A solid metal sphere inside a liquid metal shellThe outer core is liquid.
By comparing earthquake signals recorded around the world, geophysicists built the layered picture of a solid mantle, a liquid outer core, and a solid inner core.
If the solid inner core formed relatively recently in geological terms, then the earlier geodynamo had to be powered differently.
Over decades, that global record has made the inner core one of the most constrained inaccessible objects in planetary science.
As that heat leaves, the solid inner core grows at the expense of the liquid outer core.
The centre of Earth is easy to imagine wrongly. It is not a cavern, not a furnace of molten rock, and not a smaller version of the mantle pressed into a ball. The inner core is a metal object, inferred from seismic waves and mineral physics: a solid iron-rich sphere sitting inside a liquid iron-rich ocean.
The result is a finding from several lines of evidence, not a single measurement made by drilling. No instrument has visited the core, and no sample has come back from it. The picture comes from earthquake waves, density models, high-pressure experiments, and calculations of how iron alloys behave under pressures far beyond anything at the surface.
The standard reference point is the Preliminary Reference Earth Model published in Physics of the Earth and Planetary Interiors in 1981, which gave geophysicists a widely used radial model of Earth’s interior. In that model, the inner core begins about 5,150 kilometres below the surface and extends to the centre of the planet, giving it a radius of roughly 1,220 kilometres.
That makes it smaller than the Moon in radius. The Moon’s mean radius is about 1,737 kilometres and its mass is about 7.35 times 10 to the 22 kilograms, according to NASA’s Moon fact sheet. The inner core is smaller across, but it is far denser. Using PREM-scale densities, its mass is usually estimated at roughly 9 times 10 to the 22 kilograms, which is why the phrase “probably more massive than the Moon” is reasonable even though the comparison sounds strange.
A solid metal sphere inside a liquid metal shell
The outer core is liquid. The inner core is solid. Both are dominated by iron, with nickel and lighter elements mixed in, but they sit on different sides of a pressure-temperature boundary. At the inner-core boundary, pressure is so high that iron-rich metal can remain solid even at temperatures of several thousand degrees.
A 2014 PNAS paper by James Badro, Alexander Cote, and John Brodholt described a seismologically consistent model of Earth’s core composition, treating the core as iron alloyed with nickel and lighter elements such as oxygen, silicon, sulfur, or carbon. The details remain debated, partly because small changes in composition can change density, melting behaviour, and seismic velocities.
The simplest statement is therefore the safest one: the inner core is iron-rich metal, not rock. Its exact recipe is not settled. It is not pure laboratory iron. Seismic models show that the core is less dense than pure iron would be under the same conditions, which is why lighter elements are part of most compositional models.
Its solidity was inferred from the way seismic waves travel through the planet. Some waves pass through solids and liquids differently. Others do not travel through liquids at all. By comparing earthquake signals recorded around the world, geophysicists built the layered picture of a solid mantle, a liquid outer core, and a solid inner core.
Why it freezes as Earth cools
The inner core is not a relic that has always been its present size. It is thought to be growing. As Earth slowly loses heat to space through the mantle and surface, the liquid outer core cools enough at its inner boundary for iron-rich metal to crystallise. That solid metal is added to the surface of the inner core.
This is a slow process in human terms and a consequential one in planetary terms. A 2012 Nature paper by Monica Pozzo, Chris Davies, David Gubbins, and Dario Alfe framed Earth as a heat engine whose magnetic field is powered partly by heat released as the solid inner core grows, and partly by chemical convection as light elements are expelled from the freezing metal into the surrounding liquid.
The millimetre-per-year figure should be read as an order-of-magnitude estimate, not as a measured annual tick. Inner-core growth depends on the heat flowing out of the core, the melting curve of iron alloys, the composition of the liquid, and the thermal history of the mantle above it. Different assumptions can shift the inferred rate.
Still, the scale is useful. A radius increase of roughly one millimetre per year sounds tiny, but spread across a sphere more than 2,400 kilometres across, it represents a vast amount of metal crystallising over geological time. The process is slow enough to be invisible in a human lifetime and large enough to help shape the magnetic life of the planet.
The freezing does more than add metal
When iron-rich metal freezes onto the inner core, it does not simply make the solid sphere larger. It changes the liquid just above it. Some lighter elements are not easily incorporated into the solid and are left behind in the outer core liquid. That liquid becomes chemically different from its surroundings, which can help drive convection.
Convection in the outer core matters because it is tied to the geodynamo, the process that maintains Earth’s magnetic field. The field is not produced by the inner core alone. It is generated by moving, electrically conducting liquid metal in the outer core. But the growth of the inner core is one of the energy and buoyancy sources that can help keep that liquid moving.
This is why the age of the inner core is such a difficult and important question. If the solid inner core formed relatively recently in geological terms, then the earlier geodynamo had to be powered differently. If it is older, models of core cooling and conductivity have to account for that. The answer affects the history of Earth’s magnetic shielding, not just the size of a hidden metal sphere.
A 2001 Earth and Planetary Science Letters paper by Stephane Labrosse, Jean-Paul Poirier, and Jean-Louis Le Mouel discussed the age of the inner core through thermal evolution models, while a 2016 Geophysical Journal International paper by Aleksey Smirnov and colleagues showed why palaeomagnetic evidence and core thermal conductivity leave the age problem unresolved. The phrase “the unknown age of the inner core” is not rhetorical. It is an active constraint problem.
A hidden object measured by waves
The inner core’s strangeness is partly practical. It is closer to us than the Moon, but less directly accessible than the Moon. The Apollo missions brought back lunar samples. The deepest boreholes have barely scratched the crust. Everything below the mantle is inferred indirectly.
That does not make the inner core imaginary or speculative in the loose sense. Seismology is a powerful remote-sensing tool. Earthquakes send waves through the planet from many directions. Those waves speed up, slow down, reflect, refract, and disappear depending on what they pass through. Over decades, that global record has made the inner core one of the most constrained inaccessible objects in planetary science.
It also remains complicated. Studies have reported anisotropy, hemispherical differences, possible texturing, and changes in how seismic waves sample the inner core over time. A 2018 Journal of Geophysical Research: Solid Earth paper, for example, examined evidence for lateral heterogeneity near the lowermost outer core, the region just outside the inner-core boundary. The boundary is not simply a smooth diagram line in a textbook.
The planet is still becoming itself
The most useful way to think about the inner core is not as a static ball placed at Earth’s centre when the planet formed. It is part of an evolving system. Earth formed hot. It separated into metal and silicate. Its metal sank inward, its rocky mantle developed convection and plate tectonics, and its core continued to lose heat.
As that heat leaves, the solid inner core grows at the expense of the liquid outer core. A millimetre a year is barely a fingernail’s pace. Over a billion years, it becomes a planetary architecture.
So the odd comparison holds. At the centre of Earth is a solid metal sphere smaller than the Moon, probably heavier than it, and still accreting new layers from a surrounding ocean of liquid metal. It is not a separate world, but it behaves like a record of one: the deep history of Earth’s cooling, written in iron where no one can go.
