Editing Geochemistry (section)

===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}}