{{For|the cores of other planetary bodies|Planetary core}} {{short description|The innermost part of the Earth, a solid ball of iron-nickel alloy}} {{broader|Structure of the Earth#Core}} [[File:Earth poster.svg|thumb|upright=1.6|The internal structure of Earth]] '''Earth's inner core''' is the innermost [[structure of the Earth|geologic layer]] of the [[Earth]] . It is primarily a [[solid]] [[ball (mathematics)|ball]] with a [[radius]] of about {{convert|1220|km|mi|abbr=off}}, 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 mantle|Earth's mantle]]. The information that we have about it mostly comes from analysis of [[seismic waves]] and the [[Earth's magnetic field|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 {{convert|5700|K|C}} or 9806 °F, which is about the temperature at the surface of the [[Sun|Sun]]. ==Discovery== The Earth was discovered to have a solid inner core distinct from its molten [[Earth's outer core|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 wave]]s reflect off the boundary of the inner core and can be detected by sensitive [[seismographs]] on the Earth's surface. This boundary is known as the [[Keith Edward Bullen|Bullen]] discontinuity, or sometimes as the Lehmann discontinuity. A few years later, in 1940, it was hypothesized that this inner core was made of solid iron; its rigidity was confirmed in 1971. [[Earth's outer core]] was determined to be molten from observations showing that [[compressional wave]]s pass through it, but elastic [[simple shear|shear]] waves do not – or do so only very weakly. The solidity of the inner core had been difficult to establish because the elastic [[shear wave]]s that are expected to pass through a solid mass are very weak and difficult for seismographs on the Earth's surface to detect, since they become so attenuated on their way from the inner core to the surface by their passage through the liquid outer core. Dziewonski and Gilbert established that measurements of [[normal modes|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. The size of the current inner core has been determined by the travel time of the [[seismic wave]]s reflected at the inner core-outer core boundary. Those observations put its radius at about 1200 km, which is about 19% of the radius of the Earth and 70% of that of the Moon. ==Physical properties== ===Seismic wave velocity=== 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 [[seismograph]]s as they reach the surface, all over the globe. Seismic waves include "P" (primary or pressure) waves, that can travel through solid or liquid materials, and "S" (secondary or 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. 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. Thanks to this phenomeon, it is known that the inner core can propagate S waves, and therefore must be solid. In fact, 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).{{rp|fig.2}} 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.{{rp|fig.2}} 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. ===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). Its shape is believed to be very close to an [[oblate ellipsoid]] of revolution, like the surface of the earth, only that more spherical: the [[equatorial bulge|flattening]] ''f'' is estimated to be between 1/400 and 1/416;{{rp|f.2}} 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 very close to 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 {{convert|330|to|360|GPa|atm}}. 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 (about 10 m/s2). ===Density=== The density of the inner core is believed to vary smoothly from about 13.0 kg/L (= g/cm3) 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. ===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 other estimated its temperature as between {{convert|5400|K|sigfig=2}} and {{convert|5700|K|sigfig=2}}. However, in 2013 S. Anzellini and others obtained experimentally a substantially higher temperature for the melting point of iron, 6230 ± 500 K. 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 A. Buffett|B. Buffet]] determined that the average [[geomagnetic field|magnetic field]] in the liquid outer core is about 2.5 [[millitesla]]s (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 [[ocean]]s on the surface. He observed that motion of the liquid through the local magnetic field creates [[electric current]]s, that dissipate energy as heat according to [[Ohm's law]]. This dissipation in turn dampens 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 was solid, they cannot distingush between a perfectly solid material from an extremely [[viscosity|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 [[Pascal (unit)|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 [[Formation and evolution of the Solar System#Terrestrial planets|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 known pressures and estimated 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. Also, if the inner core grows by precipitation of frozen particles falling onto its surface, then some liquid can also be trapped in the pore spaces. In that case, some of this residual fluid may still persist to some small degree in much of its interior.