The scientific consensus that Earth’s core is predominantly composed of iron and nickel is the result of multiple, independent lines of evidence. The core, extending from approximately 2,900 kilometers below the surface to the planet’s center, is physically inaccessible, existing under immense pressure and heat. Researchers must rely on indirect measurements to determine its composition. By combining geophysical observations with the principles of planetary formation, scientists have established that iron is the only element that satisfies all known physical constraints of the deep interior.
Seismic Wave Analysis
The primary tool for probing Earth’s interior is seismology, which involves analyzing how waves generated by earthquakes travel through the planet. The speed and path of these waves change dramatically when they encounter boundaries between layers, allowing scientists to map the internal structure. This technique revealed the presence of a liquid outer core and a solid inner core.
The analysis hinges on two types of body waves: P-waves (compressional) and S-waves (shear). S-waves cannot travel through liquids, and the observation of a large S-wave “shadow zone” proves that the outer core, extending from 2,900 km to 5,150 km depth, is in a molten state. P-waves travel through both solids and liquids but slow down and refract sharply at the core-mantle boundary (CMB).
The abrupt change in P-wave behavior indicates an enormous jump in density from the silicate rock of the lower mantle (around \(5.0\text{ g/cm}^3\)) to the outer core (around \(11.0\text{ g/cm}^3\)). This density increase requires a material far heavier than the surrounding silicate minerals. Iron is the only abundant element that can account for this density and is the only plausible candidate for the core’s main constituent.
Earth’s Magnetic Field
The existence of Earth’s global magnetic field provides a powerful constraint on the core’s composition, demanding a highly conductive, fluid material. The magnetic field is generated by the geodynamo, which involves the movement of electrically conductive liquid metal in the outer core. Without this metallic, liquid outer core, the magnetic field would rapidly decay over geological time.
The outer core is a vast, churning ocean of molten metal driven by convection, powered by heat escaping from the inner core and its continuous solidification. As this liquid iron alloy moves, it generates electrical currents that sustain the magnetic field through a self-exciting dynamo mechanism. Iron and its alloy partner nickel are the only cosmochemically abundant elements possessing the high electrical conductivity necessary to power the dynamo under the core’s extreme pressure and temperature conditions.
Cosmochemical Clues
Evidence from the solar system’s formation history supports the iron core hypothesis, as Earth formed from the same primordial cloud of dust and gas as the Sun and other planets. Iron is the most abundant heavy element in the solar system, ranking fourth overall after hydrogen, helium, and oxygen. This suggests a large reservoir of iron was available during Earth’s accretion.
Scientists study iron meteorites, which are fragments of ancient asteroids that melted early in their history and underwent differentiation. In these small bodies, dense metal sank to the center, forming a metallic core surrounded by a silicate mantle, a process analogous to Earth’s layering. Iron meteorites are predominantly composed of iron-nickel alloys, typically containing 5 to 10% nickel, consistent with the inferred composition of Earth’s core.
Mass and Density Calculations
Gravitational measurements provide quantitative proof that a high-density core must exist to explain Earth’s total mass. By measuring the planet’s gravitational influence, scientists calculate Earth’s total mass (approximately \(5.972\times 10^{24}\text{ kg}\)). Dividing this mass by the planet’s volume yields an average density of about \(5.5\text{ g/cm}^3\).
The density of the silicate rocks that make up the crust and mantle averages only \(3.0\text{ g/cm}^3\) to \(5.5\text{ g/cm}^3\). For the planet to achieve an average density of \(5.5\text{ g/cm}^3\), the core must be significantly denser to compensate for the lighter outer layers. This mathematical necessity points to a core density reaching \(13.0\text{ g/cm}^3\) in the inner core, a density that only iron and nickel can achieve under core pressures.
High-pressure physics experiments, utilizing devices like diamond anvil cells, compress samples of iron and iron-nickel alloys to the immense pressures found deep within the Earth, up to \(1.7\text{ million atmospheres}\). These experiments confirm that the density and sound speed properties of iron and its alloys under these extreme conditions closely match the seismic observations, solidifying the conclusion that Earth’s core is overwhelmingly metallic iron.