The inner core of the Earth is a primarily solid sphere about 1220 km in radius. Its diameter is only about 70% that of the Moon, and it may be hotter than the Sun’s surface.
The inner core is a solid ball of nickel and iron, which has the pressure of the whole Earth, situated at Earth’s center.
The existence of an inner core distinct from the liquid outer core was discovered in 1936 by seismologist Inge Lehmann using observations of earthquake-generated seismic waves that partly reflect from its boundary and can be detected by sensitive seismographs on the Earth’s surface.
The outer core was thought to be liquid due to its inability to transmit elastic shear waves because only compressional waves are observed to pass through it. The solidity of the inner core has been difficult to establish, because the elastic shear waves that are expected to pass through it are very weak and difficult to detect.
Dziewonski and Gilbert established the consistency of this hypothesis using normal modes of vibration of Earth caused by large earthquakes. Recent claims of detections of inner core transmitted shear waves were initially controversial but are now gaining acceptance.
Based on the abundance of chemical elements in the solar system, their physical properties, and other chemical constraints regarding the remainder of Earth’s volume, the inner core is believed to be composed primarily of a nickel-iron alloy, with very small amounts of some other elements.
Because it is less dense than pure iron, Francis Birch judged that the outer core contains about 10% of a mixture of lighter elements, although these are expected to be less abundant in the solid inner core.