Earth's inner core grows by about 80,000 tonnes of iron every second
Nearly 2,900 kilometres beneath our feet lies an ocean of molten iron and nickel about 2,260 kilometres deep. As the planet slowly cools, its base freezes onto the solid inner core, roughly a millimetre a year. Its churning generates the magnetic field that shields life from radiation.
The outer core begins at the core-mantle boundary around 2,889 kilometres down and ends at about 5,150 kilometres, where the solid inner core starts. Seismic measurements put its outer radius at 3,483 kilometres, give or take 5, and the inner core's at about 1,220. We know it is liquid because seismic shear waves cannot pass through it. It shares much of the inner core's makeup but stays molten because the pressure there is not high enough to force it solid.
Temperatures are estimated at about 3,000 to 4,500 kelvin near its top and 4,000 to 8,000 kelvin close to the inner core. At that heat it behaves as a runny, turbulently convecting fluid. Yet it cannot be pure iron or iron-nickel: it is some 5 to 10 percent less dense than those metals would be under the same conditions, so lighter elements must be mixed in. Nobody can sample it, so researchers estimate them through high-pressure experiments, seismic calculations, models of how Earth formed and comparisons with primitive meteorites thought to preserve early Solar System proportions.
Those light elements matter for Earth's history. Candidates had to be plentiful during formation, dissolve in liquid iron and avoid boiling away. Oxygen, silicon and sulfur govern the reactions that stripped iron-loving elements from the mantle, and possible hydrogen in the core hints that water arrived before the final stages of Earth's assembly, absorbed through a hydrous magma ocean.
The magnetic field depends on motion. Heat drives convection, and so does chemistry: light elements rejected by the freezing inner core rise while denser material sinks, releasing energy for the geodynamo. Claims from 2020 that iron conducts heat far better than assumed would limit thermal convection's role. One estimate says the core will not freeze for about 91 billion years, long after the Sun expands.
Source: Earth's outer core
