Jump to content

Earth's outer core

From Wikipedia, the free encyclopedia
(Redirected fromOuter core)
For broader coverage of this topic, seeInternal structure of Earth § Core.
Earth and atmosphere structure

Earth's outer coreis a fluid layer about 2,260 km (1,400 mi) thick, composed of mostlyironandnickelthat lies above Earth's solidinner coreand below itsmantle.[1][2][3]The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at thecore-mantle boundaryand ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.[4]

Properties

[edit]

The outer core of Earth isliquid,unlike itsinner core,which is solid.[5]Evidence for a fluid outer core includesseismologywhich shows thatseismicshear-wavesare 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 wavesandnormal modesconstrain 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 thetemperatureof 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-viscosityfluid that convectsturbulently.[8]Thedynamo theoryseeseddy currentsin the nickel-iron fluid of the outer core as the principal source ofEarth's magnetic field.The averagemagnetic fieldstrength 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]

Light elements of Earth's outer core

[edit]

Composition

[edit]

Earth's outer core cannot be entirely constituted of iron or iron-nickelalloybecause their densities are higher than geophysical measurements of thedensityof Earth's outer core.[12][13][14][15]In fact, Earth's outer core is approximately 5 to 10 percent lower density thanironat Earth's coretemperaturesandpressures.[15][16][17]Hence it has been proposed that lightelementswith lowatomic numberscompose 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 lightelementscan be meaningfully constrained by high-pressureexperiments, calculations based onseismicmeasurements, models ofEarth's accretion,andcarbonaceous chondrite meteoritecomparisons withbulk silicate Earth (BSE).[12][14][15][16][18][19]Recent estimates are that Earth's outer core is composed ofironalong with 0 to 0.26 percenthydrogen,0.2 percentcarbon,0.8 to 5.3 percentoxygen,0 to 4.0 percentsilicon,1.7 percentsulfur,and 5 percentnickelby weight, and thetemperatureof thecore-mantle boundaryand the inner core boundary ranges from 4,137 to 4,300Kand from 5,400 to 6,300Krespectively.[14]

Constraints

[edit]
Accretion
[edit]
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 byEarth's accretion.[16]Namely, the light elements contained must have been abundant during Earth's formation, must be able to partition intoliquidiron at lowpressures,and must not volatilize and escape during Earth's accretionary process.[14][16]

CI chondrites
[edit]

CI chondritic meteoritesare believed to contain the same planet-forming elements in the sameproportionsas in the earlySolar System,[14]so differences between CI meteorites andBSEcan provide insights into the light element composition of Earth's outer core.[20][14]For instance, the depletion ofsiliconin 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 andinner coreis still possible.[14][21][22]

Implications for Earth's accretion and core formation history

[edit]

Tighter constraints on the concentrations of light elements in Earth's outer core would provide a better understanding ofEarth's accretionandcore formationhistory.[14][19][23]

Consequences for Earth's accretion

[edit]

Models of Earth's accretion could be better tested if we had better constraints on light elementconcentrationsin 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 thatoxidizedmaterial from the outerSolar Systemwas accreted towards the conclusion ofEarth's accretion.[14][19]If we could better constrain the concentrations ofhydrogen,oxygen,andsiliconin 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

[edit]
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 ofsiderophile elementsinEarth's mantlecompared to chondritic meteorites is attributed to metal-silicate reactions during formation of Earth's core.[24]These reactions are dependent onoxygen,silicon,andsulfur,[14][25][24]so better constraints onconcentrationsof these elements in Earth's outer core will help elucidate the conditions of formation ofEarth's core.[14][23][25][24][26]

In another example, the possible presence ofhydrogenin Earth's outer core suggests that theaccretionof Earth’swater[14][27][28]was not limited to the final stages ofEarth's accretion[23]and thatwatermay have been absorbed into core-forming metals through a hydrousmagma ocean.[14][29]

Implications for Earth's magnetic field

[edit]
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 ofmagnesium oxide,silicon dioxide,andiron(II) oxide.

Earth's magnetic fieldis driven bythermal convectionand also by chemical convection, the exclusion of light elements from the inner core, which float upward within the fluid outer core whiledenserelements sink.[17][30]This chemical convection releasesgravitational energythat is then available to power thegeodynamothat produces Earth's magnetic field.[30]Carnot efficiencieswith 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, recent claims that thethermal conductivityofironat coretemperaturesand 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 becomesupersaturatedin these light elements that would thenprecipitateinto thelower mantleformingoxidesleading 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][better source needed]

