Editing Earth's outer core (section)

== 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.<ref name=":2">{{Cite journal |last=Birch |first=Francis |date=1952 |title=Elasticity and constitution of the Earth's interior |url=http://doi.wiley.com/10.1029/JZ057i002p00227 |journal=Journal of Geophysical Research |language=en |volume=57 |issue=2 |pages=227–286 |doi=10.1029/JZ057i002p00227|bibcode=1952JGR....57..227B |url-access=subscription }}</ref><ref name=":3">{{Cite journal |last=Birch |first=Francis |date=1964-10-15 |title=Density and composition of mantle and core |url=http://doi.wiley.com/10.1029/JZ069i020p04377 |journal=Journal of Geophysical Research |language=en |volume=69 |issue=20 |pages=4377–4388 |doi=10.1029/JZ069i020p04377|bibcode=1964JGR....69.4377B |url-access=subscription }}</ref><ref name=":0">{{Cite journal |last1=Hirose |first1=Kei |last2=Wood |first2=Bernard |last3=Vočadlo |first3=Lidunka |date=2021 |title=Light elements in the Earth's core |url=https://www.nature.com/articles/s43017-021-00203-6 |journal=Nature Reviews Earth & Environment |language=en |volume=2 |issue=9 |pages=645–658 |doi=10.1038/s43017-021-00203-6 |s2cid=237272150 |issn=2662-138X}}</ref><ref name=":5">{{Cite journal |last1=Wood |first1=Bernard J. |last2=Walter |first2=Michael J. |last3=Wade |first3=Jonathan |date=2006 |title=Accretion of the Earth and segregation of its core |url=https://www.nature.com/articles/nature04763 |journal=Nature |language=en |volume=441 |issue=7095 |pages=825–833 |doi=10.1038/nature04763 |pmid=16778882 |bibcode=2006Natur.441..825W |s2cid=8942975 |issn=1476-4687|url-access=subscription }}</ref> In fact, Earth's outer core is approximately 5 to 10 percent lower density than [[iron]] at Earth's core [[temperature]]s and [[pressure]]s.<ref name=":5" /><ref name=":4">{{Cite journal |last=Poirier |first=Jean-Paul |date=1994-09-01 |title=Light elements in the Earth's outer core: A critical review |url=https://dx.doi.org/10.1016/0031-9201%2894%2990120-1 |journal=Physics of the Earth and Planetary Interiors |language=en |volume=85 |issue=3 |pages=319–337 |doi=10.1016/0031-9201(94)90120-1 |bibcode=1994PEPI...85..319P |issn=0031-9201|url-access=subscription }}</ref><ref name=":7">{{Cite journal |last1=Mittal |first1=Tushar |last2=Knezek |first2=Nicholas |last3=Arveson |first3=Sarah M. |last4=McGuire |first4=Chris P. |last5=Williams |first5=Curtis D. |last6=Jones |first6=Timothy D. |last7=Li |first7=Jie |date=2020-02-15 |title=Precipitation of multiple light elements to power Earth's early dynamo |journal=Earth and Planetary Science Letters |language=en |volume=532 |pages=116030 |doi=10.1016/j.epsl.2019.116030 |bibcode=2020E&PSL.53216030M |s2cid=213919815 |issn=0012-821X|doi-access=free }}</ref> Hence it has been proposed that light [[chemical element|elements]] with low [[atomic number]]s compose part of Earth's outer core, as the only feasible way to lower its density.<ref name=":0" /><ref name=":5" /><ref name=":4" /> Although Earth's outer core is inaccessible to direct sampling,<ref name=":0" /><ref name=":5" /><ref name=":1">{{Cite journal |last1=Zhang |first1=Youjun |last2=Sekine |first2=Toshimori |last3=He |first3=Hongliang |last4=Yu |first4=Yin |last5=Liu |first5=Fusheng |last6=Zhang |first6=Mingjian |date=2016-03-02 |title=Experimental constraints on light elements in the Earth's outer core |journal=Scientific Reports |language=en |volume=6 |issue=1 |pages=22473 |doi=10.1038/srep22473 |issn=2045-2322 |pmc=4773879 |pmid=26932596|bibcode=2016NatSR...622473Z }}</ref> the composition of light [[Chemical element|elements]] can be meaningfully constrained by high-[[pressure]] experiments, calculations based on [[seismic]] measurements, models of [[Accretion (astrophysics)|Earth's accretion]], and [[chondrite|carbonaceous chondrite meteorite]] comparisons with [[Primitive mantle|bulk silicate Earth (BSE)]].<ref name=":2" /><ref name=":0" /><ref name=":5" /><ref name=":4" /><ref name=":1" /><ref name=":8">{{Cite journal |last1=Suer |first1=Terry-Ann |last2=Siebert |first2=Julien |last3=Remusat |first3=Laurent |last4=Menguy |first4=Nicolas |last5=Fiquet |first5=Guillaume |date=2017-07-01 |title=A sulfur-poor terrestrial core inferred from metal–silicate partitioning experiments |url=https://www.sciencedirect.com/science/article/pii/S0012821X17301954 |journal=Earth and Planetary Science Letters |language=en |volume=469 |pages=84–97 |doi=10.1016/j.epsl.2017.04.016 |bibcode=2017E&PSL.469...84S |issn=0012-821X|url-access=subscription }}</ref> 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 [[Kelvin|K]] and from 5,400 to 6,300 [[Kelvin|K]] respectively.<ref name=":0" />

