Earth's outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostlyiron andnickel that lies above Earth's solidinner core and below itsmantle.[1][2][3] The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at thecore-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.[4]
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The outer core of Earth is liquid, unlike itsinner core, which is solid.[5] Evidence for a fluid outer core includesseismology which shows thatseismicshear-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 ofbody waves andnormal 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]: 94
Estimates for thetemperature 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 convectsturbulently.[8] Thedynamo theory seeseddy currents in the nickel-iron fluid of the outer core as the principal source ofEarth's magnetic field. The averagemagnetic field strength in Earth's outer core is estimated to be 2.5millitesla, 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. This is approximately 80,000 tonnes of iron per second.[11]
Earth's outer core cannot be entirely constituted of iron or iron-nickelalloy because their densities are higher than geophysical measurements of thedensity of Earth's outer core.[12][13][14][15] The outer core is approximately 5 to 10 percent lower density thaniron at Earth's coretemperatures andpressures.[15][16][17] Hence it has been proposed that lightelements with lowatomic numbers compose part of Earth's outer core, as the only feasible way to lower its density.[14][15][16]
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 byEarth's accretion.[16] Namely, the light elements contained must have been abundant during Earth's formation, must be able to partition intoliquid iron at lowpressures, and must not volatilize and escape during Earth's accretionary process.[14][16]
CI chondritic meteorites are believed to contain the same planet-forming elements in the sameproportions as in the earlySolar System,[14] so differences between CI meteorites andBSE can provide insights into the light element composition of Earth's outer core.[20][14] For instance, the depletion ofsilicon in Earth'sprimitive mantle 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 andinner 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 ofEarth's accretion andcore formation history.[14][19][23]
Models of Earth's accretion could be better tested if we had better constraints on light elementconcentrations 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 thatoxidized material from the outerSolar System was accreted towards the conclusion ofEarth's accretion.[14][19] If we could better constrain the concentrations ofhydrogen,oxygen, andsilicon in Earth's outer core, models of Earth's accretion that match these concentrations would presumably better constrain Earth's formation.[14]
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.
A diagram of Earth's geodynamo and magnetic field, which could have been driven in Earth's early history by the crystallization ofmagnesium oxide,silicon dioxide, andiron(II) oxide.
Earth's magnetic field is driven bythermal convection and also by chemical convection, the exclusion of light elements from the inner core, which float upward within the fluid outer core whiledenser elements sink.[17][30] This chemical convection releasesgravitational energy that is then available to power thegeodynamo 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 ofEarth's inner core, Earth's geodynamo was mainly driven by thermal convection.[30] However, as of 2020[update], claims that thethermal conductivity ofiron at coretemperatures 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 dissolveoxygen,magnesium,silicon, and other light elements.[17] As the Earth's core began to cool, it would becomesupersaturated in these light elements that would thenprecipitate into thelower mantle formingoxides leading to a different variant of chemical convection.[14][17]
The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in thesolar wind. The rate of cooling by conduction and convection is uncertain,[31] but one estimate is that the core would not be expected to freeze up for approximately 91 billion years, which is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out.[32]
^Wassel, Lauren; Irving, Jessica; Dues, Arwen (2011). "Reconciling the hemispherical structure of Earth's inner core with its super-rotation".Nature Geoscience.4 (4):264–267.Bibcode:2011NatGe...4..264W.doi:10.1038/ngeo1083.
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