The Earth’s structure features a solid, metallic sphere at its deepest point, known as the inner core. Located over 5,000 kilometers beneath the surface, it is subjected to immense pressure and heat. Scientists use sophisticated methods to determine its makeup, revealing a unique alloy of heavy metals and lighter elements that differs significantly from the surrounding crust and mantle.
The Primary Metallic Components
The foundational material of the inner core is a dense alloy of iron (Fe) and nickel (Ni), elements that sank to the planet’s center during its formation through a process called planetary differentiation. Iron accounts for approximately 85% to 90% of the core’s mass, with nickel making up roughly 5% to 10%. This composition is consistent with the abundance of these metals in the materials that formed Earth.
Although the inner core’s temperature is estimated to be between 5,400 to 5,700 degrees Celsius, comparable to the heat on the surface of the Sun, it remains solid. This is due to the crushing pressure exerted by the overlying layers of the planet. Pressures within the inner core range from 330 to 360 gigapascals, over three million times the pressure at sea level.
This incredible pressure elevates the melting point of the alloy, forcing the atoms into a tightly packed crystalline structure. This structure is believed to be either hexagonal close-packed (hcp) iron or body-centered cubic (bcc) iron, accommodating the high-pressure conditions. The continual crystallization of this iron alloy from the molten outer core expands the inner core’s radius by about one millimeter each year.
Identifying the Less Dense Elements
The inner core is not composed purely of iron and nickel. Data derived from seismic wave analysis indicates a significant disparity, often referred to as the “density paradox.” The core is approximately 3% less dense than a pure iron-nickel alloy would be under these extreme conditions. This difference implies that a portion of the inner core’s mass must be composed of elements lighter than iron.
The search for these light elements is a central focus of modern core research. Candidates include:
- Silicon (Si)
- Oxygen (O)
- Sulfur (S)
- Carbon (C)
- Hydrogen (H)
These elements are thought to be dissolved into the iron crystal lattice, much like carbon is dissolved into iron to make steel, which reduces the overall density of the alloy. Experiments show that significant amounts of silicon and sulfur can be incorporated into iron under core-like conditions.
Scientists generally agree that a combination of several lighter elements, rather than just one, is responsible for the density deficit. Oxygen is a strong candidate, as models suggest the liquid outer core contains 8% to 13% oxygen, much of which is excluded as the inner core solidifies. Determining the precise ratio of these elements is challenging, as their presence affects how the iron lattice forms and transmits energy.
How Scientists Determine Inner Core Composition
Directly sampling the inner core is impossible, as it lies thousands of kilometers beneath the surface. The primary tool for peering into the planet’s interior is seismology, which studies the behavior of seismic waves generated by earthquakes. The speed of these P-waves (compressional waves) and S-waves (shear waves) traveling through the Earth is directly related to the density and rigidity of the material they pass through.
Seismologists map the inner core’s physical properties by tracking how these waves are reflected and refracted at layer boundaries. For example, S-waves disappear completely upon reaching the outer core, confirming its liquid state. The speed of P-waves through the inner core reveals its solid state and density. By comparing measured wave speeds with speeds calculated for various alloys, scientists constrain the range of materials that could make up the core.
The second method involves high-pressure and high-temperature laboratory experiments, often using specialized devices like the diamond anvil cell. This apparatus uses two opposing diamonds to compress a tiny material sample to pressures exceeding those found at the core. Researchers then use lasers to heat the sample while under compression, simulating the inner core’s extreme conditions.
These experiments allow scientists to measure properties like melting points, crystal structures, and sound velocities of iron-nickel alloys mixed with light elements. By matching the experimental properties of these synthetic core materials to the geophysical data from seismic studies, researchers refine their models. Computational models, such as ab initio molecular dynamics, also play a complementary role by predicting how iron and its alloys will behave at the core’s specific pressure and temperature range.