==== Constraints ====
===== Accretion =====
[[File:Earth_formation.jpg|alt=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.|thumb|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 [[accretion (astrophysics)|Earth's accretion]].<ref name=":4" /> Namely, the light elements contained must have been abundant during Earth's formation, must be able to partition into [[liquid]] iron at low [[pressure]]s, and must not volatilize and escape during Earth's accretionary process.<ref name=":0" /><ref name=":4" />

===== CI chondrites =====
[[CI chondrite|CI chondritic meteorites]] are believed to contain the same planet-forming elements in the same [[ratio|proportions]] as in the early [[Solar System]],<ref name=":0" /> so differences between CI meteorites and [[primitive mantle|BSE]] can provide insights into the light element composition of Earth's outer core.<ref name=":122">{{Cite journal |last1=Zhang |first1=Youjun |last2=Sekine |first2=Toshimori |last3=He |first3=Hongliang |last4=Yu |first4=Yin |last5=Liu |first5=Fusheng |last6=Zhang |first6=Mingjian |date=2014-07-15 |title=Shock compression of Fe-Ni-Si system to 280 GPa: Implications for the composition of the Earth's outer core |journal=Geophysical Research Letters |volume=41 |issue=13 |pages=4554–4559 |doi=10.1002/2014gl060670 |bibcode=2014GeoRL..41.4554Z |s2cid=128528504 |issn=0094-8276|doi-access=free }}</ref><ref name=":0" /> For instance, the depletion of [[silicon]] in Earth's [[primitive 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 and [[Earth's inner core|inner core]] is still possible.<ref name=":0" /><ref>{{Cite journal |last1=Georg |first1=R. Bastian |last2=Halliday |first2=Alex N. |last3=Schauble |first3=Edwin A. |last4=Reynolds |first4=Ben C. |date=2007 |title=Silicon in the Earth's core |url=https://www.nature.com/articles/nature05927 |journal=Nature |language=en |volume=447 |issue=7148 |pages=1102–1106 |doi=10.1038/nature05927 |pmid=17597757 |bibcode=2007Natur.447.1102G |s2cid=1892924 |issn=1476-4687|url-access=subscription }}</ref><ref>{{Cite journal |last1=Dauphas |first1=Nicolas |last2=Poitrasson |first2=Franck |last3=Burkhardt |first3=Christoph |last4=Kobayashi |first4=Hiroshi |last5=Kurosawa |first5=Kosuke |date=2015-10-01 |title=Planetary and meteoritic Mg/Si and δ30Si variations inherited from solar nebula chemistry |url=https://www.sciencedirect.com/science/article/pii/S0012821X15004355 |journal=Earth and Planetary Science Letters |language=en |volume=427 |pages=236–248 |doi=10.1016/j.epsl.2015.07.008 |arxiv=1507.02922 |bibcode=2015E&PSL.427..236D |s2cid=20744455 |issn=0012-821X}}</ref>

