The Earth is structured in layers, moving inward from the crust through the mantle to the metallic core. The core is divided into a liquid outer layer and a solid inner sphere. This innermost layer, the Earth’s inner core, is the deepest region of our planet, situated over 5,000 kilometers beneath the surface. Its existence was first inferred in 1936 by seismologist Inge Lehmann, who detected an unexpected reflection of seismic waves passing through the planet’s center.
Composition and Physical State
The inner core is primarily composed of a metallic alloy of iron and nickel. Seismic data suggests this alloy is less dense than pure iron and nickel, indicating the likely presence of lighter elements. Scientists hypothesize these lighter elements could include small amounts of silicon, oxygen, or sulfur mixed into the dominant iron matrix. This solid sphere has an estimated radius of about 1,221 kilometers, making it roughly 70% the size of the Moon.
A defining characteristic of the inner core is its solid physical state, despite being intensely hot. This solidity is maintained by the extraordinary pressure exerted by the overlying layers of the Earth. The pressure compacts the atoms so tightly that they cannot transition into a liquid state. The inner core is separated from the surrounding liquid outer core by a transition zone known as the Inner Core Boundary.
Extreme Temperature and Pressure
Temperature estimates at the inner core boundary range between 5,400 and 5,700 Kelvin, comparable to the temperature found on the surface of the sun. The pressure reaches an estimated 330 to 360 Gigapascals, over three million times the atmospheric pressure at sea level. This combination of high pressure and temperature creates the stable solid state of the iron alloy.
The source of this heat includes residual heat from the planet’s formation. An ongoing source is the release of latent heat as the liquid iron of the outer core slowly crystallizes onto the solid inner core. This continuous crystallization drives thermal and compositional convection in the liquid outer core.
The Inner Core’s Unique Rotation
The inner core rotates independently of the planet’s mantle and crust, engaging in “super-rotation” by spinning slightly faster than the Earth’s surface. This differential rotation is driven by electromagnetic and gravitational forces generated by the churning liquid iron in the outer core. Recent seismic studies indicate that this rotation rate is not constant; the inner core can slow down, stop, or even rotate in the opposite direction relative to the surface.
The growth of the inner core, which occurs at a rate of approximately 0.5 millimeters per year, is fundamental to the Earth’s magnetic field. As the metal crystallizes, lighter elements are expelled into the surrounding liquid outer core. This expulsion generates compositional buoyancy, powering the convective flow of the liquid metal—a process known as the geodynamo.
How Scientists Study the Deepest Earth
Since direct observation is impossible, scientists rely on indirect methods, primarily seismology—the study of seismic waves generated by earthquakes. Seismologists analyze how two main types of waves, compressional P-waves and shear S-waves, travel through the planet.
By monitoring the arrival times and paths of these waves at seismic stations, researchers infer the composition and physical state of the deep materials. S-waves cannot travel through liquid but can travel through solid material, which provides evidence that the inner core is solid while the outer core is liquid.
Laboratory experiments utilizing diamond anvil cells also simulate the extreme conditions of the core. These cells compress materials to millions of atmospheres of pressure while heating them with lasers, allowing scientists to test theoretical models.