Carbon: The Hidden Ingredient Behind Earth’s Inner Core
A collaborative study by researchers from the University of Oxford, University of Leeds, and University College London has unveiled a surprising factor in the long-standing question of how Earth’s inner core formed. The team finds that carbon, present in the core, would be essential to the crystallization process that created the solid iron-dominated heart of our planet. Their key result suggests the inner core could have begun to solidify if the core contains about 3.8% carbon by mass, a proportion that would make the freezing process viable under the planet’s historical cooling rates. Published in Nature Communications, the work challenges prior assumptions that the core was composed mainly of iron and that crystallization required excessive supercooling.
The inner core sits at the planet’s center as a dense, iron-rich solid that grows as the surrounding molten outer core cools and solidifies. How this transition began has long fascinated scientists because it connects to the magnetic field, seismology, and the deep chemistry of Earth’s interior. The new finding argues that carbon’s presence could have provided the right conditions for solidification without invoking unrealistically large cooling. This insight helps explain why seismologists observe a solid inner core coexisting with a surprisingly less dense core, hinting at a more complex chemical makeup than pure iron would suggest.
How scientists probed the deep Earth
Direct access to Earth’s inner depths is impossible, so the researchers turned to computer simulations to recreate the freezing process at pressures and temperatures found in the inner core. They tested the influence of several elements that the mantle could have donated to the core during Earth’s history, notably silicon, sulfur, oxygen, and carbon. As co-author Associate Professor Andrew Walker from Oxford explained, these elements would have been incorporated into the core as the planet differentiated, potentially shaping the core’s physical and chemical properties today.
The surprising role of carbon versus other elements
The simulations tracked how often small crystal-like clusters formed from a supercooled liquid, a process called nucleation that starts freezing. Contrary to expectations, silicon and sulfur appeared to slow the onset of crystallization, meaning more supercooling would have been required if these elements dominated the core. In contrast, carbon demonstrated a marked effect: higher carbon content reduced the required supercooling, making solidification more plausible under early Earth conditions.
Quantifying carbon’s impact on supercooling
When the model included 2.4% carbon by mass, the necessary supercooling dropped to about 420°C. While this is still substantial, it presented a closer fit to what would be feasible in Earth’s cooling history. Extrapolating to 3.8% carbon yielded a much more favorable 266°C of supercooling, aligning with the observed size and duration of inner-core growth. This is the first time a composition has matched both the nucleation process and the current inner-core size in simulations, positioning carbon as a key driver of the event that created Earth’s solid inner core.
Implications for Earth’s history and future
These findings imply that carbon may be more abundant in the core than previously thought, and that its presence could have been crucial to forming a durable inner core without extreme cooling. If carbon aided crystallization, it also affects interpretations of the core’s density and seismic properties, helping to reconcile observations with theoretical models. The work also feeds into the broader debate about when the inner core began to solidify, a topic that has ranged from more than two billion years ago to potentially much younger ages. With this new information, scientists can place tighter constraints on the core’s chemistry, structure, and long-term evolution.
About the study and its significance
The research, led by Alfred J. Wilson and colleagues Christopher J. Davies, Andrew M. Walker, and Dario Alfè, was published in Nature Communications on September 4, 2025. Funded by the Natural Environment Research Council (NERC), the study advances our understanding of Earth’s deep interior by linking chemical composition to physical processes that shaped the planet’s hidden core. The results emphasize how subtle shifts in elemental makeup could drive major planetary-scale phenomena, including magnetic field stability and seismic behavior.
Bottom line
By identifying carbon as a potential catalyst for inner-core crystallization, the study offers a compelling explanation for how Earth’s solid inner core could form under modest cooling. It also opens new avenues for investigating how other planetary bodies may crystallize their cores, revealing the chemistry of worlds beneath our feet and beyond.