References

[edit]
  1. ^"Earth's Interior".Science & Innovation.National Geographic. 18 January 2017. Archived fromthe originalon May 6, 2017.Retrieved14 November2018.
  2. ^Sue, Caryl (2015-08-17). Evers, Jeannie (ed.)."Core".National Geographic Society.Retrieved2022-02-25.
  3. ^Zhang, Youjun; Sekine, Toshimori; He, Hongliang; Yu, Yin; Liu, Fusheng; Zhang, Mingjian (2014-07-15)."Shock compression of Fe-Ni-Si system to 280 GPa: Implications for the composition of the Earth's outer core".Geophysical Research Letters.41(13): 4554–4559.Bibcode:2014GeoRL..41.4554Z.doi:10.1002/2014gl060670.ISSN0094-8276.S2CID128528504.
  4. ^Young, C J; Lay, T (1987)."The Core-Mantle Boundary".Annual Review of Earth and Planetary Sciences.15(1): 25–46.Bibcode:1987AREPS..15...25Y.doi:10.1146/annurev.ea.15.050187.000325.ISSN0084-6597.
  5. ^Gutenberg, Beno (2016).Physics of the Earth's interior.Academic Press. pp. 101–118.ISBN978-1-4832-8212-1.
  6. ^Jeffreys, Harold (1 June 1926)."The Rigidity of the Earth's Central Core".Monthly Notices of the Royal Astronomical Society.1:371–383.Bibcode:1926GeoJ....1..371J.doi:10.1111/j.1365-246X.1926.tb05385.x.ISSN1365-246X.
  7. ^Ahrens, Thomas J., ed. (1995).Global earth physics a handbook of physical constants(3rd ed.). Washington, DC:American Geophysical Union.ISBN9780875908519.
  8. ^abDe Wijs, Gilles A.; Kresse, Georg; Vočadlo, Lidunka; Dobson, David; Alfè, Dario; Gillan, Michael J.; Price, Geoffrey D. (1998)."The viscosity of liquid iron at the physical conditions of the Earth's core"(PDF).Nature.392(6678): 805.Bibcode:1998Natur.392..805D.doi:10.1038/33905.S2CID205003051.
  9. ^Staff writer (17 December 2010)."First Measurement Of Magnetic Field Inside Earth's Core".Science 2.0.Retrieved14 November2018.
  10. ^Buffett, Bruce A. (2010). "Tidal dissipation and the strength of the Earth's internal magnetic field".Nature.468(7326): 952–4.Bibcode:2010Natur.468..952B.doi:10.1038/nature09643.PMID21164483.S2CID4431270.
  11. ^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.
  12. ^abBirch, Francis (1952)."Elasticity and constitution of the Earth's interior".Journal of Geophysical Research.57(2): 227–286.Bibcode:1952JGR....57..227B.doi:10.1029/JZ057i002p00227.
  13. ^Birch, Francis (1964-10-15)."Density and composition of mantle and core".Journal of Geophysical Research.69(20): 4377–4388.Bibcode:1964JGR....69.4377B.doi:10.1029/JZ069i020p04377.
  14. ^abcdefghijklmnopqrstuHirose, Kei; Wood, Bernard; Vočadlo, Lidunka (2021)."Light elements in the Earth's core".Nature Reviews Earth & Environment.2(9): 645–658.doi:10.1038/s43017-021-00203-6.ISSN2662-138X.S2CID237272150.
  15. ^abcdeWood, Bernard J.; Walter, Michael J.; Wade, Jonathan (2006)."Accretion of the Earth and segregation of its core".Nature.441(7095): 825–833.Bibcode:2006Natur.441..825W.doi:10.1038/nature04763.ISSN1476-4687.PMID16778882.S2CID8942975.
  16. ^abcdePoirier, Jean-Paul (1994-09-01)."Light elements in the Earth's outer core: A critical review".Physics of the Earth and Planetary Interiors.85(3): 319–337.Bibcode:1994PEPI...85..319P.doi:10.1016/0031-9201(94)90120-1.ISSN0031-9201.
  17. ^abcdefMittal, Tushar; Knezek, Nicholas; Arveson, Sarah M.; McGuire, Chris P.; Williams, Curtis D.; Jones, Timothy D.; Li, Jie (2020-02-15)."Precipitation of multiple light elements to power Earth's early dynamo".Earth and Planetary Science Letters.532:116030.Bibcode:2020E&PSL.53216030M.doi:10.1016/j.epsl.2019.116030.ISSN0012-821X.S2CID213919815.
  18. ^abZhang, Youjun; Sekine, Toshimori; He, Hongliang; Yu, Yin; Liu, Fusheng; Zhang, Mingjian (2016-03-02)."Experimental constraints on light elements in the Earth's outer core".Scientific Reports.6(1): 22473.Bibcode:2016NatSR...622473Z.doi:10.1038/srep22473.ISSN2045-2322.PMC4773879.PMID26932596.