=== 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 [[accretion (astrophysics)|Earth's accretion]] and [[internal structure of Earth|core formation]] history.<ref name=":0" /><ref name=":8" /><ref name=":9">{{Cite journal |last1=Rubie |first1=D. C. |last2=Jacobson |first2=S. A. |last3=Morbidelli |first3=A. |last4=O’Brien |first4=D. P. |last5=Young |first5=E. D. |last6=de Vries |first6=J. |last7=Nimmo |first7=F. |last8=Palme |first8=H. |last9=Frost |first9=D. J.| author9-link=Daniel Frost (earth scientist) |date=2015-03-01 |title=Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water |url=https://www.sciencedirect.com/science/article/pii/S0019103514005545 |journal=Icarus |language=en |volume=248 |pages=89–108 |doi=10.1016/j.icarus.2014.10.015 |arxiv=1410.3509 |bibcode=2015Icar..248...89R |s2cid=37592339 |issn=0019-1035}}</ref>

==== Consequences for Earth's accretion ====
Models of Earth's accretion could be better tested if we had better constraints on light element [[concentration]]s in Earth's outer core.<ref name=":0" /><ref name=":9" /> For example, accretionary models based on core-mantle element partitioning tend to support proto-Earths constructed from reduced, condensed, and volatile-free material,<ref name=":0" /><ref name=":8" /><ref name=":9" /> despite the possibility that [[redox|oxidized]] material from the outer [[Solar System]] was accreted towards the conclusion of [[accretion (astrophysics)|Earth's accretion]].<ref name=":0" /><ref name=":8" /> 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.<ref name=":0" />

==== Consequences for Earth's core formation ====
[[File:Differentiation_white.png|alt=A diagram of Earth's differentiation. The diagram displays Earth's different layers and how dense materials move towards Earth's core.|thumb|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 [[Goldschmidt classification#Siderophile elements|siderophile elements]] in [[Earth's mantle]] compared to chondritic meteorites is attributed to metal-silicate reactions during formation of Earth's core.<ref name=":11">{{Cite journal |last1=Badro |first1=James |last2=Brodholt |first2=John P. |last3=Piet |first3=Hélène |last4=Siebert |first4=Julien |last5=Ryerson |first5=Frederick J. |date=2015-10-06 |title=Core formation and core composition from coupled geochemical and geophysical constraints |journal=Proceedings of the National Academy of Sciences |language=en |volume=112 |issue=40 |pages=12310–12314 |doi=10.1073/pnas.1505672112 |issn=0027-8424 |pmc=4603515 |pmid=26392555|bibcode=2015PNAS..11212310B |doi-access=free }}</ref> These reactions are dependent on [[oxygen]], [[silicon]], and [[sulfur]],<ref name=":0" /><ref name=":10">{{Cite journal |last1=Fischer |first1=Rebecca A. |last2=Nakajima |first2=Yoichi |last3=Campbell |first3=Andrew J. |last4=Frost |first4=Daniel J.|author4-link=Daniel Frost (earth scientist) |last5=Harries |first5=Dennis |last6=Langenhorst |first6=Falko |last7=Miyajima |first7=Nobuyoshi |last8=Pollok |first8=Kilian |last9=Rubie |first9=David C. |date=2015-10-15 |title=High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O |journal=Geochimica et Cosmochimica Acta |language=en |volume=167 |pages=177–194 |doi=10.1016/j.gca.2015.06.026 |bibcode=2015GeCoA.167..177F |issn=0016-7037|doi-access=free }}</ref><ref name=":11" /> so better constraints on [[Concentration|concentrations]] of these elements in Earth's outer core will help elucidate the conditions of formation of [[Earth's core]].<ref name=":0" /><ref name=":9" /><ref name=":10" /><ref name=":11" /><ref>{{Cite journal |last1=Wade |first1=J. |last2=Wood |first2=B. J. |date=2005-07-30 |title=Core formation and the oxidation state of the Earth |url=https://www.sciencedirect.com/science/article/pii/S0012821X05003286 |journal=Earth and Planetary Science Letters |language=en |volume=236 |issue=1 |pages=78–95 |doi=10.1016/j.epsl.2005.05.017 |bibcode=2005E&PSL.236...78W |issn=0012-821X|url-access=subscription }}</ref>

