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Is the Earth's core as hot as the sun?

Scientists have taken new measurements that suggest that the core of the Earth burns at 6000 degrees Celsius — as hot as the surface of the sun.


(Jordens inre image by Mats Halldin, CC BY-SA 3.0)

Scientists have taken new measurements that suggest that the core of the Earth burns at 6000 degrees Celsius — as hot as the surface of the sun.

Previous measurements had put the solid crystalline core at around 5000 degrees Celsius, but scientists at French government-funded research agency CEA believe the temperature could be as high as 6000 degrees Celsius, giving a melting point of 6230 ± 500 degrees Celsius.

For context, the surface of the sun is estimated to be around 5800 degrees Celsius (never fear; the sun's core is around 15 million degrees Celsius).

There are two parts to the Earth's core: the molten outer core, comprising an iron-nickel alloy, and the solid inner core made of the same materials. Although the core is extremely hot, it remains solid due to the slow cooling of the outer core and compression, and it is believed that, inside its molten home, the inner core spins faster than the rotation of the Earth by a fraction of a degree per year. The movement created by thermal activity inside the Earth is what creates the planet's magnetosphere — but the temperature difference between the core and the mantle at the previous measurements didn't seem high enough (1500 degrees Celsius) to cause the required activity.

(As an aside, there's also zero gravity at the Earth's core. I learned this from a no-prize-winning letter at the back of an Amazing Spider-Man comic when I was a kid, disputing the density characters gained when visiting the centre of the Earth. This is indisputable proof that comics are good for you.)

The structure of the Earth. (Earth poster image by Kelvinsong, CC BY-SA 3.0)

The farthest, however, we've ever been able to drill is 12 kilometres deep, which is not even halfway through the planet's 35-kilometre-deep crust. Most of what we know about the structure of the Earth has been gleaned from seismic activity. In order to collect this new information, the team at CEA developed a new diffraction x-ray technique that takes measurements more quickly, within as little as a second.

They then crushed a piece of iron between a diamond anvil cell (DAC) to replicate the intense pressure at the centre of the Earth, which is 330 gigapascals (GPa), around 3.3 million times atmospheric pressure. The team set the DAC for 200GPa, heated it with a laser and x-rayed the results to see how it reacted under these conditions.

They determined the melting point at 4800 degrees Celsius at 200GPa, and extrapolated from there that the melting point at 330GPa would be 6230 ± 500 degrees Celsius.

"We are of course very satisfied that our experiment validated today's best theories on heat transfer from the Earth's core and the generation of the Earth's magnetic field," said Agnès Dewaele, who led the research team. "I am hopeful that in the not-so-distant future, we can reproduce in our laboratories, and investigate with synchrotron x-rays, every state of matter inside the Earth."

You can read the full study, "Melting of Iron at Earth's Inner Core Boundary Based on Fast X-ray Diffraction", online at Science.