The Earth’s inner core is the planet’s deepest and most inaccessible layer, situated over 5,150 kilometers beneath the surface. This sphere represents the final boundary toward the planet’s center, a region of extreme conditions impossible to probe directly. The inner core has a radius estimated to be about 1,230 kilometers, making it roughly 70% the size of the Moon. All knowledge about this highly pressurized realm must be inferred through indirect scientific observation.
Primary Elements of the Inner Core
The inner core is primarily composed of an iron-nickel alloy, with iron (Fe) being the dominant element. This composition is consistent with the high density and magnetic properties observed within the core region. Based on estimates derived from the study of meteorites and the overall composition of the solar system, nickel (Ni) is thought to constitute up to 10% of the alloy by weight.
Scientists hypothesize the presence of other, lighter elements within this metallic mixture to account for the inner core’s observed characteristics. Seismological data indicates the core’s density is approximately 3% to 5% less than that of pure iron or iron-nickel under the same extreme pressure conditions. This density deficit strongly suggests that a small percentage of low atomic mass elements must be dissolved within the iron crystal structure.
Candidates for these light elements include Silicon (Si), Oxygen (O), Sulfur (S), and Carbon (C), which would have been incorporated during the planet’s formation. Estimates suggest the inner core contains a small fraction of these elements, perhaps up to 2.3% Silicon or 1.3% Carbon by weight. These elements reduce the overall density of the alloy, providing a match between theoretical models and the physical properties determined by seismic wave analysis.
The Solid State Under Extreme Pressure
Despite being the hottest region of the planet, the inner core exists in a solid, crystalline state. The estimated temperature at the inner core boundary is approximately 5,700 Kelvin (5,430 °C), a heat level comparable to the surface of the sun. Iron and nickel would typically melt at this temperature, but the massive pressure exerted by the overlying layers prevents this from happening.
The pressure ranges from about 330 to 360 Gigapascals, which is over three million times the atmospheric pressure at Earth’s surface. This immense force squeezes the metal atoms so tightly that it raises the melting point of the iron-nickel alloy far above the surrounding temperature. The high pressure forces the metal into a tightly packed, solid crystal structure, most likely hexagonal close-packed iron.
This physical state distinguishes the solid inner core from the liquid outer core that surrounds it. The outer core has a similar high temperature, but the pressure is slightly lower, allowing the iron-nickel alloy there to remain molten and flow freely. The transition from liquid to solid at the inner core boundary is a direct consequence of the pressure exceeding the melting point of the metal alloy at that depth.
Seismic Evidence and Discovery Methods
Scientists rely on seismology—the study of earthquake-generated waves—to determine the inner core’s composition and state. These waves travel through the Earth’s interior, and their speed and path change based on the density and rigidity of the material they pass through. Two primary types of body waves are used: P-waves (Primary or compressional waves) and S-waves (Secondary or shear waves).
P-waves can travel through both solids and liquids, but S-waves can only propagate through solid material, as liquids do not support shear motion. The initial discovery of the core’s structure came from Danish seismologist Inge Lehmann in 1936, who observed unexpected P-wave arrivals that implied a distinct inner boundary within the core. The subsequent detection of S-waves passing through the deepest region provided definitive proof that the innermost sphere is solid.
Analysis of the velocity of these seismic waves provides further constraints on the core’s makeup. The measured P-wave velocity is slower than what would be expected for pure iron under those conditions, supporting the hypothesis that lighter elements are mixed into the iron-nickel alloy. High-pressure laboratory experiments, which simulate core conditions, and density modeling are also used to match the properties of various iron alloys with the observed seismic data.