In another example, the possible presence of [[hydrogen]] in Earth's outer core suggests that the [[Accretion (astrophysics)|accretion]] of Earth’s [[water]]<ref name=":0" /><ref>{{Cite journal |last1=Sato |first1=Takao |last2=Okuzumi |first2=Satoshi |last3=Ida |first3=Shigeru |date=2016-05-01 |title=On the water delivery to terrestrial embryos by ice pebble accretion |url=https://www.aanda.org/articles/aa/abs/2016/05/aa27069-15/aa27069-15.html |journal=Astronomy & Astrophysics |language=en |volume=589 |pages=A15 |doi=10.1051/0004-6361/201527069 |arxiv=1512.02414 |bibcode=2016A&A...589A..15S |s2cid=55107839 |issn=0004-6361}}</ref><ref>{{Cite journal |last1=Raymond |first1=Sean N. |last2=Quinn |first2=Thomas |last3=Lunine |first3=Jonathan I. |date=2007-02-01 |title=High-Resolution Simulations of The Final Assembly of Earth-Like Planets. 2. Water Delivery And Planetary Habitability |url=https://www.liebertpub.com/doi/10.1089/ast.2006.06-0126 |journal=Astrobiology |volume=7 |issue=1 |pages=66–84 |doi=10.1089/ast.2006.06-0126 |pmid=17407404 |arxiv=astro-ph/0510285 |bibcode=2007AsBio...7...66R |s2cid=10257401 |issn=1531-1074}}</ref> was not limited to the final stages of [[Accretion (astrophysics)|Earth's accretion]]<ref name=":9" /> and that [[water]] may have been absorbed into core-forming metals through a hydrous [[magma ocean]].<ref name=":0" /><ref>{{Cite journal |last1=Tagawa |first1=Shoh |last2=Sakamoto |first2=Naoya |last3=Hirose |first3=Kei |last4=Yokoo |first4=Shunpei |last5=Hernlund |first5=John |last6=Ohishi |first6=Yasuo |last7=Yurimoto |first7=Hisayoshi |date=2021-05-11 |title=Experimental evidence for hydrogen incorporation into Earth's core |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=2588 |doi=10.1038/s41467-021-22035-0 |issn=2041-1723 |pmc=8113257 |pmid=33976113|bibcode=2021NatCo..12.2588T }}</ref>

=== Implications for Earth's magnetic field ===
[[File:Dynamo Theory - Outer core convection and magnetic field generation.svg|alt=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.|thumb|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 [[convection (heat transfer)|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 [[density|denser]] elements sink.<ref name=":7" /><ref name=":6">{{Cite journal |last=Buffett |first=Bruce A. |date=2000-06-16 |title=Earth's Core and the Geodynamo |url=https://www.science.org/doi/abs/10.1126/science.288.5473.2007 |journal=Science |volume=288 |issue=5473 |pages=2007–2012 |language=EN |doi=10.1126/science.288.5473.2007|pmid=10856207 |bibcode=2000Sci...288.2007B |url-access=subscription }}</ref> This chemical convection releases [[gravitational energy]] that is then available to power the [[dynamo theory|geodynamo]] that produces Earth's magnetic field.<ref name=":6" /> [[Carnot cycle|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.<ref name=":6" /> Traditionally it was thought that prior to the formation of [[Earth's inner core#Age|Earth's inner core]], Earth's geodynamo was mainly driven by thermal convection.<ref name=":6" /> However, recent claims that the [[thermal conductivity]] of [[iron]] at core [[temperature]]s 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.<ref name=":0" /><ref name=":7" /> This conundrum is known as the new "core paradox."<ref name=":0" /><ref name=":7" /> 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.<ref name=":7" /> As the Earth's core began to cool, it would become [[supersaturation|supersaturated]] in these light elements that would then [[precipitation (chemistry)|precipitate]] into the [[Lower mantle (Earth)|lower mantle]] forming [[oxide]]s leading to a different variant of chemical convection.<ref name=":0" /><ref name=":7" />

The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the [[solar wind]]. The rate of cooling by conduction and convection is uncertain,<ref>{{cite web |url=https://www.nbcnews.com/science/science-news/earths-core-cooling-faster-previously-thought-researchers-say-rcna12732 |title=Earth's core cooling faster than previously thought, researchers say |date=19 January 2022 |author=David K. Li |publisher=[[NBC News]]}}</ref> 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.<ref>{{cite web |url=https://education.nationalgeographic.org/resource/core/ |access-date=15 July 2024 |title=Core |publisher=[[National Geographic]]}}</ref>{{better source needed|date=July 2024}}