We may finally understand what caused the inner core of the Earth to freeze.

The inner core is a sphere of iron approximately 2,400 km (1,500 miles) in diameter, enveloped by a molten outer core. Its growth is responsible for generating the Earth’s magnetic field, which shields the planet from harmful solar radiation. However, the precise process by which the core first crystallized has remained unclear.

Recent research published in Nature Communications suggests a mechanism that hinges on deep Earth chemistry. By utilizing advanced computer simulations, scientists examined how various factors influence the freezing of iron under extreme pressure and temperature at the planet’s center.

They found that incorporating carbon allows iron to solidify under realistic conditions, positioning it as a key component in understanding the ingredients that contributed to the formation of the inner core billions of years ago.

“By investigating how Earth’s inner core formed, we gain insights not only into the planet’s history,” said Dr. Alfred Wilson from the University of Leeds, who led the study.

“We get rare insights into the chemistry of a region that we can never physically reach, and we can only speculate on how it might change in the future.”

At the extreme pressures found 5,000 km beneath our feet, iron doesn’t simply freeze when it drops below its melting point; it requires “super-cooling” of the crystals before they form. Pure iron must be cooled to as low as 1,000°C (1832°F), resulting in a significantly larger core than the one we see today.

New computer modeling indicates that the presence of carbon alters this equation. With less than 4% carbon in the mix, iron can crystallize at much lower temperatures, producing a core that aligns with seismic observations.

Scientists believe that the Earth’s center likely continues to host a mixture of elements. However, this research firmly highlights the critical role of carbon in one of geology’s greatest mysteries.

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