{{cn|date=March 2019}} ==Structure== Many scientists had initially expected that the inner core would be found to be [[homogeneous mixture|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 traight 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, that found the direction of maximum speed is as close to the Earth's rotation axis as can be determined. ===Transverse anisotropy=== However, other authors claim that P-wave speed is faster in other directions, at least in some regons of the inner core. ===Causes of anisotropy=== It has been determined that, at inner core conditions, iron is believed to crystallize as [[allotropes of iron|ε-iron]], a form with the hexagonal close-packed (hcp) structure, whose crystal are strongly anistropic too. A preference for the crystals in the core to align in the north-south direction could account for the observed seismic anomaly. In 1996, S. Yoshida and others proposed that the anisotropy could be caused by a 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. That flow would cause the crystals to partially reorient themselves accoding to the direction of the flow. T. Yukutake claimed in 1998 that [[thermal convection]] inside the inner core did not seem to be a plausible explanation. In 1998, S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time. ===Multiple layers=== In 2002, M. Ishii and A. Dziewoński presented evidence that the solid inner core contained an "innermost inner core" (IMIC) with somewhat different properties than the shell around it. The nature of the differences and radius of the IMIC are still unresolved as of 2019, with proposals for the latter ranging from 300 km to 750 km. A. Wang and X. Song recently proposed a three-layer model, with an "inner inner core" (IIC) with about 500 km radius, an "outer inner core" (OIC) layer about 600 km thick, and an isotropic shell 100 km thick. In this model, the "faster P-wave" direction would be parallel to the Earth's axis in the OIC, but perpendicular to that axis in the IIC. However, conclusion has been disputed by claims that there need not be sharp discontinuities in the inner core, only a gradual change of properties with depth. ===Lateral variation=== In 1997, S. Tanaka and H. Hamaguchi claimed, on the basis of seismic data, that the anisotropy of the inner core material, while oriented N-S, was more pronounced in "eastern" hemisphere of the inner core (at about 110 °E longitude, roughly under [[Indonesia]]) than in the "western" hemisphere (at about 70 °W, roughly under [[Colombia]]. Alboussère and others proposed that it could be due to melting in the Eastern hemisphere and re-crystallization in the Western one. C. Finlay conjectured that this process could explain the asymmetry in the Earth's magnetic field. ===Other structure=== Other researchers claim that he properties of the inner core's surface vary from place to place across distances as small as 1 km. This variation is surprising, since lateral temperature variations along the inner-core boundary are known to be extremely small (this conclusion is confidently constrained by [[magnetic field]] observations).{{cn|date=March 2019}} ==Growth== [[File:Outer_core_convection_rolls.jpg|thumb|upright=1.25|Schematic of the Earth's inner core and outer core motion and the magnetic field it generates.]] The Earth's inner core is thought to be slowly growing as the liquid outer core at the boundary with the inner core cools and solidifies due to the gradual cooling of the Earth's interior (about 100 degrees Celsius per billion years). According to calculations by Alfé and others, as the iron crystallizes onto the inner core, the liquid just above it becomes enriched in oxygen, and therefore less dense than the rest of the outer core. This process creates convection currents in the outer core, which are thought to be the prime driver for the currents that create the Earth's magnetic field. The existence of the inner core also affects the dynamic motions of liquid in the outer core, and thus may help fix the magnetic field.{{Citation needed|date=May 2007}} ==Dynamics== Because the inner core is not rigidly connected to the Earth's solid [[mantle (geology)|mantle]], the possibility that it [[rotation|rotates]] slightly more quickly or slowly than the rest of Earth has long been entertained. In the 1990s, seismologists made various claims about detecting this kind of super-rotation by observing changes in the characteristics of [[seismic waves]] passing through the inner core over several decades, using the aforementioned property that it transmits waves more quickly in some directions. Estimates of this super-rotation are around one degree of extra rotation per year. == Age == Theories about the age of the core are necessarily part of theories of the [[history of Earth]] as a whole. This has been a long debated topic and is still under discussion at the present time. It is widely believed that the Earth’s solid inner core formed out of an initially completely liquid core as the Earth cooled down. However, there is still no firm evidence about the time when this process started. {|class="table floatright" |+'''Age estimates from
different studies and methods''' | {|class="wikitable" |+align=bottom style="font-weight:normal; text-align:center;|'''T''' = thermodynamic modeling
'''P''' = paleomagnetism analysis
