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= Earth's_inner_core =
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Introduction
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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
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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 1400 km for the inner core, not far from the
currently accepted value of 1221 km. In 1938, Beno Gutenberg and
Charles 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 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.
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 world.
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
* the Earth's magnetic field. While it seems to be generated mostly by
fluid and electric currents in the outer core, those currents are
strongly affected by the presence of the solid inner core and by the
heat that flows out of it. (Although made of iron, the core is not
ferromagnetic, due to being above the Curie temperature.)
* the Earth's mass, its gravitational field, and its angular inertia.
These are all affected by the density and dimensions of the inner
layers.
* the natural oscillation frequencies and modes of the whole Earth
oscillations, when large earthquakes make the planet "ring" like a
bell. These oscillations also depend strongly on the inner layers'
density, size, and shape.
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 (), which is about (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 is estimated to be between and , 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 , 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 330 to.
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 (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 5400
K and 5700 K. 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 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 ()
structure. That structure can still admit the inclusion of small
amounts of nickel and other elements.
Structure
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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 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 by slow
thermal convection inside the inner core. T. Yukutake claimed in 1998
that such convective motions were unlikely. However, B. Buffet in 2009
estimated the viscosity of the inner core and found that such
convection could have happened, especially when the core was smaller.
On the other hand, M. Bergman in 1997 proposed that the anisotropy was
due to an observed tendency of iron crystals to grow faster when their
crystallographic axes are aligned with the direction of the cooling
heat flow. He, therefore, proposed that the heat flow out of the inner
core would be biased towards the radial direction.
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 proposed, in 2018, 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, the 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.
In 2023, a study reported new evidence "for an
anisotropically-distinctive innermost inner core" - a ~650-km thick
innermost ball - "and its transition to a weakly anisotropic outer
shell, which could be a fossilized record of a significant global
event from the past." They suggest that atoms in the IIC atoms are
[packed] slightly differently than its outer layer, causing seismic
waves to pass through the IIC at different speeds than through the
surrounding core (P-wave speeds ~4% slower at ~50° from the Earth’s
rotation axis).
* News article about the study:
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 Borneo) than in the "western"
hemisphere (at about 70 °W, roughly under Colombia).
Alboussère and others proposed that this asymmetry 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.
However, in 2017 Frost and Romanowicz disputed those earlier
inferences, claiming that the data shows only a weak anisotropy, with
the speed in the N−S direction being only 0.5% to 1.5% faster than in
equatorial directions, and no clear signs of E−W variation.
Other structure
=================
Other researchers claim that the 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).
Growth
======================================================================
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.
Dynamics
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Because the inner core is not rigidly connected to the Earth's solid
mantle, the possibility that it rotates slightly more quickly or
slowly than the rest of Earth has long been considered. 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. In 1996, X. Song and P. Richards estimated this
"super-rotation" of the inner core relative to the mantle as about one
degree per year. In 2005, they and J. Zhang compared recordings of
"seismic doublets" (recordings by the same station of earthquakes
occurring in the same location on the opposite side of the Earth,
years apart), and revised that estimate to 0.3 to 0.5 degree per year.
In 2023, it was reported that the core stopped spinning faster than
the planet's surface around 2009 and likely is now rotating slower
than it. This is not thought to have major effects and one cycle of
the oscillation is thought to be about seven decades, coinciding with
several other geophysical periodicities, "especially the length of day
and magnetic field".
In 1999, M. Greff-Lefftz and H. Legros noted that the gravitational
fields of the Sun and Moon that are responsible for ocean tides also
apply torques to the Earth, affecting its axis of rotation and a
slowing down of its rotation rate. Those torques are felt mainly by
the crust and mantle, so that their rotation axis and speed may differ
from overall rotation of the fluid in the outer core and the rotation
of the inner core. The dynamics is complicated because of the currents
and magnetic fields in the inner core. They find that the axis of the
inner core wobbles (nutates) slightly with a period of about 1 day.
With some assumptions on the evolution of the Earth, they conclude
that the fluid motions in the outer core would have entered resonance
with the tidal forces at several times in the past (3.0, 1.8, and 0.3
billion years ago). During those epochs, which lasted 200-300 million
years each, the extra heat generated by stronger fluid motions might
have stopped the growth of the inner core.
Age
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Theories about the age of the core are part of theories of the history
of Earth. It is widely believed that the Earth's solid inner core
formed out of an initially completely liquid core as the Earth cooled.
However, the time when this process started is unknown.
|**T** = thermodynamic modeling **P** = paleomagnetism analysis
**(R)** = with radioactive elements **(N)** = without them
Age estimates in billion years from different studies and methods {
!Date !Authors !Age !Method
|**T(N)**
|2001 |Labrosse et al. |1±0.5
|**T(R)**
|2003 |Labrosse |~2
|**P**
|2011 |Smirnov et al. |2-3.5
|**T**
|2014 |Driscoll and Bercovici |0.65
|**T**
|2015 |Labrosse |< 0.7
|**P**
|2015 |Biggin et al. |1-1.5
|**T**
|2016 |Ohta et al. |< 0.7
|**T**
|2016 |Konôpková et al. |< 4.2
|**P**
|2019 |Bono et al. |0.5
|}
Two main approaches have been used to infer the age of the inner core:
thermodynamic modeling of the cooling of the Earth, and analysis of
paleomagnetic evidence. The estimates yielded by these methods vary
from 0.5 to 2 billion years old.
Thermodynamic evidence
========================
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
radioactive elements 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 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.
In 2014, Driscoll and Bercovici published a thermal history of the
Earth that avoided the so-called mantle 'thermal catastrophe' and 'new
core paradox' by invoking 3 TW of radiogenic heating by the decay of
in the core. Such high abundances of K in the core are not supported
by experimental partitioning studies, so such a thermal history
remains highly debatable.
Paleomagnetic evidence
========================
Another way to estimate the age of the Earth is to analyze changes in
the 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 different
dynamic processes in the core that 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 (2.8-2.5 billion years ago) and the Proterozoic (2.5-0.541
billion). They found that the geomagnetic 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 may have been due to growth of the
solid inner core between 3.5-2.0 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
1.0-1.5 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.
In 2016, P. Driscoll published a numerical 'evolving dynamo' model
that made a detailed prediction of the paleomagnetic field evolution
over 0.0-2.0 Ga. The 'evolving dynamo' model was driven by
time-variable boundary conditions produced by the thermal history
solution in Driscoll and Bercovici (2014). The 'evolving dynamo' model
predicted a strong-field dynamo prior to 1.7 Ga that is multipolar, a
strong-field dynamo from 1.0-1.7 Ga that is predominantly dipolar, a
weak-field dynamo from 0.6-1.0 Ga that is a non-axial dipole, and a
strong-field dynamo after inner core nucleation from 0.0-0.6 Ga that
is predominantly dipolar.
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 that provides support for the
predictions by Driscoll (2016). Considering other evidence of high
frequency of 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. A 'News and
Views' by P. Driscoll summarizes the state of the field following the
Bono results. New paleomagnetic data from the Cambrian appear to
support this hypothesis.
See also
======================================================================
* Geodynamics
* Internal structure of Earth
* Iron meteorite
* Thermal history of Earth
* Travel to the Earth's center
References
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{{reflist|30em|refs=
}}
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=========
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