Earth's outer core

• 1. Introduction

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  Earth's outer core is a churning ocean of liquid metal that no human will ever see, beginning roughly 2,889 kilometres beneath your feet. It is about 2,260 kilometres thick, made mostly of iron and nickel, and it stays liquid for a strange reason. There simply is not enough pressure down there to squeeze it solid, even though the inner core just below it is frozen rock-hard. How do we know any of this about a place we can never reach? And why should a buried layer of molten metal matter to anything living on the surface? The answers run from earthquakes that pass through the planet, to the invisible shield that keeps our atmosphere from blowing away, to a question scientists call the new core paradox.

• 2. Reading A Hidden Ocean

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  Seismic shear-waves refuse to travel through the outer core, and that refusal is the central clue to its liquid state. Shear-waves cannot move through fluids, so their disappearance tells geophysicists the layer must be molten rather than solid like the inner core. From these waves and from the planet's natural ringing modes, scientists have pinned the radius of the outer core at 3,483 kilometres, give or take 5 kilometres. The inner core radius, by comparison, sits at 1,220 kilometres with an uncertainty of 10. The whole layer ends 5,150 kilometres down, at the inner core boundary. Temperatures climb across that span, estimated at roughly 3,000 to 4,500 kelvin in the outer region and 4,000 to 8,000 kelvin near the inner core. Such heat makes the metal a low-viscosity fluid that convects turbulently, never settling into stillness.

• 3. A Frozen Front Advancing

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  Roughly 80,000 tonnes of iron freeze solid every second at the boundary between the two cores. As Earth's core slowly cools, the liquid at the inner core boundary crystallises, so the solid inner core grows at the expense of the outer one. The estimated pace is about 1 millimetre per year, a glacial creep across geological time. This freezing does more than thicken the inner core. As iron locks into solid form, it expels the lighter elements mixed into the fluid, and those castoffs drift upward while heavier material sinks. That sorting of light from heavy is not a mere side effect. It feeds one of the most consequential machines on the planet.

• 4. The Missing Lightness

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  Pure iron is too dense to match what geophysical measurements actually find in the outer core. The layer turns out to be about 5 to 10 percent less dense than iron would be at the temperatures and pressures down there. The only feasible way to explain that gap is to mix in light elements with low atomic numbers. By weight, estimates put the outer core at iron plus 0 to 0.26 percent hydrogen, 0.2 percent carbon, 0.8 to 5.3 percent oxygen, 0 to 4.0 percent silicon, 1.7 percent sulfur, and about 5 percent nickel. Nobody has ever sampled this material directly. Instead the recipe is reconstructed from high-pressure experiments, calculations tied to seismic measurements, models of how Earth gathered itself together, and comparisons with carbonaceous chondrite meteorites against the bulk silicate Earth.

• 5. Clues From The Solar System's Childhood

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  CI chondritic meteorites are thought to preserve the same planet-forming elements, in the same proportions, as the early Solar System. That makes them a baseline. Where the bulk silicate Earth differs from these meteorites, scientists read a story about what sank into the core. The depletion of silicon in Earth's primitive mantle, set against CI meteorites, may mean silicon was pulled down into the core. Even so, a wide range of silicon concentrations across the outer and inner core remains possible. The light elements that could be present were constrained by the conditions of Earth's accretion itself. They had to be abundant during formation, able to dissolve into liquid iron at low pressures, and stable enough not to volatilize and escape as the young planet came together.

• 6. What Hydrogen Would Mean

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  The possible presence of hydrogen in the outer core hints that Earth's water did not all arrive at the very end of its formation. If hydrogen really sits down there, water may have been absorbed into core-forming metals through a hydrous magma ocean. The depletion of siderophile elements in Earth's mantle, compared with chondritic meteorites, is blamed on metal-silicate reactions during core formation. Those reactions hinge on oxygen, silicon, and sulfur, so tighter limits on these elements would sharpen the picture of how the core formed. Accretionary models based on core-mantle element partitioning tend to favour a proto-Earth built from reduced, condensed, volatile-free material. Yet oxidized material from the outer Solar System may have been added near the close of accretion, and better constraints on hydrogen, oxygen, and silicon would test which story holds.

• 7. The Planet's Magnetic Engine

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  Eddy currents swirling through the nickel-iron fluid are, under dynamo theory, the principal source of Earth's magnetic field. The average field strength inside the outer core is estimated at 2.5 millitesla, fifty times stronger than the field measured at the surface. Two forces drive the engine. Thermal convection moves heat, while chemical convection works by ejecting light elements from the inner core, releasing gravitational energy as they float up and denser material falls. Carnot efficiency estimates, carrying large uncertainties, suggest compositional convection supplies about 80 percent of the geodynamo's power and thermal convection about 20 percent. This field is not a curiosity. It shields life from interplanetary radiation and stops the solar wind from stripping the atmosphere away.

• 8. The New Core Paradox

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  For a long time, scientists assumed thermal convection ran the geodynamo before the inner core ever formed. Then came claims that iron's thermal conductivity at core temperatures and pressures is far higher than once believed. If true, the core cooled mostly by conduction rather than convection, which would have starved thermal convection of its power to drive the dynamo. That tension is now called the new core paradox. One proposed escape route requires the early core to have been hot enough to dissolve oxygen, magnesium, silicon, and other light elements. As it cooled and grew supersaturated, those elements would precipitate into the lower mantle as oxides, powering a different flavour of chemical convection. The rate of cooling stays uncertain, but one estimate holds the core would not freeze for roughly 91 billion years, long after the Sun expands, sterilizes the surface, and burns out.