Earth's inner core
Introduction
The Earth's inner core is the innermost part of our planet, a solid ball of iron-nickel alloy with minor elements. It is located beneath the outer core, and its radius is roughly one-fifth of Earth's. The existence of the inner core was proposed by Danish seismologist Inge Lehmann in 1936, based on observations of seismic waves.
Discovery
The discovery of the inner core is credited to Inge Lehmann, who deduced its presence from the seismic data that was available in the 1930s. She observed that some P-waves, which should have been deflected by the core, were in fact recorded at seismic stations. Lehmann interpreted this as evidence of a central core that reflected these waves.
Composition
The inner core is primarily composed of an iron–nickel alloy, with small amounts of elements such as oxygen, silicon, and sulfur. These lighter elements are believed to be present due to the extreme pressure conditions that exist in the core, which allows these elements to mix with the predominantly iron-nickel material.
Structure and Properties
The inner core is a solid sphere with a radius of about 1,220 kilometers, which is about 70% of the Moon's radius. It is surrounded by the liquid outer core, and is under immense pressure, estimated to be around 330 to 360 gigapascals. This is over 3 million times more atmospheric pressure than at sea level. The temperature of the inner core is estimated to be between 5700 and 7000 K, which is hotter than the surface of the Sun.
Rotation
The inner core is thought to rotate at a different speed than the rest of the planet. This was first proposed in the 1990s, and has been supported by various studies since. The exact rate of this rotation is still a matter of ongoing research, but it is thought to be approximately 0.1–0.5 degrees per year faster than the surface rotation.
Seismic Activity
Seismic waves provide the primary means of studying the inner core, as direct observation and sampling are currently impossible. Two types of seismic waves are particularly important: P-waves, which are compressional waves that travel through solids and liquids, and S-waves, which are shear waves that only travel through solids. The behavior of these waves as they pass through the Earth gives scientists valuable information about the properties of the inner core.
Magnetic Field
The Earth's magnetic field is believed to be generated by the dynamo effect in the liquid outer core, but the solid inner core plays a crucial role in this process. The rotation of the inner core, along with convection currents in the outer core, help to drive the dynamo that generates the magnetic field.
Inner Core Boundary
The boundary separating the inner core from the outer core, known as the Inner Core Boundary (ICB), is a transition zone that is of great interest to geophysicists. The ICB is marked by a sharp change in seismic wave velocities, indicating a change from the liquid outer core to the solid inner core.