The center of the Earth would glow brighter than the surface of the sun. At roughly 6,150 Kelvin (about 10,600°F), the inner core is hotter than the sun’s visible surface, meaning the material there would radiate an intense white-yellow light if you could somehow see it. But no human or camera could survive the journey. Everything we know about the middle of the Earth comes from indirect observation, primarily the behavior of earthquake waves as they pass through the planet.
What Color Would the Core Glow?
Any material heated to extreme temperatures emits light based on how hot it is. Think of a stove burner going from black to dull red to bright orange as it heats up. At around 5,800 Kelvin, an object glows yellow, which is why the sun appears the color it does. The inner core sits at roughly 6,150 Kelvin, so it would glow a brilliant white with a slight yellow tint, similar to sunlight but seen from impossibly close range.
There would be no landscape to admire, though. You wouldn’t see caverns, rocky formations, or anything resembling a surface. The inner core is a solid ball of metal compressed under roughly 3.6 million times atmospheric pressure. The outer core surrounding it is liquid metal. If you could somehow peer into the boundary between them, you’d see an ocean of blindingly bright molten iron giving way to a solid mass glowing at nearly the same intensity.
What the Inner Core Is Made Of
The inner core is a dense sphere about 1,216 kilometers (roughly 756 miles) in radius, making it slightly smaller than the moon. It’s composed almost entirely of iron (about 85% by weight) and nickel (about 5%), with the remaining fraction made up of lighter elements like silicon, carbon, and sulfur. Despite temperatures that would melt iron at the Earth’s surface many times over, the crushing pressure at the center keeps the metal locked in a solid crystalline structure.
This material is extraordinarily dense. A cubic centimeter of the inner core weighs between 12.6 and 13 grams, roughly 70% denser than lead and about four to five times denser than the rock beneath your feet. If you could hold a basketball-sized piece of it, it would weigh over 100 kilograms. The inner core is also nearly the same temperature throughout, varying by only about 20 degrees from its center to its outer edge.
The Liquid Metal Ocean Surrounding It
Wrapped around the inner core is the outer core, a churning layer of liquid iron alloy roughly 2,250 kilometers thick. It begins at a depth of about 2,900 kilometers below the surface and extends down to where the solid inner core starts at around 5,155 kilometers deep. This is the largest ocean on Earth, though no water is involved. It’s a vast shell of molten metal under enormous pressure.
The viscosity of this liquid metal varies dramatically depending on depth. Near the top of the outer core, the fluid may flow with a viscosity on the order of a few thousand pascal-seconds, somewhat thicker than honey but far from sluggish at the scale of planetary flows. Deeper down, near the inner core boundary, estimates suggest the viscosity could be a trillion times higher. These flowing currents of electrically conducting liquid iron are what generate Earth’s magnetic field. As the liquid rises and circulates, influenced by the planet’s rotation, it creates electrical currents that sustain a self-reinforcing magnetic dynamo.
The Strange Boundary Where Mantle Meets Core
Before you reach the liquid outer core, there’s a transition zone that scientists find particularly puzzling. The D” (pronounced “D double-prime”) layer sits at the very bottom of the rocky mantle, just above the molten iron. It’s roughly 250 kilometers thick and has properties that don’t match the mantle above or the core below.
This layer contains a mineral called post-perovskite that can absorb large amounts of iron, dramatically changing how seismic waves travel through it. Scattered within this zone are patches called ultralow-velocity zones, thin regions just 5 to 40 kilometers thick where seismic waves slow down by 10 to 30 percent compared to surrounding rock. These patches may represent partially molten material, pockets where the intense heat from the core below has softened the overlying rock. If you were descending toward the center of the Earth, this would be where solid rock transitions into something far more alien.
How Scientists “See” the Core
No drill has come close to the core. The deepest borehole ever made, Russia’s Kola Superdeep Borehole, reached only about 12 kilometers, barely scratching the crust. Everything known about the deep interior comes from seismic waves generated by earthquakes.
Earthquakes produce two key types of waves. P-waves (primary waves) are compression waves that can travel through both solids and liquids, like sound moving through air and water. S-waves (secondary waves) are shear waves that move rock side to side, and they cannot pass through liquid. When a large earthquake occurs, seismometers on the opposite side of the planet detect P-waves that have traveled through the core but find a “shadow zone” where S-waves disappear entirely. This is how scientists confirmed the outer core is liquid: S-waves simply cannot make it through.
At a depth of about 5,155 kilometers, P-waves suddenly speed up, indicating they’ve hit something solid again. That’s the inner core. By mapping thousands of earthquakes and the wave patterns they produce, scientists have built a detailed picture of the layers, their densities, and their physical states, all without ever seeing them directly.
A Core That’s Still Growing
The inner core hasn’t always existed. Early in Earth’s history, the entire core was liquid. As the planet slowly cooled, iron began to crystallize at the center, and the solid inner core started to form. It’s still growing today. The current best estimates put that growth rate at roughly a few millimeters per year under normal cooling conditions, though an initial rapid phase may have frozen supercooled liquid iron at rates closer to centimeters per year.
One of the ongoing puzzles is how the inner core first nucleated. Calculations suggest that spontaneously forming a solid crystal in the liquid iron would require at least 450 degrees of supercooling below the melting point. But geophysical models indicate the actual supercooling was probably less than 100 degrees. Something helped the process along, whether it was impurities in the iron acting as seeds for crystallization or some other mechanism scientists are still working out. What’s clear is that the inner core is a relatively young feature of the planet, likely less than a billion years old, and it will continue growing for billions of years to come as Earth keeps slowly losing heat to space.