Editing Geochemistry (section)

===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}}