Earth’s outer core is primarily liquid iron, making up roughly 85% of its mass, alloyed with about 5% nickel and a mix of lighter elements that account for the remainder. This molten metallic layer sits about 2,900 kilometers beneath the surface and is 2,200 kilometers thick, making it a massive shell of superheated liquid metal surrounding the solid inner core. Its composition is central to one of the planet’s most important features: the magnetic field that shields life from solar radiation.
Iron, Nickel, and the “Missing” Elements
The bulk of the outer core is iron with a smaller fraction of nickel, a combination scientists call a nickel-iron (NiFe) alloy. This ratio closely matches what’s found in metallic meteorites, which are fragments of the iron cores of ancient planetesimals that broke apart billions of years ago. The similarity isn’t a coincidence. Earth formed from the same cloud of material as those early planetary bodies, and the heaviest metals sank to the center as the young planet was still molten.
Pure iron-nickel at outer core pressures, however, would be denser than what seismic observations actually show. Sound waves traveling through the outer core move about 4 to 5% faster than they would through pure liquid iron, indicating that something lighter is mixed in. This gap between the expected density and the measured density is known as the “density deficit,” and it has driven decades of research into which light elements round out the outer core’s recipe.
Which Light Elements Fill the Gap
The leading candidates for the outer core’s light element inventory are oxygen, silicon, sulfur, carbon, and hydrogen. Not all of them contribute equally, and pinning down their exact proportions remains one of the harder problems in deep-Earth science.
A compositional model published in the Proceedings of the National Academy of Sciences found that oxygen is required as a major component in every viable solution. In fact, a core containing oxygen as the only light element, at about 5.4% by weight, can satisfy seismic observations on its own. The study’s best overall fit was a core with 3.7% oxygen and 1.9% silicon, with no sulfur or carbon needed. It also placed upper limits on other elements: silicon can’t exceed about 4.5%, and sulfur can’t exceed about 2.4%.
More recent work published in Nature has added hydrogen to the picture. That study concluded that hydrogen and silicon are the preferred light elements, with silicon content estimated at around 4.1% by weight. Hydrogen is particularly interesting because it strongly favors staying in the liquid outer core rather than crystallizing into the solid inner core. The researchers estimated the outer core holds hydrogen equivalent to roughly 50 Earth oceans worth of the element, locked inside the metal alloy under extreme pressure. Carbon and sulfur, by contrast, appear to play minor roles, likely falling below the 1% threshold.
The debate isn’t fully settled. Different modeling approaches emphasize different elements depending on assumptions about how Earth formed and how chemically reduced or oxidized the early planet was. But most current models converge on oxygen and silicon as the dominant light ingredients, with hydrogen as a possible third player.
A Liquid Layer Under Extreme Conditions
The outer core is liquid, not solid, and the evidence for this is straightforward. Earthquakes generate two main types of seismic waves. Compression waves (P-waves) can travel through both solids and liquids, while shear waves (S-waves) can only move through solids because liquids don’t resist shearing motion. Seismometers on the opposite side of the Earth from a large earthquake detect P-waves that have been slowed and bent, but S-waves vanish entirely beyond about 103 degrees of arc distance from the quake. In 1914, geophysicist Beno Gutenberg explained this shadow zone as the result of a molten layer beginning at roughly 2,900 kilometers depth. That interpretation still holds.
Temperatures in the outer core range from about 4,500°C at its top (the boundary with the mantle) to roughly 5,500°C at its base (where it meets the solid inner core). These temperatures are comparable to the surface of the Sun. The pressures are equally staggering, reaching into the hundreds of gigapascals. For context, one gigapascal is about 10,000 times atmospheric pressure at sea level.
Despite being called “liquid,” the outer core doesn’t flow like water in the everyday sense. Scientists have struggled to pin down its viscosity, with estimates spanning a remarkable 14 orders of magnitude. Some models suggest the liquid iron flows relatively easily, while others point to much higher resistance. This range reflects how difficult it is to recreate outer core conditions in a laboratory, though recent high-pressure experiments are narrowing the uncertainty.
How This Composition Creates the Magnetic Field
The outer core’s liquid, electrically conductive iron is what makes Earth’s magnetic field possible. The mechanism, called the geodynamo, works through convection. Heat escaping from the inner core and the mantle above creates temperature differences that drive the liquid metal to circulate, much like water boiling in a pot. On top of that, as the inner core slowly solidifies, lighter elements are expelled into the liquid outer core, creating chemical buoyancy that adds another source of upward flow.
These convection currents move electrically conductive fluid through an existing magnetic field, which induces electric currents, which in turn regenerate the magnetic field. Earth’s rotation organizes the flow into patterns that sustain this cycle. Without a liquid, iron-rich outer core, the planet would have no global magnetic field, no magnetosphere to deflect charged particles from the Sun, and a very different atmosphere.
The composition matters for more than just the field’s existence. The specific mix of light elements affects the density contrast between the inner and outer core, which controls how vigorously convection operates. A core richer in oxygen, for example, would behave differently from one dominated by silicon, because these elements partition differently between the solid inner core and liquid outer core as crystallization proceeds. This partitioning is what drives the chemical buoyancy that helps power the dynamo in the first place.
How Scientists Study a Layer They Can’t Reach
No drill has come close to the outer core. The deepest borehole ever drilled, the Kola Superdeep Borehole in Russia, reached only about 12 kilometers, barely scratching the crust. Instead, scientists rely on indirect methods.
Seismic waves from earthquakes act as a natural imaging tool. By measuring how fast these waves travel and how they bend at layer boundaries, geophysicists can calculate the density and rigidity of each layer. The speed of compression waves through the outer core tells researchers its density is lower than pure iron, pointing to the presence of lighter elements.
Laboratory experiments complement seismic data. Using diamond anvil cells, researchers squeeze tiny samples to pressures exceeding 200 gigapascals while heating them with lasers, recreating conditions found thousands of kilometers below the surface. These experiments measure how iron alloys behave at extreme conditions, letting scientists test which mixtures of elements match the seismic observations. Meteorite analysis provides a third line of evidence, offering a direct sample of the iron-nickel alloys that formed in the cores of early solar system bodies with compositions similar to Earth’s building blocks.