The Earth’s core, the deepest layer of our planet, remains inaccessible to direct observation, yet it plays a profound role in making life possible. This immense sphere is a powerhouse of heat and motion. Its dynamic processes generate the Earth’s magnetic field, a protective shield that extends far into space. This field deflects harmful charged particles from the sun, preventing them from stripping away our atmosphere. Understanding the core’s composition and state is fundamental to comprehending the entire planetary system.
Defining the Inner and Outer Core
The core is distinctly divided into two layers based on their physical state. The outer core is a vast, molten sea of metal that begins about 2,890 kilometers below the surface. Despite enormous pressure, the temperature, estimated between 4,000 and 5,500 degrees Celsius, keeps the material liquid.
This liquid state allows the metal to flow. The constant movement of this molten iron-nickel alloy generates the electrical currents that drive the geodynamo, the process creating the Earth’s magnetic field.
Below this liquid expanse lies the inner core, a super-hot, solid sphere extending to the center of the Earth. Temperatures here are even higher, potentially reaching 5,200 to 6,000 degrees Celsius. However, the immense pressure, reaching over three million times the atmospheric pressure, overrides the intense heat. This pressure forces the iron-nickel alloy into a solid, crystalline structure. The boundary separating the liquid outer core from the solid inner core is known as the Lehmann discontinuity.
The Elemental Composition of the Core
The core is overwhelmingly composed of metal, specifically an alloy of Iron (Fe) and Nickel (Ni). These dense elements sank toward the center of the planet during its formation, a process known as planetary differentiation. Iron is the most abundant element, constituting about 80% or more of the core’s total mass, with Nickel making up the remainder of the dominant metallic components.
However, the core is not composed of pure iron and nickel. It is slightly less dense than models predict a pure alloy would be, a “density deficit” that indicates the presence of lighter elements mixed into the metallic solution. These lighter elements are estimated to make up around 5% to 10% of the core’s mass, though the exact mix remains intensely researched.
Candidate lighter elements include:
- Sulfur
- Oxygen
- Silicon
- Carbon
These elements lower the alloy’s melting point and density, allowing scientists to reconcile theoretical models with observed seismic data. For example, some studies suggest Oxygen may be a major light element in the outer core, while others find that Silicon and Sulfur concentrations must be kept relatively low.
How Scientists Determine Core Makeup
Since no probe has ever reached the core, scientists rely on indirect methods to determine its makeup and state. The most powerful tool is seismology, which involves analyzing the waves generated by earthquakes. These seismic waves act like an ultrasound, providing data on the Earth’s interior structure.
Scientists track two main types of waves: P-waves (compressional waves) and S-waves (shear waves). P-waves travel through solids and liquids, but they slow down and refract when crossing into the liquid outer core. Crucially, S-waves cannot transmit through a liquid medium at all. The observation that S-waves stop completely at the core-mantle boundary provided the initial evidence that the outer core is liquid.
To simulate the core’s extreme conditions—pressures millions of times greater than the surface and temperatures comparable to the sun—scientists use high-pressure laboratory experiments. The diamond anvil cell is one such device. It uses two tiny diamonds to squeeze a sample of material, such as iron, while lasers heat it.
By observing how various iron alloys behave under these simulated conditions, researchers can test which compositions best match the density and wave-speed data gathered by seismology. These models are then used to predict the properties of the core, refining the estimates of its elemental composition.