'''(R)''' = with radioactive elements
'''(N)''' = without them !Date !Authors !Age !Method |- |2001 |Labrosse et al. |1±0.5 |'''T(N)''' |- |2003 |Labrosse |~2 |'''T(R)''' |- |2011 |Smirnov et al. |2–3.5 |'''P''' |- |2015 |Labrosse |< 0.7 |'''T''' |- |2015 |Biggin et al. |1–1.5 |'''P''' |- |2016 |Ohta et al. |< 0.7 |'''T''' |- |2016 |style="white-space: nowrap;"|Konôpková et al. |< 4.2 |'''T''' |- |2019 |Bono et al. |0.5 |'''P''' |} |} Two main approaches have been used to infer the age of the inner core: [[thermodynamics|thermodynamic]] modeling of the cooling of the Earth, and analysis of [[paleomagnetism|paleomagnetic]] evidence. The estimates yielded by these methods still vary over a large range, from 0.5 to 2 billion years old. === Thermodynamic evidence === [[File:Heat_flow_of_the_inner_earth.jpg|thumb|upright=1.35|Heat flow of the inner earth, according to S. T. Dye and R. Arevalo.]] One of the ways to estimate the age of the inner core is by modeling the cooling of the Earth, constrained by a minimum value for the [[heat flux]] at the [[core–mantle boundary]] (CMB). That estimate is based on the prevailing theory that the Earth's magnetic field is primarily triggered by [[convection]] currents in the liquid part of the core, and the fact that a minimum heat flux is required to sustain those currents. The heat flux at the CMB at present time can be reliably estimated because it is related to the measured heat flux at Earth’s surface and to the measured rate of [[mantle convection]]. In 2001, S. Labrosse and others, assuming that there were no [[radionuclide|radioactive element]]s in the core, gave an estimate of 1±0.5 billion years for the age of the inner core — considerably less than the estimated age of the Earth and of its liquid core (about 4.5 billion years) In 2003, the same group concluded that, if the core contained a reasonable amount of radioactive elements, the inner core's age could be a few hundred million years older. In 2012, theoretical computations by M. Pozzo and others indicated that the [[electrical resistivity and conductivity|electrical conductivity]] of iron and other hypothetical core materials, at the high pressures and temperatures expected there, were two or three times higher than assumed in previous research. These predictions were confirmed in 2013 by measurements by Gomi and others. The higher values for electrical conductivity led to increased estimates of the [[thermal conductivity]], to 90 W/m/K; which, in turn, lowered estimates of its age to less than 700 million years old. However, in 2016 Konôpková and others directly measured the thermal conductivity of solid iron at inner core conditions, and obtained a much lower value, 18-44 W/m/K. With those values, they obtained an upper bound of 4.2 billion years for the age of the inner core, compatible with the paleomagnetic evidence. === Paleomagnetic evidence === Another way to estimate the age of the Earth is to analyze changes in the [[geomagnetic field|magnetic field of Earth]] during its history, as trapped in rocks that formed at various times (the "paleomagnetic record"). The presence or absence of the solid inner core could result in very different dynamic processes in the core which could lead to noticeable changes in the magnetic field. In 2011, Smirnov and others published an analysis of the paleomagnetism in a large sample of rocks that formed in the [[Neoarchean]] (2.8 to 2.5 billion years ago) and the [[Proterozoic]] (2.5 to 0.541 billion). They found that the gomagnetic field was closer to that of a magnetic [[dipole]] during the Neoarchean than after it. They interpreted that change as evidence that the dynamo effect was more deeply seated in the core during that epoch, whereas in the later time currents closer to the core-mantle boundary grew in importance. They further speculate that the change my have been due to growth of the solid inner core between 3.5 an 2 billion years ago. In 2015, Biggin and others published the analysis of an extensive and carefully selected set of [[Precambrian]] samples and observed a prominent increase in the Earth's magnetic field strength and variance around 1–1.5 billion years ago. This change had not been noticed before due to the lack of sufficient robust measurements. They speculated that the change could be due to the birth of Earth’s solid inner core. From their age estimate they derived a rather modest value for the thermal conductivity of the outer core, that allowed for simpler models of the Earth's thermal evolution. An analysis of rock samples from the [[Ediacaran]] epoch (formed about 565 million years ago), published by Bono and others in 2019, revealed unusually low intensity and two distinct directions for the geomagnetic field during that time. Considering other evidence of high frequency of [[geomagnetic reversal|magnetic field reversals]] around that time, they speculate that those anomalies could be due to the onset of formation of the inner core, which would then be 0.5 billion years old. ==See also== {{Wikibooks |Historical Geology|Structure of the Earth}} * [[Geodynamics]] * [[Iron meteorite]] * [[Structure of the Earth]] * ''[[Travel to the Earth's center]]'' ==References== {{cite journal | author=J.A. 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