Editing Geochemistry (section)

==Abundance of elements==
{{main|Abundance of the chemical elements}}

===Solar System===
[[File:ElementalAbundance.svg|thumb|upright=1.7|Abundances of Solar System elements.<ref>Data from table 6 of {{cite journal|last1=Cameron|first1=A.G.W.|title=Abundances of the elements in the solar system|journal=Space Science Reviews|date=September 1973|volume=15|issue=1|pages=121|doi=10.1007/BF00172440|bibcode=1973SSRv...15..121C|s2cid=120201972}}</ref>]]
The composition of the [[Solar System]] is similar to that of many other stars, and aside from small anomalies it can be assumed to have formed from a [[solar nebula]] that had a uniform composition, and the composition of the [[Sun]]'s [[photosphere]] is similar to that of the rest of the Solar System. The composition of the photosphere is determined by fitting the [[Spectral line|absorption lines]] in its [[spectrum]] to models of the Sun's atmosphere.<ref name=Palme>{{cite book|first1=H.|last1=Palme|first2=A.|last2=Jones|chapter=1.03 &ndash; Solar system abundance of the elements|editor-last1=Holland|editor-first1=H.D.|editor-last2=Turekian|editor-first2=K.K.|title=Treatise on Geochemistry|volume=1: Meteorites, Comets and Planets|date=2003|publisher=Elsevier Science|location=Oxford|isbn=9780080437514|doi=10.1016/B0-08-043751-6/01060-4|pages=41&ndash;61|edition=1st|chapter-url=https://www.elsevier.ca/brochures/treatiseongeochemistry/contents/sample1.pdf|access-date=3 October 2017|archive-date=3 October 2017|archive-url=https://web.archive.org/web/20171003225547/https://www.elsevier.ca/brochures/treatiseongeochemistry/contents/sample1.pdf|url-status=dead}}</ref> By far the largest two elements by fraction of total mass are hydrogen (74.9%) and [[helium]] (23.8%), with all the remaining elements contributing just 1.3%.<ref>{{cite journal|last1=Lodders|first1=Katharina|author1-link=Katharina Lodders|title=Solar System Abundances and Condensation Temperatures of the Elements|journal=The Astrophysical Journal|date=10 July 2003|volume=591|issue=2|pages=1220–1247|doi=10.1086/375492|bibcode=2003ApJ...591.1220L|citeseerx=10.1.1.695.5451|s2cid=42498829 }}</ref> There is a general trend of [[Exponential decay|exponential decrease]] in abundance with increasing atomic number, although elements with even atomic number are more common than their odd-numbered neighbors (the [[Oddo–Harkins rule]]). Compared to the overall trend, [[lithium]], [[boron]] and [[beryllium]] are depleted and iron is anomalously enriched.<ref>{{cite book|last1=Middlemost|first1=Eric A. K.|title=Magmas, Rocks and Planetary Development: A Survey of Magma/Igneous Rock Systems|date=2014|publisher=Routledge|isbn=9781317892649}}</ref>{{rp|284&ndash;285}}

The pattern of elemental abundance is mainly due to two factors. The hydrogen, helium, and some of the lithium were [[Big Bang nucleosynthesis|formed in about 20 minutes after the Big Bang]], while the rest were [[Stellar nucleosynthesis|created in the interiors of stars]].<ref name=McSween/>{{rp|316&ndash;317}}

===Meteorites===
[[Meteorite]]s come in a variety of compositions, but chemical analysis can determine whether they were once in [[planetesimal]]s that [[melting|melted]] or [[Planetary differentiation|differentiated]].<ref name=Palme/>{{rp|45}} [[Chondrite]]s are undifferentiated and have round mineral inclusions called [[chondrule]]s. With the ages of 4.56 billion years, they date to the [[Formation and evolution of the Solar System|early solar system]]. A particular kind, the [[CI chondrite]], has a composition that closely matches that of the Sun's photosphere, except for depletion of some volatiles (H, He, C, N, O) and a group of elements (Li, B, Be) that are destroyed by nucleosynthesis in the Sun.<ref name=McSween/>{{rp|318}}<ref name=Palme/> Because of the latter group, CI chondrites are considered a better match for the composition of the early Solar System. Moreover, the chemical analysis of CI chondrites is more accurate than for the photosphere, so it is generally used as the source for chemical abundance, despite their rareness (only five have been recovered on Earth).<ref name=Palme/>

