Innermost part of Earth, a solid ball of iron-nickel alloy
Earth's inner core is the innermost geologic layer of the planet Earth. It is primarily a solid ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius.
There are no samples of the core accessible for direct measurement, as there are for Earth's mantle. The characteristics of the core have been deduced mostly from measurements of seismic waves and Earth's magnetic field. The inner core is believed to be composed of an iron–nickel alloy with some other elements. The temperature at its surface is estimated to be approximately , about the temperature at the surface of the Sun.
The inner core is solid at high temperature because of its high pressure, in accordance with the Simon-Glatzel equation.
Scientific history.
Earth was discovered to have a solid inner core distinct from its molten Earth's outer core in 1936, by the Danish seismologist Inge Lehmann's study of seismograms from earthquakes in New Zealand, detected by sensitive seismographs on the Earth's surface. She deduced that the seismic waves reflect off the boundary of the inner core and inferred a radius of for the inner core, not far from the currently accepted value of . In 1938, Beno Gutenberg and Charles Richter analyzed a more extensive set of data and estimated the thickness of the outer core as with a steep but continuous thick transition to the inner core, implying a radius between for the inner core.
A few years later, in 1940, it was hypothesized that this inner core was made of solid iron. In 1952, Francis 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 Keith Edward Bullen, has been proposed, but its use seems to be rare. The rigidity of the inner core was confirmed in 1971.
Adam Dziewonski and James Freeman 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.
Data sources.
Seismic waves.
Almost all measurements that scientists have about the physical properties of the inner core are the seismic waves that pass through it. Deep earthquakes generate the most informative waves, 30 km or more below the surface of the Earth (where the mantle is relatively more homogeneous) and are recorded by seismographs as they reach the surface, all over the globe.
Seismic waves include "P" (primary or pressure) 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 the liquid core (K) again, 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.
While S waves cannot reach or leave the inner core as such, P waves can be converted into S waves, and vice versa, as they hit the boundary between the inner and outer core at an oblique angle. The "PKJKP" waves are similar to the PKIKP waves, but are converted into S waves when they enter the inner core, travel through it as S waves (J), and are converted again into P waves when they exit the inner core. Thanks to this phenomenon, it is known that the inner core can propagate S waves, and therefore must be solid.
Other sources.
Other sources of information about the inner core include
Physical properties.
Seismic wave velocity.
The velocity of the S waves in the core varies smoothly from about 3.7 km/s at the center to about 3.5 km/s at the surface. That is considerably less than the velocity of S waves in the lower crust (about 4.5 km/s) and less than half the velocity in the deep mantle, just above the outer core (about 7.3 km/s).
The velocity of the P-waves in the core also varies smoothly through the inner core, from about 11.4 km/s at the center to about 11.1 km/s at the surface. Then the speed drops abruptly at the inner-outer core boundary to about 10.4 km/s.
Size and shape.
On the basis of the seismic data, the inner core is estimated to be about 1221 km in radius (2442 km in diameter), which is about 19% of the radius of the Earth and 70% of the radius of the Moon.
Its volume is about 7.6 billion cubic km (7.6 × 1018 m3), which is about ;templatestyles src="Fraction/styles.css" / ;1⁄146 (0.69%) of the volume of the whole Earth.
Its shape is believed to be close to an oblate ellipsoid of revolution, like the surface of the Earth, only more spherical: the flattening  "f"  is estimated to be between ;templatestyles src="Fraction/styles.css" / ;1⁄400 and ;templatestyles src="Fraction/styles.css" / ;1⁄416, meaning that the radius along the Earth's axis is estimated to be about 3 km shorter than the radius at the equator. In comparison, the flattening of the Earth as a whole is close to ;templatestyles src="Fraction/styles.css" / ;1⁄300, and the polar radius is 21 km shorter than the equatorial one.
Pressure and gravity.
The pressure in the Earth's inner core is slightly higher than it is at the boundary between the outer and inner cores: It ranges from about .
The acceleration of gravity at the surface of the inner core can be computed to be 4.3 m/s2; which is less than half the value at the surface of the Earth (9.8 m/s2).
Density and mass.
The density of the inner core is believed to vary smoothly from about 13.0 kg/L (= g/cm3 = t/m3) at the center to about 12.8 kg/L at the surface. As it happens with other material properties, the density drops suddenly at that surface: The liquid just above the inner core is believed to be significantly less dense, at about 12.1 kg/L. For comparison, the average density in the upper 100 km of the Earth is about 3.4 kg/L.
That density implies a mass of about 1023 kg for the inner core, which is ;templatestyles src="Fraction/styles.css" / ;1⁄60 (1.7%) of the mass of the whole Earth.