  19. ^abcdSuer, Terry-Ann; Siebert, Julien; Remusat, Laurent; Menguy, Nicolas; Fiquet, Guillaume (2017-07-01)."A sulfur-poor terrestrial core inferred from metal–silicate partitioning experiments".Earth and Planetary Science Letters.469:84–97.Bibcode:2017E&PSL.469...84S.doi:10.1016/j.epsl.2017.04.016.ISSN0012-821X.
  20. ^Zhang, Youjun; Sekine, Toshimori; He, Hongliang; Yu, Yin; Liu, Fusheng; Zhang, Mingjian (2014-07-15)."Shock compression of Fe-Ni-Si system to 280 GPa: Implications for the composition of the Earth's outer core".Geophysical Research Letters.41(13): 4554–4559.Bibcode:2014GeoRL..41.4554Z.doi:10.1002/2014gl060670.ISSN0094-8276.S2CID128528504.
  21. ^Georg, R. Bastian; Halliday, Alex N.; Schauble, Edwin A.; Reynolds, Ben C. (2007)."Silicon in the Earth's core".Nature.447(7148): 1102–1106.Bibcode:2007Natur.447.1102G.doi:10.1038/nature05927.ISSN1476-4687.PMID17597757.S2CID1892924.
  22. ^Dauphas, Nicolas; Poitrasson, Franck; Burkhardt, Christoph; Kobayashi, Hiroshi; Kurosawa, Kosuke (2015-10-01)."Planetary and meteoritic Mg/Si and δ30Si variations inherited from solar nebula chemistry".Earth and Planetary Science Letters.427:236–248.arXiv:1507.02922.Bibcode:2015E&PSL.427..236D.doi:10.1016/j.epsl.2015.07.008.ISSN0012-821X.S2CID20744455.
  23. ^abcdeRubie, D. C.; Jacobson, S. A.; Morbidelli, A.; O’Brien, D. P.; Young, E. D.; de Vries, J.; Nimmo, F.; Palme, H.;Frost, D. J.(2015-03-01)."Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water".Icarus.248:89–108.arXiv:1410.3509.Bibcode:2015Icar..248...89R.doi:10.1016/j.icarus.2014.10.015.ISSN0019-1035.S2CID37592339.
  24. ^abcBadro, James; Brodholt, John P.; Piet, Hélène; Siebert, Julien; Ryerson, Frederick J. (2015-10-06)."Core formation and core composition from coupled geochemical and geophysical constraints".Proceedings of the National Academy of Sciences.112(40): 12310–12314.Bibcode:2015PNAS..11212310B.doi:10.1073/pnas.1505672112.ISSN0027-8424.PMC4603515.PMID26392555.
  25. ^abFischer, Rebecca A.; Nakajima, Yoichi; Campbell, Andrew J.;Frost, Daniel J.;Harries, Dennis; Langenhorst, Falko; Miyajima, Nobuyoshi; Pollok, Kilian; Rubie, David C. (2015-10-15)."High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O".Geochimica et Cosmochimica Acta.167:177–194.Bibcode:2015GeCoA.167..177F.doi:10.1016/j.gca.2015.06.026.ISSN0016-7037.
  26. ^Wade, J.; Wood, B. J. (2005-07-30)."Core formation and the oxidation state of the Earth".Earth and Planetary Science Letters.236(1): 78–95.Bibcode:2005E&PSL.236...78W.doi:10.1016/j.epsl.2005.05.017.ISSN0012-821X.
  27. ^Sato, Takao; Okuzumi, Satoshi; Ida, Shigeru (2016-05-01)."On the water delivery to terrestrial embryos by ice pebble accretion".Astronomy & Astrophysics.589:A15.arXiv:1512.02414.Bibcode:2016A&A...589A..15S.doi:10.1051/0004-6361/201527069.ISSN0004-6361.S2CID55107839.
  28. ^Raymond, Sean N.; Quinn, Thomas; Lunine, Jonathan I. (2007-02-01)."High-Resolution Simulations of The Final Assembly of Earth-Like Planets. 2. Water Delivery And Planetary Habitability".Astrobiology.7(1): 66–84.arXiv:astro-ph/0510285.Bibcode:2007AsBio...7...66R.doi:10.1089/ast.2006.06-0126.ISSN1531-1074.PMID17407404.S2CID10257401.
  29. ^Tagawa, Shoh; Sakamoto, Naoya; Hirose, Kei; Yokoo, Shunpei; Hernlund, John; Ohishi, Yasuo; Yurimoto, Hisayoshi (2021-05-11)."Experimental evidence for hydrogen incorporation into Earth's core".Nature Communications.12(1): 2588.Bibcode:2021NatCo..12.2588T.doi:10.1038/s41467-021-22035-0.ISSN2041-1723.PMC8113257.PMID33976113.
  30. ^abcdBuffett, Bruce A. (2000-06-16)."Earth's Core and the Geodynamo".Science.288(5473): 2007–2012.Bibcode:2000Sci...288.2007B.doi:10.1126/science.288.5473.2007.PMID10856207.
  31. ^David K. Li (19 January 2022)."Earth's core cooling faster than previously thought, researchers say".NBC News.
  32. ^"Core".National Geographic.Retrieved15 July2024.
[edit]
The WikibookHistorical Geologyhas a page on the topic of:Structure of the Earth
Retrieved from "https://en.wikipedia.org/w/index.php?title=Earth%27s_outer_core&oldid=1249894961"