Unraveling the Mystery of Earth's Inner Core: Could "3.8% Carbon" Protect the World? A Small Amount of Carbon That Moved Earth's Deepest Part

The Earth's inner core has traditionally been thought to be composed of "primarily iron with a small amount of light elements," but there remained a contradiction in explaining its "initial freezing," as pure iron would require too much supercooling. A new study published in Nature Communications on September 4, 2025, focuses on the Fe–C (iron-carbon) system, demonstrating through molecular dynamics that even a few percent of carbon can accelerate nucleation and reduce the required supercooling to realistic levels. Specifically, crystallization of the inner core can begin with supercooling of about 420°C at 2.4% carbon and about 266°C at 3.8% carbon. In contrast, elements like Si and S work to delay nucleation. This finding aligns with the density differences estimated from seismic waves and the observed size of the inner core, providing important implications for the long-term history of the dynamo supporting Earth's magnetic field and plate tectonics. A Phys.org explanation on September 17 organized these findings in a straightforward manner for the general public. On social media, a succinct summary that "carbon formed the inner core" spread widely, while misunderstandings that the "inner core is primarily carbon" were also observed. Meanwhile, the expert community engaged in calm discussions focusing on the mechanisms.