===Giant planets===
[[File: Gas Giant Interiors.jpg|thumb|upright=2|Cutaways illustrating models of the interiors of the giant planets.]]
The planets of the Solar System are divided into two groups: the four inner planets are the [[terrestrial planet]]s ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]]), with relatively small sizes and rocky surfaces. The four outer planets are the [[giant planets]], which are dominated by hydrogen and helium and have lower mean densities. These can be further subdivided into the [[gas giant]]s ([[Jupiter]] and [[Saturn]]) and the [[ice giant]]s ([[Uranus]] and [[Neptune]]) that have large icy cores.<ref>{{cite book|last1=Encrenaz|first1=Therese|author1-link=Thérèse Encrenaz|last2=Bibring|first2=Jean-Pierre|last3=Blanc|first3=M.|last4=Barucci|first4=Maria-Antonietta|last5=Roques|first5=Francoise|last6=Zarka|first6=Philippe|title=The solar system|date=2004|publisher=Springer|location=Berlin|isbn=9783540002413|edition=3rd}}</ref>{{rp|26&ndash;27,283&ndash;284}}

Most of our direct information on the composition of the giant planets is from [[spectroscopy]]. Since the 1930s, Jupiter was known to contain hydrogen, [[methane]] and [[ammonium]]. In the 1960s, [[interferometry]] greatly increased the resolution and sensitivity of spectral analysis, allowing the identification of a much greater collection of molecules including [[ethane]], [[acetylene]], water and [[carbon monoxide]].<ref name=Lewis>{{cite book|last1=Lewis|first1=John|title=Physics and Chemistry of the Solar System|date=1995|publisher=Elsevier Science|location=Burlington|isbn=9780323145848}}</ref>{{rp|138&ndash;139}} However, Earth-based spectroscopy becomes increasingly difficult with more remote planets, since the reflected light of the Sun is much dimmer; and spectroscopic analysis of light from the planets can only be used to detect vibrations of molecules, which are in the [[infrared]] frequency range. This constrains the abundances of the elements H, C and N.<ref name=Lewis/>{{rp|130}} Two other elements are detected: phosphorus in the gas [[phosphine]] (PH<sub>3</sub>) and germanium in [[germane]] (GeH<sub>4</sub>).<ref name=Lewis/>{{rp|131}}

The helium atom has vibrations in the [[ultraviolet]] range, which is strongly absorbed by the atmospheres of the outer planets and Earth. Thus, despite its abundance, helium was only detected once spacecraft were sent to the outer planets, and then only indirectly through collision-induced absorption in hydrogen molecules.<ref name=Lewis/>{{rp|209}} Further information on Jupiter was obtained from the [[Galileo Probe|''Galileo'' probe]] when it was sent into the atmosphere in 1995;<ref>{{cite journal|last1=Atreya|first1=S.K|last2=Mahaffy|first2=P.R|last3=Niemann|first3=H.B|last4=Wong|first4=M.H|last5=Owen|first5=T.C|title=Composition and origin of the atmosphere of Jupiter—an update, and implications for the extrasolar giant planets|journal=Planetary and Space Science|date=February 2003|volume=51|issue=2|pages=105–112|doi=10.1016/S0032-0633(02)00144-7|bibcode=2003P&SS...51..105A}}</ref><ref name=Fortney>{{cite journal|last1=Fortney|first1=Jonathan|title=Viewpoint: Peering into Jupiter|journal=Physics|volume=3|page=26|date=22 March 2010|doi=10.1103/Physics.3.26|doi-access=free}}</ref> and the [[Cassini retirement|final mission]] of the [[Cassini–Huygens|Cassini probe]] in 2017 was to enter the atmosphere of Saturn.<ref name="Bittersweet ending">{{Cite news |url=http://www.latimes.com/science/sciencenow/la-sci-sn-cassini-ends-scene-20170915-story.html |title=As NASA's Cassini mission flames out over Saturn, scientists mark bittersweet end of mission |last=Netburn |first=Deborah |date=15 September 2017 |newspaper=The Los Angeles Times |access-date=10 October 2017 |archive-date=16 November 2017 |archive-url=https://web.archive.org/web/20171116091911/http://www.latimes.com/science/sciencenow/la-sci-sn-cassini-ends-scene-20170915-story.html |url-status=live }}</ref> In the atmosphere of Jupiter, He was found to be depleted by a factor of 2 compared to solar composition and Ne by a factor of 10, a surprising result since the other noble gases and the elements C, N and S were enhanced by factors of 2 to 4 (oxygen was also depleted but this was attributed to the unusually dry region that Galileo sampled).<ref name=Fortney/>