Temperature.
The temperature of the inner core can be estimated from the melting temperature of impure iron at the pressure which iron is under at the boundary of the inner core (about 330 GPa). From these considerations, in 2002, D. Alfè and others estimated its temperature as between and . However, in 2013, S. Anzellini and others obtained experimentally a substantially higher temperature for the melting point of iron, .
Iron can be solid at such high temperatures only because its melting temperature increases dramatically at pressures of that magnitude (see the Clausius–Clapeyron relation).
Magnetic field.
In 2010, Bruce Buffett determined that the average magnetic field in the liquid outer core is about 2.5 milliteslas (25 gauss), which is about 40 times the maximum strength at the surface. He started from the known fact that the Moon and Sun cause tides in the liquid outer core, just as they do on the oceans on the surface. He observed that motion of the liquid through the local magnetic field creates electric currents, that dissipate energy as heat according to Ohm's law. This dissipation, in turn, damps the tidal motions and explains previously detected anomalies in Earth's nutation. From the magnitude of the latter effect he could calculate the magnetic field. The field inside the inner core presumably has a similar strength. While indirect, this measurement does not depend significantly on any assumptions about the evolution of the Earth or the composition of the core.
Viscosity.
Although seismic waves propagate through the core as if it were solid, the measurements cannot distinguish a solid material from an extremely viscous one. Some scientists have therefore considered whether there may be slow convection in the inner core (as is believed to exist in the mantle). That could be an explanation for the anisotropy detected in seismic studies. In 2009, B. Buffett estimated the viscosity of the inner core at 1018 Pa·s, which is a sextillion times the viscosity of water, and more than a billion times that of pitch.
Composition.
There is still no direct evidence about the composition of the inner core. However, based on the relative prevalence of various chemical elements in the Solar System, the theory of planetary formation, and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of an iron–nickel alloy.
At the estimated pressures and temperatures of the core, it is predicted that pure iron could be solid, but its density would exceed the known density of the core by approximately 3%. That result implies the presence of lighter elements in the core, such as silicon, oxygen, or sulfur, in addition to the probable presence of nickel. Recent estimates (2007) allow for up to 10% nickel and 2–3% of unidentified lighter elements.
According to computations by D. Alfè and others, the liquid outer core contains 8–13% of oxygen, but as the iron crystallizes out to form the inner core the oxygen is mostly left in the liquid.
Laboratory experiments and analysis of seismic wave velocities seem to indicate that the inner core consists specifically of ε-iron, a crystalline form of the metal with the hexagonal close-packed (HCP) structure. That structure can still admit the inclusion of small amounts of nickel and other elements.
Structure.
Many scientists had initially expected that the inner core would be found to be homogeneous, because that same process should have proceeded uniformly during its entire formation. It was even suggested that Earth's inner core might be a single crystal of iron.
Axis-aligned anisotropy.
In 1983, G. Poupinet and others observed that the travel time of PKIKP waves (P waves that travel through the inner core) was about 2 seconds less for straight north–south paths than straight paths on the equatorial plane. Even taking into account the flattening of the Earth at the poles (about 0.33% for the whole Earth, 0.25% for the inner core) and crust and upper mantle heterogeneities, this difference implied that P waves (of a broad range of wavelengths) travel through the inner core about 1% faster in the north–south direction than along directions perpendicular to that.
This P wave speed anisotropy has been confirmed by later studies, including more seismic data and study of the free oscillations of the whole Earth. Some authors have claimed higher values for the difference, up to 4.8%; however, in 2017 Daniel Frost and Barbara Romanowicz confirmed that the value is between 0.5% and 1.5%.
Non-axial anisotropy.
Some authors have claimed that P wave speed is faster in directions that are oblique or perpendicular to the N−S axis, at least in some regions of the inner core. However, these claims have been disputed by Frost and Romanowicz, who instead claim that the direction of maximum speed is as close to the Earth's rotation axis as can be determined.
Causes of anisotropy.
Laboratory data and theoretical computations indicate that the propagation of pressure waves in the HCP crystals of ε-iron are strongly anisotropic, too, with one "fast" axis and two equally "slow" ones. A preference for the crystals in the core to align in the north–south direction could account for the observed seismic anomaly.
One phenomenon that could cause such partial alignment is slow flow ("creep") inside the inner core, from the equator towards the poles or vice versa. That flow would cause the crystals to partially reorient themselves according to the direction of the flow. In 1996, S. Yoshida and others proposed that such a flow could be caused by higher rate of freezing at the equator than at polar latitudes. An equator-to-pole flow then would set up in the inner core, tending to restore the isostatic equilibrium of its surface.
Others suggested that the required flow could be caused