The deepest layer of Earth, the inner core, is generally accepted to be in a solid state of matter. This sphere, with a radius of approximately 1,220 kilometers, resides at the center of the planet. Despite the extreme environment, this region behaves mechanically like a solid, a conclusion derived from indirect measurements and the behavior of seismic waves.
The Solid State: Pressure Versus Temperature
The inner core is subject to extraordinary heat, with temperatures estimated between 5,000°C and 6,000°C, which is hot enough to melt any known material at surface pressure. However, the state of matter is determined by a balance between temperature and the immense pressure exerted by the overlying layers. The pressure at the inner core is staggering, reaching up to 3.6 million atmospheres (about 330 to 360 GigaPascals).
This colossal compression prevents the iron-nickel alloy from transitioning into a liquid state. The melting point of a substance increases dramatically as pressure rises. This effect means the temperature required to melt iron at the core’s pressure is far higher than the ambient temperature found there.
The inner core’s actual temperature is below the melting point of the iron alloy at that specific pressure, forcing the material to remain solid. The pressure overrides the intense thermal energy, locking the atoms into a crystalline structure. Modeling suggests the melting temperature of iron at the inner core boundary is around 5,950 Kelvin (5,677°C) to 6,700 Kelvin (6,427°C).
Contrasting the Inner and Outer Core
The Earth’s core is divided into two distinct regions: the solid inner core and the liquid outer core. The difference in state is a direct consequence of the pressure gradient. The outer core surrounds the inner core and is composed of a similar iron-nickel alloy, but it is in a molten state.
Although the outer core is extremely hot, its pressure is significantly lower than the inner core, reaching about 1.4 million atmospheres at the core-mantle boundary. This pressure is insufficient to elevate the melting point of the iron alloy above its current temperature. Therefore, the outer core’s material exists as a fluid capable of flow and convection, which generates Earth’s magnetic field.
The boundary between the liquid outer core and the solid inner core, known as the Inner Core Boundary (ICB), marks the specific depth where pressure finally becomes high enough to force the material into a solid phase. This transition from liquid to solid occurs because the temperature at the ICB is just below the pressure-dependent melting point of the iron-nickel alloy. The outer core provides the necessary context for understanding the inner core’s state, as it demonstrates that the material’s phase is entirely controlled by the pressure-temperature relationship at that specific depth.
Seismic Evidence Revealing the State
The understanding of the inner core’s solid state comes from analyzing how seismic waves travel through Earth’s interior. These waves change speed and direction when they encounter boundaries between materials of different properties. Scientists use two main types of body waves: P-waves (Primary or compressional waves) and S-waves (Secondary or shear waves).
P-waves travel by compressing and expanding the material, passing through both solids and liquids, though they slow down in liquid media. S-waves travel by shearing the material and cannot propagate through liquids because fluids lack rigidity. The distinct behavior of these waves allowed scientists to determine the state of matter for each layer.
Seismologists observed that S-waves completely stop at the boundary between the mantle and the outer core, confirming the outer core is liquid. P-waves traveling through the liquid outer core that hit the inner core boundary convert into new P and S waves within the inner core. The detection of these regenerated S-waves provides unambiguous proof that the inner core possesses the shear strength characteristic of a solid. Furthermore, the sharp increase in P-wave velocity upon entering the inner core reinforces the conclusion that it is a dense, solid structure.
Composition and Density
The inner core is primarily composed of an alloy of iron and nickel, a conclusion supported by the relative abundance of these elements in the solar system and laboratory experiments. Iron is estimated to account for about 80% to 90% of the inner core’s composition. Nickel, a heavy element that readily bonds with iron, makes up most of the remainder.
However, the core’s density, calculated from seismic wave propagation, is slightly lower than what would be expected for a pure iron-nickel alloy under those pressures. This density deficit suggests the mixture includes a small percentage of lighter elements, such as sulfur, silicon, oxygen, or carbon. These lighter elements are dissolved within the iron-nickel crystal lattice, slightly lowering the overall density.
The extreme density of the inner core, estimated to be around 13 grams per cubic centimeter, is a result of the pressure compressing the iron-nickel alloy. This composition is fundamental to the inner core’s solid state, as the high-pressure behavior and melting characteristics of iron are what determine the phase transition at the boundary with the liquid outer core. Some research even suggests the possibility of the iron atoms existing in a superionic state, where light elements move freely within a solid iron framework, leading to a structure that is solid yet surprisingly pliable.