Spectroscopic methods only penetrate the atmospheres of Jupiter and Saturn to depths where the pressure is about equal to 1 [[bar (unit)|bar]], approximately Earth's [[atmospheric pressure]] at [[sea level]].<ref name=Lewis/>{{rp|131}} The Galileo probe penetrated to 22 bars.<ref name=Fortney/> This is a small fraction of the planet, which is expected to reach pressures of over 40 Mbar. To constrain the composition in the interior, thermodynamic models are constructed using the information on temperature from infrared emission spectra and equations of state for the likely compositions.<ref name=Lewis/>{{rp|131}} High-pressure experiments predict that hydrogen will be a metallic liquid in the interior of Jupiter and Saturn, while in Uranus and Neptune it remains in the molecular state.<ref name=Lewis/>{{rp|135&ndash;136}} Estimates also depend on models for the formation of the planets. Condensation of the presolar nebula would result in a gaseous planet with the same composition as the Sun, but the planets could also have formed when a solid core captured nebular gas.<ref name=Lewis/>{{rp|136}}

In current models, the four giant planets have cores of rock and ice that are roughly the same size, but the proportion of hydrogen and helium decreases from about 300 Earth masses in Jupiter to 75 in Saturn and just a few in Uranus and Neptune.<ref name=Lewis/>{{rp|220}} Thus, while the gas giants are primarily composed of hydrogen and helium, the ice giants are primarily composed of heavier elements (O, C, N, S), primarily in the form of water, methane, and ammonia. The surfaces are cold enough for molecular hydrogen to be liquid, so much of each planet is likely a hydrogen ocean overlaying one of heavier compounds.<ref>{{cite book|chapter=11. Uranus and Neptune|last1=Lang|first1=Kenneth R.|title=NASA's Cosmos|date=2010|publisher=Tufts University|chapter-url=https://ase.tufts.edu/cosmos/print_chapter.asp?id=11|access-date=11 October 2017|archive-date=9 September 2018|archive-url=https://web.archive.org/web/20180909093838/https://ase.tufts.edu/cosmos/print_chapter.asp?id=11|url-status=live}}</ref> Outside the core, Jupiter has a mantle of liquid metallic hydrogen and an atmosphere of molecular hydrogen and helium. Metallic hydrogen does not mix well with helium, and in Saturn, it may form a separate layer below the metallic hydrogen.<ref name=Lewis/>{{rp|138}}

===Terrestrial planets===
{{See also|Composition of Mars}}
Terrestrial planets are believed to have come from the same nebular material as the giant planets, but they have lost most of the lighter elements and have different histories. Planets closer to the Sun might be expected to have a higher fraction of refractory elements, but if their later stages of formation involved collisions of large objects with orbits that sampled different parts of the Solar System, there could be little systematic dependence on position.<ref name=Anderson>{{cite book|last1=Anderson|first1=Don L.|title=New Theory of the Earth|date=2007|publisher=Cambridge University Press|isbn=9781139462082}}</ref>{{rp|3&ndash;4}}

