Earth’s inner core is primarily composed of iron, with roughly 5 to 10% nickel and 2 to 3% lighter elements mixed in. This solid metallic ball sits at the center of the planet, about 1,215 kilometers in radius, under pressures between 330 and 365 gigapascals and temperatures near 6,150 kelvin (about 5,877°C). Despite being hotter than the surface of the Sun, the inner core remains solid because of the immense pressure squeezing it from all sides.
Iron and Nickel Form the Base
Iron is by far the dominant element in the inner core. Geochemical models estimate that nickel makes up about 5.5% by weight, though laboratory experiments show the iron lattice can accommodate up to 10% nickel while maintaining the same crystal structure. That structure is called hexagonal close-packed, meaning the iron atoms arrange themselves in tightly stacked layers, the most efficient packing geometry possible at these extreme pressures.
Nobody has drilled to the core, so these estimates come from indirect evidence. The two main lines of reasoning are cosmochemistry (analyzing the composition of iron meteorites, which are remnants of other planetary cores) and high-pressure physics experiments that recreate core conditions in the lab using diamond anvil cells.
The Light Element Mystery
Pure iron under inner core conditions would be 4 to 5% denser than what seismic data actually show. This “density deficit” tells scientists that something lighter is mixed in, bringing the overall density down to the observed range of 12.8 to 13.1 grams per cubic centimeter. Identifying exactly which light elements fill that gap is one of the biggest open questions in earth science.
The leading candidates are silicon, sulfur, oxygen, carbon, and hydrogen. Of these, silicon appears to be the strongest contender, with recent compositional models proposing the inner core is an iron-nickel-silicon alloy containing 1 to 2% silicon. Oxygen and silicon together are thought to be the main light components of the broader core system, while volatile elements like carbon and hydrogen likely fall below 1% in the inner core.
Pinning down the exact recipe matters because even small compositional differences change how fast seismic waves travel through the core. Scientists compare the wave speeds and densities they measure with seismographs against the wave speeds and densities of various iron alloys tested in the lab. When a lab alloy matches the seismic observations, it becomes a strong compositional candidate.
How Scientists “See” the Inner Core
Seismic waves from large earthquakes pass through every layer of the planet. Compressional waves (P-waves) travel through both solids and liquids, while shear waves (S-waves) only move through solids. The inner core transmits both types: P-waves at about 11.0 to 11.3 km/s and S-waves at about 3.5 to 3.7 km/s. The fact that S-waves exist there at all is the primary evidence that the inner core is solid rather than liquid like the outer core surrounding it.
Models that include a solid inner core fit observed seismic data dramatically better than models with a fully liquid core. One recent reanalysis found that the statistical misfit drops from roughly 500,000 to about 200 when a solid inner core is included, leaving little room for doubt about its physical state.
A Possible Superionic Layer
Recent simulations have introduced a twist: some light elements in the inner core may behave like a liquid even though the iron lattice around them stays solid. Under inner core conditions, hydrogen, oxygen, and carbon atoms can sit between the iron atoms and diffuse freely through the rigid metal framework, similar to how ions move through a battery electrolyte. This “superionic” state means the inner core could be simultaneously solid (in its iron structure) and partly fluid (in its light element behavior), which may help explain some puzzling seismic observations, including why the core absorbs shear wave energy more strongly than a simple solid would.
Structure Within the Inner Core
The inner core is not uniform throughout. Seismic waves travel faster along the north-south axis (parallel to Earth’s rotation) than along the equator, a property called anisotropy. This suggests the iron crystals are preferentially aligned rather than randomly oriented. There is also an east-west asymmetry, where the eastern and western hemispheres of the inner core show slightly different seismic properties, possibly reflecting differences in crystal size or texture.
Deeper still, a region a few hundred kilometers in radius at the very center, sometimes called the “innermost inner core,” shows even stronger anisotropy with a different direction of slowest wave speed. This inner layer may represent a shift in how the core grew over time, perhaps recording a change in crystallization patterns or in the convection currents of the surrounding liquid outer core. The transition between this innermost region and the rest of the inner core appears gradual rather than sharp.
How Old the Inner Core Is
Earth is about 4.5 billion years old, but the inner core is much younger. For most of the planet’s history, the entire core was liquid. Laboratory experiments recreating core pressures and temperatures put the age of the solid inner core at roughly 1 to 1.3 billion years, placing it at the younger end of estimates that have ranged as high as 4.5 billion years and as low as 565 million years. The inner core continues to grow today as the outer core slowly cools and iron crystallizes onto its surface.
This ongoing crystallization is not just a geological curiosity. When iron solidifies onto the inner core, it releases latent heat and expels lighter elements back into the liquid outer core. Both effects drive convection currents in the outer core, and those currents of electrically conductive liquid iron are what generate Earth’s magnetic field. Without the inner core’s gradual growth, the geodynamo that shields the planet from solar radiation would be significantly weaker or might not function at all.
Why Composition Affects the Magnetic Field
The inner core’s electrical conductivity gives it a stabilizing role in the magnetic field. Because the field cannot easily diffuse through a solid conductor, the inner core dampens out rapid fluctuations in the geodynamo, acting as a kind of magnetic flywheel. This stabilizing effect likely influences how often Earth’s magnetic poles flip. As the inner core has grown over the past billion-plus years, its increasing size may have changed the frequency and character of these polarity reversals. Some researchers believe the inner core also provides a “memory” for the dynamo, helping explain why certain patterns in the magnetic field repeat across successive pole flips observed at widely separated locations on Earth’s surface.