Researchers have made a discovery involving how the Earth’s core initially froze and crystallized millions of years ago, suggesting that a previously unexpected amount of carbon, as revealed in a new paper published in Nature Communications.
Accounting for 3.8% of the Earth’s interior, such a percentage is far higher than researchers believed was likely to exist in the planet’s core, which is primarily believed to be rich in iron. Presently, it continues to grow as more of its molten surroundings cool to join the core’s mass in a process that scientists have debated for decades.
Understanding The Earth’s Inner Core
Forming a solid inner core requires more than just cooling—it depends on the precise chemistry of the molten material. The process can be compared to hail formation: water droplets in clouds must reach temperatures well below water’s normal freezing point before crystallizing into ice. Similarly, molten iron in Earth’s core must become “supercooled,” dropping roughly 800–1000 °C below its normal freezing point to solidify.
However, geological evidence suggests Earth’s core has never cooled this far. Simulations show such extreme cooling would have caused runaway core growth and likely collapsed the planet’s magnetic field—an event that never appears to have happened. Data instead indicate the core cooled by less than 250 °C below its melting point.
Reconciling A Warm Core
To explain how the core could crystallize without extreme supercooling, researchers from the University of Oxford, the University of Leeds, and University College London turned to computer simulations. Lacking direct access to the planet’s center, they modeled how elements like carbon, oxygen, sulfur, and silicon—known to exist in the mantle—would influence crystallization if present in the core.
“Each of these elements exist in the overlying mantle and could therefore have been dissolved into the core during Earth’s history,” said co-author Associate Professor Andrew Walker of the University of Oxford. “As a result, these could explain why we have a solid inner core with relatively little supercooling at this depth. The presence of one or more of these elements could also rationalise why the core is less dense than pure iron, a key observation from seismology.”
The First Steps Of Freezing
The team simulated atomic-scale interactions, tracking how roughly 100,000 atoms behaved under high pressure and supercooled conditions similar to those in the inner core. They focused on “nucleation,” the moment when small crystal clusters first form, marking the beginning of freezing.
The results were striking. Elements like silicon and sulfur, once thought to be major components of the core, actually slowed freezing and required greater supercooling. Carbon, by contrast, accelerated freezing, bringing models closer to observed conditions. A 2.4% carbon mixture still fell short, but at 3.8% carbon, the simulations aligned perfectly with expectations—producing an inner core temperature requiring just 266 °C of supercooling. No other combination reproduced the core’s observed nucleation and size.
Continuing to Investigate the Core
“It is exciting to see how atomic-scale processes control the fundamental structure and dynamics of our planet,” said lead author Dr Alfred Wilson of the University of Leeds. “By studying how Earth’s inner core formed, we are not just learning about our planet’s past. 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.”
The study also lends new data to the ongoing debate over the age of Earth’s core. Some scientists argue it began crystallizing more than two billion years ago, while others suggest it solidified less than half a billion years ago. The new evidence, pointing to carbon’s crucial role, may help resolve this long-standing question.
The paper, “Constraining Earth’s Core Composition from Inner Core Nucleation,” appeared in Nature Communications on September 4, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.