What is Earth's inner core made of?

Answer

Intense pressure keeps it solid even though it is thought to be as hot as the Sun's surface. Temperature figures are estimates.

Context

The inner core is a ball about 1,220 km in radius, a little smaller than the Moon, lying roughly 5,150 km beneath the surface. Its temperature is estimated at around 5,000 to 6,000 degrees Celsius, comparable to the Sun's surface, yet it is solid. The reason is pressure: at more than three million times atmospheric pressure, the melting point of iron rises so high that the metal stays crystalline despite the heat.

Nobody has sampled it, so the evidence is seismic. In 1936 the Danish seismologist Inge Lehmann explained odd earthquake waves by proposing a distinct inner region inside the liquid outer core. Later studies of waves and of the planet's free oscillations indicated the region is solid. Its density and wave speeds suggest iron-nickel alloy mixed with a small share of lighter elements such as sulfur, silicon or oxygen, whose exact identities remain uncertain. Iron meteorites, thought to be fragments of ancient asteroid cores, serve as a rough compositional analogue.

The core is growing slowly as the liquid outer core cools and freezes onto it, by roughly a millimeter per year according to common estimates, and its age is often put at around one billion years, though the figure is debated. Movement of the liquid outer core around it generates Earth's magnetic field. Whether the inner core rotates slightly faster or slower than the mantle has been argued in several seismic studies with differing conclusions.