Earth's outer core is a fluid layer about thick, composed of mostly
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
that lies above Earth's solid
inner core and below its
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
.
The outer core begins approximately beneath Earth's surface at the
core-mantle boundary and ends beneath Earth's surface at the
inner core boundary.
Properties
Unlike Earth's
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
,
inner core, its outer core is
liquid. Evidence for a fluid outer core includes
seismology which shows that
seismic shear-waves are not transmitted through the outer core. Although having a composition similar to Earth's solid
inner core, the outer core remains
liquid as there is not enough pressure to keep it in a
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
state.
Seismic inversions of
body waves
Body may refer to:
In science
* Physical body, an object in physics that represents a large amount, has mass or takes up space
* Body (biology), the physical material of an organism
* Body plan, the physical features shared by a group of animal ...
and
normal modes constrain the radius of the outer core to be 3483 km with an uncertainty of 5 km, while that of the inner core is 1220±10 km.
Estimates for the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
of the outer core are about in its outer region and near the
inner core.
Modeling has shown that the outer core, because of its high
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
, is a low-
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the int ...
fluid that convects
turbulently.
The
dynamo theory sees
eddy currents in the
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
-
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
fluid of the outer core as the principal source of
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...
. The average
magnetic field strength in Earth's outer core is estimated to be 2.5
millitesla, 50 times stronger than the
magnetic field at the surface.
As
Earth's core cools, the
liquid at the
inner core boundary freezes, causing the solid
inner core to grow at the expense of the outer core, at an estimated rate of 1 mm per year. This is approximately 80,000 tonnes of iron per second.
Light elements of Earth's outer core
Composition
Earth's outer core cannot be entirely constituted of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
or
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
-
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductili ...
because their
densities
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
are higher than geophysical measurements of the
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of Earth's outer core.
In fact, Earth's outer core is approximately 5 to 10 percent lower
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
than
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
at
Earth's core temperatures
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
and
pressures.
Hence it has been proposed that light
elements with low
atomic numbers
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of ever ...
comprise part of Earth's outer core, as the only feasible way to lower its
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
.
Although Earth's outer core is inaccessible to direct sampling,
the composition of light
elements can be meaningfully constrained by high-
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
experiments, calculations based on
seismic measurements, models of
Earth's accretion, and
carbonaceous chondrite meteorite comparisons with
bulk silicate Earth (BSE).
Recent estimates are that Earth's outer core is composed of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
along with 0 to 0.26 percent
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
, 0.2 percent
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
, 0.8 to 5.3 percent
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
, 0 to 4.0 percent
silicon, 1.7 percent
sulfur, and 5 percent
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
by weight, and the
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
of the
core-mantle boundary and the
inner core boundary ranges from 4,137 to 4,300
K and from 5,400 to 6,300
K respectively.
Constraints
= Accretion
=
The variety of light
elements present in Earth's outer core is constrained in part by
Earth's accretion.
Namely, the light
elements contained must have been abundant during Earth's formation, must be able to partition into
liquid iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
at low
pressures, and must not volatilize and escape during Earth's
accretionary process.
= CI chondrites
=
CI chondritic meteorites are believed to contain the same planet-forming
elements in the same
proportions as in the early
Solar System,
so differences between
CI meteorites and
BSE can provide insights into the light
element composition of Earth's outer core.
For instance, the depletion of
silicon in
BSE compared to
CI meteorites may indicate that
silicon was absorbed into
Earth's core; however, a wide range of
silicon concentrations in Earth's outer and
inner core is still possible.
Implications for Earth's accretion and core formation history
Tighter constraints on the concentrations of light
elements in Earth's outer core would provide a better understanding of
Earth's accretion and
core formation history.
Consequences for Earth's accretion
Models of
Earth's accretion could be better tested if we had better constraints on light
element concentrations in Earth's outer core.
For example,
accretionary models based on core-mantle element partitioning tend to support proto-Earths constructed from reduced, condensed, and volatile-free material,
despite the possibility that
oxidized
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
material from the outer
Solar System was
accreted towards the conclusion of
Earth's accretion.
If we could better constrain the
concentrations of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
,
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
, and
silicon in Earth's outer core, models of
Earth's accretion that match these
concentrations would presumably better constrain Earth’s formation.
Consequences for Earth's core formation
The depletion of
siderophile elements in
Earth's mantle compared to
chondritic meteorites is attributed to metal-silicate reactions during formation of
Earth's core.
These reactions are dependent on
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
,
silicon, and
sulfur,
so better constraints on
concentrations of these elements in Earth's outer core will help elucidate the conditions of formation of
Earth's core.
In another example, the possible presence of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
in Earth's outer core suggests that the
accretion
Accretion may refer to:
Science
* Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity
* Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...
of Earth’s
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
was not limited to the final stages of
Earth's accretion and that
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
may have been absorbed into core-forming metals through a hydrous magma ocean.
Implications for Earth's magnetic field
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...
is driven by
thermal convection
Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes o ...
and also by
chemical convection, the exclusion of light
elements from the
inner core, which float upward within the fluid outer core while
denser
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
elements sink.
This
chemical convection releases
gravitational energy that is then available to power the
geodynamo that produces
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...
.
Carnot efficiencies with large uncertainties suggest that compositional and
thermal convection
Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes o ...
contribute about 80 percent and 20 percent respectively to the power of Earth's
geodynamo.
Traditionally it was thought that prior to the formation of
Earth's inner core Earth's
geodynamo was mainly driven by
thermal convection
Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes o ...
.
However, recent claims that the
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
at
core temperatures
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
and
pressures is much higher than previously thought imply that core cooling was largely by conduction not convection, limiting the ability of
thermal convection
Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes o ...
to drive the
geodynamo.
This conundrum is known as the new "core paradox."
An alternative process that could have sustained Earth's
geodynamo requires
Earth's core to have initially been hot enough to dissolve
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
,
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
,
silicon, and other light
elements.
As the
Earth's core began to cool, it would become
supersaturated
In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a li ...
in these light
elements that would then
precipitate into the
lower mantle forming
oxides leading to a different variant of
chemical convection.
References
External links
{{Authority control
Structure of the Earth
Geomagnetism