Earth's Outer Core
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Earth's Outer Core
The internal structure of Earth

Earth's outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle.[1][2][3] The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at the core-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.[4]

Properties

Unlike Earth's solid, inner core, its outer core is liquid.[5] Evidence for a fluid outer core includes seismology which shows that seismic shear-waves are not transmitted through the outer core.[6] 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 state.

Seismic inversions of body waves 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.[7]

Estimates for the temperature of the outer core are about 3,000-4,500 K (2,700-4,200 °C; 4,900-7,600 °F) in its outer region and 4,000-8,000 K (3,700-7,700 °C; 6,700-14,000 °F) near the inner core.[8] Modeling has shown that the outer core, because of its high temperature, is a low-viscosity fluid that convects turbulently.[8] The dynamo theory sees eddy currents in the nickel-iron fluid of the outer core as the principal source of Earth's magnetic field. 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.[9][10]

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.[11]

Light elements of Earth's outer core

Composition

Earth's outer core cannot be entirely constituted of iron or iron-nickel alloy because their densities are higher than geophysical measurements of the density of Earth's outer core.[12][13][14][15] In fact, Earth's outer core is approximately 5 to 10 percent lower density than iron at Earth's core temperatures and pressures.[15][16][17] Hence it has been proposed that light elements with low atomic numbers comprise part of Earth's outer core, as the only feasible way to lower its density.[14][15][16] Although Earth's outer core is inaccessible to direct sampling,[14][15][18] the composition of light elements can be meaningfully constrained by high-pressure experiments, calculations based on seismic measurements, models of Earth's accretion, and carbonaceous chondrite meteorite comparisons with bulk silicate Earth (BSE).[12][14][15][16][18][19] Recent estimates are that Earth's outer core is composed of iron along with 0 to 0.26 percent hydrogen, 0.2 percent carbon, 0.8 to 5.3 percent oxygen, 0 to 4.0 percent silicon, 1.7 percent sulfur, and 5 percent nickel by weight, and the temperature 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.[14]

Constraints

Accretion
An artist's illustration of what Earth might have looked like early in its formation. In this image, the Earth looks molten, with red gaps of lava separating with jagged and seemingly-cooled plates of material.
An artist's illustration of what Earth might have looked like early in its formation.

The variety of light elements present in Earth's outer core is constrained in part by Earth's accretion.[16] Namely, the light elements contained must have been abundant during Earth's formation, must be able to partition into liquid iron at low pressures, and must not volatilize and escape during Earth's accretionary process.[14][16]

CI chondrites

CI chondritic meteorites are believed to contain the same planet-forming elements in the same proportions as in the early Solar System,[14] so differences between CI meteorites and BSE can provide insights into the light element composition of Earth's outer core.[20][14] 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.[14][21][22]

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.[14][19][23]

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.[14][23] For example, accretionary models based on core-mantle element partitioning tend to support proto-Earths constructed from reduced, condensed, and volatile-free material,[14][19][23] despite the possibility that oxidized material from the outer Solar System was accreted towards the conclusion of Earth's accretion.[14][19] If we could better constrain the concentrations of hydrogen, oxygen, and silicon in Earth's outer core, models of Earth's accretion that match these concentrations would presumably better constrain Earth's formation.[14]

Consequences for Earth's core formation

A diagram of Earth's differentiation. The diagram displays Earth's different layers and how dense materials move towards Earth's core.
A diagram of Earth's differentiation. The light elements sulfur, silicon, oxygen, carbon, and hydrogen may constitute part of the outer core due to their abundance and ability to partition into liquid iron under certain conditions.

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.[24] These reactions are dependent on oxygen, silicon, and sulfur,[14][25][24] so better constraints on concentrations of these elements in Earth's outer core will help elucidate the conditions of formation of Earth's core.[14][23][25][24][26]

In another example, the possible presence of hydrogen in Earth's outer core suggests that the accretion of Earth's water[14][27][28] was not limited to the final stages of Earth's accretion[23] and that water may have been absorbed into core-forming metals through a hydrous magma ocean.[14][29]

Implications for Earth's magnetic field

A diagram of Earth's geodynamo and magnetic field, which could have been driven in Earth's early history by the crystallization of magnesium oxide, silicon dioxide, and iron(II) oxide. Convection of Earth's outer core is displayed alongside magnetic field lines.
A diagram of Earth's geodynamo and magnetic field, which could have been driven in Earth's early history by the crystallization of magnesium oxide, silicon dioxide, and iron(II) oxide.

Earth's magnetic field is driven by thermal convection and also by chemical convection, the exclusion of light elements from the inner core, which float upward within the fluid outer core while denser elements sink.[17][30] This chemical convection releases gravitational energy that is then available to power the geodynamo that produces Earth's magnetic field.[30] Carnot efficiencies with large uncertainties suggest that compositional and thermal convection contribute about 80 percent and 20 percent respectively to the power of Earth's geodynamo.[30] Traditionally it was thought that prior to the formation of Earth's inner core Earth's geodynamo was mainly driven by thermal convection.[30] However, recent claims that the thermal conductivity of iron at core temperatures and pressures is much higher than previously thought imply that core cooling was largely by conduction not convection, limiting the ability of thermal convection to drive the geodynamo.[14][17] This conundrum is known as the new "core paradox."[14][17] An alternative process that could have sustained Earth's geodynamo requires Earth's core to have initially been hot enough to dissolve oxygen, magnesium, silicon, and other light elements.[17] As the Earth's core began to cool, it would become supersaturated in these light elements that would then precipitate into the lower mantle forming oxides leading to a different variant of chemical convection.[14][17]

References

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