Direct information on Mars, Venus and Mercury largely comes from spacecraft missions. Using [[gamma-ray spectrometer]]s, the composition of the crust of Mars has been measured by the [[Mars Odyssey]] orbiter,<ref>{{cite web|title=GRS|url=https://mars.nasa.gov/odyssey/mission/instruments/grs/|website=Jet Propulsion Laboratory|publisher=USA.gov|access-date=17 October 2017|archive-date=8 February 2018|archive-url=https://web.archive.org/web/20180208132702/https://mars.nasa.gov/odyssey/mission/instruments/grs/|url-status=live}}</ref> the crust of Venus by some of the [[Venera]] missions to Venus,<ref name=Anderson/> and the crust of Mercury by the ''[[MESSENGER]]'' spacecraft.<ref>{{cite journal|last1=Rhodes|first1=Edgar A.|last2=Evans|first2=Larry G.|last3=Nittler|first3=Larry R.|last4=Starr|first4=Richard D.|last5=Sprague|first5=Ann L.|last6=Lawrence|first6=David J.|last7=McCoy|first7=Timothy J.|last8=Stockstill-Cahill|first8=Karen R.|last9=Goldsten|first9=John O.|last10=Peplowski|first10=Patrick N.|last11=Hamara|first11=David K.|last12=Boynton|first12=William V.|last13=Solomon|first13=Sean C.|title=Analysis of MESSENGER Gamma-Ray Spectrometer data from the Mercury flybys|journal=Planetary and Space Science|date=December 2011|volume=59|issue=15|pages=1829–1841|doi=10.1016/j.pss.2011.07.018|bibcode=2011P&SS...59.1829R}}</ref> Additional information on Mars comes from meteorites that have landed on Earth (the [[Shergottite]]s, [[Nakhlite]]s, and [[Chassignite]]s, collectively known as SNC meteorites).<ref name=Kieffer>{{cite book|editor-last1=Kieffer|editor-first1=Hugh H.|title=Mars|date=1994|publisher=University of Arizona Press|location=Tucson|isbn=9780816512577|edition=2nd|url-access=registration|url=https://archive.org/details/mars0000unse}}</ref>{{rp|124}} Abundances are also constrained by the masses of the planets, while the internal distribution of elements is constrained by their moments of inertia.<ref name=McSween/>{{rp|334}}

The planets condensed from the solar nebula, and much of the details of their composition are determined by fractionation as they cooled. The phases that condense fall into five groups. First to condense are materials rich in refractory elements such as Ca and Al. These are followed by nickel and iron, then [[Talc|magnesium silicates]]. Below about 700 [[kelvin]]s (700 K), [[Iron(II) sulfide|FeS]] and volatile-rich metals and silicates form a fourth group, and in the fifth group [[Iron(II) oxide|FeO]] enter the magnesium silicates.<ref name=Morgan1980>{{cite journal|last1=Morgan|first1=John W.|last2=Anders|first2=Edward|title=Chemical composition of Earth, Venus, and Mercury|journal=Proceedings of the National Academy of Sciences of the United States of America|date=December 1980|volume=77|issue=12|pages=6973&ndash;6977|jstor=9538|bibcode=1980PNAS...77.6973M|doi=10.1073/pnas.77.12.6973|pmid=16592930|pmc=350422|doi-access=free}}</ref> The compositions of the planets and the Moon are ''chondritic'', meaning that within each group the ratios between elements are the same as in carbonaceous chondrites.<ref name=McSween/>{{rp|334}}

The estimates of planetary compositions depend on the model used. In the ''equilibrium condensation'' model, each planet was formed from a ''feeding zone'' in which the compositions of solids were determined by the temperature in that zone. Thus, Mercury formed at 1400 K, where iron remained in a pure metallic form and there was little magnesium or silicon in solid form; Venus at 900 K, so all the magnesium and silicon condensed; Earth at 600 K, so it contains FeS and silicates; and Mars at 450 K, so FeO was incorporated into magnesium silicates. The greatest problem with this theory is that volatiles would not condense, so the planets would have no atmospheres and Earth no atmosphere.<ref name=McSween/>{{rp|335&ndash;336}}

In ''chondritic mixing'' models, the compositions of chondrites are used to estimate planetary compositions. For example, one model mixes two components, one with the composition of C1 chondrites and one with just the refractory components of C1 chondrites.<ref name=McSween/>{{rp|337}} In another model, the abundances of the five fractionation groups are estimated using an index element for each group. For the most refractory group, [[uranium]] is used; iron for the second; the ratios of potassium and [[thallium]] to uranium for the next two; and the molar ratio FeO/(FeO+[[Magnesium oxide|MgO]]) for the last. Using thermal and seismic models along with heat flow and density, Fe can be constrained to within 10 percent on Earth, Venus, and Mercury. U can be constrained within about 30% on Earth, but its abundance on other planets is based on "educated guesses". One difficulty with this model is that there may be significant errors in its prediction of volatile abundances because some volatiles are only partially condensed.<ref name=Morgan1980/><ref name=McSween/>{{rp|337&ndash;338}}