was powered, the more time it would have to cool, adding years (more like aeons in this case) to the core.

Earth diagram

Zhang and his team needed to see whether they could resolve this if they were going to unearth the age of the inner core, which has greater pressure than a million atmospheres and temperatures that could be hotter than the Sun’s surface. This is when they brought out some hardcore molten metal.

“We determined the electrical and thermal conductivity of squeezing laser-heated iron as the main component of the core at simulated Earth’s core conditions through experiments and theory,” he said. “The thermal conductivity turned out to be 30-50% lower than the previous study found, and supports a geodynamo driven by thermal convection at the early stage and by both thermal and compositional convection at the present day.”

The geodynamo’s thermal conductivity might also have been affected by the presence of the lighter elements that dissolved into the molten iron when it first formed, especially silicon. This could mean that these elements are making it less thermally conductive. Lower conductivity from lighter elements could be confused with cooling time and also explain the reason why so many estimates for the age of Earth’s core were on the higher side, though one recent estimate before Zhang’s experiment was a mere 565 million years, hard to believe with a planet billions of years old. Thermal convection and compositional convection keep the geodynamo active so it can continue to prevent Earth from going Martian.

“There are three main requirements for an active dynamo,” Zhang explained. “These are an electrically conductive fluid outer core, kinetic energy by Earth’s rotation, and an internal energy source to maintain the convection motion of the fluid. The internal energy source includes thermal and compositional energy that drives the convection. Which type of convection is valid depends on the thermal conductivity of the core.”

Earth’s magnetic field is thought to have spiked around a billion years ago, which has been supported by how magnetic materials are arranged in rocks about that age. That spike probably occurred because of the drastic increase in Earth’s magnetism when the inner core formed. The composition of these rocks is more evidence for the core’s updated age.

“The early geodynamo was mainly driven by thermal convection before the inner core formation, while with the inner core growth, it is driven by both thermal and compositional convection,” said Zhang.

For those of us who wish to be immortal, we can only imagine what it would be like for a billion years to feel like almost nothing at all.

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