The inner core, Earth’s deepest layer, is definitively a solid ball of metal. Scientists cannot drill down to the planet’s center, but they use indirect methods, primarily studying how energy travels through the Earth, to determine its physical properties and build a detailed model of its layered structure.
Earth’s Internal Architecture
The Earth is structured into four primary layers: the crust, the mantle, the outer core, and the inner core. The thin crust and the thicker, mostly solid mantle, composed largely of silicate rocks, form the outer shell. Beneath the mantle, the core begins at a depth of nearly 2,900 kilometers.
The outer core is a vast layer of molten metal, predominantly liquid iron and nickel. This fluid layer generates the Earth’s magnetic field through its convective movement. The transition to the solid inner core is marked by the Inner Core Boundary (ICB).
The inner core is a dense, hot sphere with a radius of approximately 1,220 kilometers, slightly smaller than the moon.
Pressure, Temperature, and the Inner Core’s Solid State
The inner core is primarily an alloy of iron and nickel, similar in composition to the outer core, but its state is governed by thermodynamic conditions. The temperature is estimated to be between 5,200 and 6,000 degrees Celsius, rivaling the heat of the sun’s surface. Under normal pressure, this heat would melt or vaporize iron.
However, the inner core is subjected to immense confining pressure—estimated to be over 3.6 million times greater than surface pressure. This pressure dramatically increases the material’s melting point. For the iron-nickel alloy, the pressure raises the melting point far above the ambient temperature, compressing the atoms into a stable, solid crystalline structure. The inner core is solid because the mechanical force of the overlying layers prevents its atoms from moving freely.
Proving the State: Evidence from Seismology
Since direct sampling is impossible, scientists rely on seismology—the study of earthquake waves—to probe the inner core. Earthquakes generate two main types of waves: P-waves (primary, compressional) and S-waves (secondary, shear). P-waves travel through solids, liquids, and gases.
S-waves, however, cannot propagate through a liquid medium because liquids lack the shear strength required to transmit the motion. Observations show that S-waves disappear at the core-mantle boundary, confirming the outer core is liquid. P-waves traveling through the center of the Earth show distinct changes in velocity and are refracted at the inner core boundary. This pattern is consistent with a transition from a liquid to a solid medium.
Sensitive measurements have detected P-waves converting into S-waves upon entering the inner core and converting back upon exiting. The observation of S-wave behavior inside the deepest part of the Earth provides evidence of its rigidity, confirming the innermost layer is a solid ball of metal.