Editing Earth's outer core (section)

=== 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>