A team of scientists have now made the most accurate measurement of Earth's core temperature to date, and their results show that it's over 1,000 degrees hotter than they previously thought and hotter than the surface of the Sun.
The Earth is basically a metal ball at its core, surrounded by a hot layer of viscous, molten rock (the mantle), and topped off by the solid crust of rock that we live on. The metal at the core — a mixture of nickel and iron — is so hot that it's said to flow as easily as water, but with the weight of the entire planet crushing inward towards the centre, the pressure gets so high that close to the middle this liquid metal actually gets crunched back into a solid form. This separates the core into a liquid outer core (where the planet's magnetic field is generated) and the solid inner core.
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Scientists haven't been able to directly measure the temperatures in the different layers of the planet, though. They figured all this out by measuring how seismic waves (like from earthquakes) move through the different layers, and then comparing what they saw to different materials at different temperatures and pressures in the lab. So far, they've worked it all out well enough that they're fairly certain they have it right. One thing that's missing, though, is the actual temperatures.
About 20 years ago, a scientist named Reinhard Boehler performed an experiment to figure out the temperature between the solid inner core and the liquid outer core, by compressing pieces of iron until they reached the pressures expected at the boundary between those layers, and then measuring the temperature when the iron started to melt. His experiments showed that the temperature was likely around 4,500°C. However, that result caused some problems, because it wasn't hot enough to explain why Earth has a magnetic field.
The magnetic field is created by the churning motion of the liquid metal in the outer core. It's kind of like running an electric current through a loop of wire. The loop generates a magnetic field. For the planet to generate a magnetic field as strong as the one we know surrounds our world, the temperature difference between the inner core and the lower 'edge' of the mantle has to be at least 1,500 degrees. However, the temperature along the inner mantle has been figured to be around 4,000°C. That leaves only a 500 degree difference, which is just not hot enough.
Part of the problem with this kind of experiment is that it's very hard to maintain high pressure and temperature in the lab (especially at the levels that exist in the planet's core).
"In practice, many experimental challenges have to be met, as the iron sample has to be insulated thermally and also must not be allowed to chemically react with its environment," study co-author Agnès Dewaele, from the Commissariat à l’Énergie Atomique in France, explained in an ESRF statement. "Even if a sample reaches the extreme temperatures and pressures at the centre of the Earth, it will only do so for a matter of seconds. In this short timeframe it is extremely difficult to determine whether it has started to melt or is still solid."
The method used in the new experiment was to fire a beam of X-rays through the iron sample, and watch how the iron atoms caused the path of the X-rays to change. This 'diffraction' method gave them results in less than a second, whereas the older experiments used a slower method, and therefore was less accurate.
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The new experiment was only able to compress the iron up to about two-thirds of the 3.3 million atmospheres of pressure that would be experienced at the boundary between the inner and outer cores (that's 3.3 million times greater than normal air pressure), but it was a simple matter to just extrapolate the results from there, and they found that the temperature would be around 6,000 degrees (plus or minus 500 degrees). That's more than hot enough to account for the Earth's magnetic field, and is at least as hot, if not hotter, than the temperature of the surface of the Sun (which is roughly 5,500°C).
(Images courtesy: Wikimedia commons)
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