We may finally know what triggered Earth’s inner core to freeze, helping unlock one of the planet’s longest-standing geological puzzles.
The inner core is a solid ball of iron about 2,400km (1,500 miles) wide, surrounded by a molten outer core. Its growth powers Earth’s magnetic field, which shields the planet from harmful solar radiation. But exactly how the core first crystallised has never been clear.
A new study, published in Nature Communications, suggests the process depended on the chemistry of the deep Earth. Using powerful computer simulations, researchers tested how different elements might affect the freezing of iron under the immense pressures and temperatures at the planet’s centre.
They found that adding carbon allowed iron to solidify under realistic conditions – making it a strong candidate for the missing ingredient that helped the inner core form billions of years ago.
“By studying how Earth’s inner core formed, we are not just learning about our planet’s past,” said Dr Alfred Wilson from the University of Leeds, who led the study.
“We’re getting a rare glimpse into the chemistry of a region we can never hope to reach directly and learning about how it could change in the future.”
At the pressures found 5,000km beneath our feet, iron does not simply freeze once it drops below its melting point – it must be 'supercooled' before crystals appear. Pure iron would need to be cooled by up to 1,000°C (1832°F), creating a core much larger than we see today.
The new computer modelling shows that carbon changes the equation. With just under 4 per cent carbon in the mix, iron can crystallise after a far smaller drop in temperature, producing a core that matches seismic observations.
Scientists say Earth’s centre is still likely to contain a cocktail of elements. But the findings place carbon firmly at the heart of one of geology’s greatest mysteries.
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