Hidden chemistry of Earth core revealed through freezing process

Hidden chemistry of Earth core revealed through freezing process
by Sophie Jenkins
London, UK (SPX) Sep 07, 2025

A new study by researchers from Oxford, Leeds, and University College London has uncovered fresh insights into Earth's core chemistry by showing how crystallisation became possible millions of years ago. Published in Nature Communications, the research indicates that Earth's core required 3.8% carbon content to begin freezing.

This finding suggests that carbon is more abundant in the core than previously assumed and that it played a crucial role in the emergence of the inner core. The discovery helps explain how freezing began under realistic supercooling conditions, providing rare insight into the inaccessible depths of the planet.

Earth's solid iron-rich inner core continues to grow as the molten outer core slowly freezes. Yet, understanding how this process initiated has long challenged scientists. Pure iron would need 800-1000 C of supercooling before crystallising, which would have caused rapid core growth and magnetic field collapse - scenarios not supported by Earth's history.

Instead, scientists believe cooling did not exceed about 250 C below iron's melting point. The team turned to high-precision computer simulations, testing the influence of silicon, sulphur, oxygen, and carbon. While silicon and sulphur slowed crystallisation, carbon accelerated it, offering the only pathway to freezing with limited supercooling.

Atomic-scale simulations of 100,000 atoms showed that with 2.4% carbon, supercooling would still require 420 C. At 3.8% carbon, however, the threshold dropped to 266 C, aligning with observed inner core conditions. This makes carbon the critical element that enabled the inner core to exist without external "nucleation seeds," which would have dissolved or melted at such depths.

"This research shows how atomic-scale processes govern the fundamental structure of our planet," said lead author Dr Alfred Wilson of the University of Leeds. "By studying the formation of the inner core, we gain a rare glimpse into the chemistry of a region we cannot access directly."

The results tighten constraints on core composition and advance debates over when the inner core began solidifying - whether more than two billion years ago or within the past half-billion years. Understanding carbon's role brings researchers closer to resolving Earth's deep interior history.

Full image caption: Cartoon of the Earth with cutaway showing the mantle and inner and outer core. Magnetic field lines produced by the geodynamo extend into space and interact with the solar wind. The iron-rich core at Earth's centre is slowly freezing from the inside out. This growth of the solid inner core powers the magnetic field that shields our planet from harmful space weather. How and when the inner core first began to freeze remains a mystery but new research shows that solving it could reveal the core's composition, giving us a clearer picture of Earth's deep interior. Image credit: Dr Alfred Wilson.

Research Report:Constraining Earth's core composition from inner core nucleation