Inner core

Earth's inner core is the innermost geologic layer of the Earth. It is primarily a solid ball with a radius of about 1,220 kilometres (760 miles), which is about 20% of the Earth's radius and 70% of the Moon's radius. Earth's inner core is the innermost geologic layer of the Earth. It is primarily a solid ball with a radius of about 1,220 kilometres (760 miles), which is about 20% of the Earth's radius and 70% of the Moon's radius. There are no samples of the Earth's core available for direct measurement, as there are for the Earth's mantle. Information about the Earth's core mostly comes from analysis of seismic waves and the magnetic field. The inner core is believed to be composed of an iron–nickel alloy with some other elements. The temperature at the inner core's surface is estimated to be approximately 5,700 K (5,430 °C) or 9806 °F, which is about the temperature at the surface of the Sun. The Earth was discovered to have a solid inner core distinct from its molten outer core in 1936, by the Danish seismologist Inge Lehmann, who deduced its presence by studying seismograms from earthquakes in New Zealand. She observed that the seismic waves reflect off the boundary of the inner core and can be detected by sensitive seismographs on the Earth's surface. She inferred a radius of 1400 km for the inner core, not very far from the currently accepted value of 1221 km. In 1938, B. Gutenberg and C. Richter analyzed a more extensive set of data and estimated the thickness of the outer core as 1950 km with a steep but continuous 300 km thick transition to the inner core; implying a radius between 1230 and 1530 km for the inner core.:p.372 A few years later, in 1940, it was hypothesized that this inner core was made of solid iron. In 1952, F. Birch published a detailed analysis of the available data and concluded that the inner core was probably crystalline iron. The boundary between the inner and outer cores is sometimes called the 'Lehmann discontinuity', although the name usually refers to another discontinuity. The name 'Bullen' or 'Lehmann-Bullen discontinuity', after K. Bullen has been proposed, but its use seems to be rare. The rigidity of the inner core was confirmed in 1971. Dziewoński and Gilbert established that measurements of normal modes of vibration of Earth caused by large earthquakes were consistent with a liquid outer core. In 2005, shear waves were detected passing through the inner core; these claims were initially controversial, but are now gaining acceptance. Almost all direct measurements that we have about the physical properties of the inner core are the seismic waves that pass through it. The most informative waves are generated by deep earthquakes, 30 km or more below the surface of the Earth (where the mantle is relatively more homogeneous) and recorded by seismographs as they reach the surface, all over the globe. Seismic waves include 'P' (primary or pressure) waves, compressional waves that can travel through solid or liquid materials, and 'S' (secondary or shear) shear waves that can only propagate through rigid elastic solids. The two waves have different velocities and are damped at different rates as they travel through the same material. Of particular interest are the so-called 'PKiKP' waves—pressure waves (P) that start near the surface, cross the mantle-core boundary, travel through the core (K), are reflected at the inner core boundary (i), cross again the liquid core (K), cross back into the mantle, and are detected as pressure waves (P) at the surface. Also of interest are the 'PKIKP' waves, that travel through the inner core (I) instead of being reflected at its surface (i). Those signals are easier to interpret when the path from source to detector is close to a straight line—namely, when the receiver is just above the source for the reflected PKiKP waves, and antipodal to it for the transmitted PKIKP waves.