Editing Moon (section)

===Structure===
{{Main|Internal structure of the Moon|Geology of the Moon}}
[[File:Return of the moon diagram.svg|thumb|upright=1.5|Moon's internal structure: solid inner core (iron-metallic), molten outer core, hardened mantle and crust. The crust on the Moon's near side permanently facing Earth is thinner, featuring larger areas flooded by material of the once molten mantle forming today's [[lunar mare]].]]
The Moon is a [[planetary differentiation|differentiated]] body that was initially in [[hydrostatic equilibrium]] but has since departed from this condition.<ref>{{cite journal |title=Interpretation of lunar potential fields |first=Stanley Keith |last=Runcorn |date=March 31, 1977 |journal=Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences |doi=10.1098/rsta.1977.0094 |bibcode=1977RSPTA.285..507R |volume=285 |issue=1327 |pages=507–516 |s2cid=124703189}}</ref> It has a [[geochemically]] distinct [[Crust (geology)|crust]], [[Mantle (geology)|mantle]], and [[Planetary core|core]]. The Moon has a solid iron-rich inner core with a radius possibly as small as {{convert|240|km}} and a fluid outer core primarily made of liquid iron with a radius of roughly {{convert|300|km}}. Around the core is a partially molten boundary layer with a radius of about {{convert|500|km}}.<ref>{{cite web |last1=Brown |first1=D. |last2=Anderson |first2=J. |website=NASA |url=http://www.nasa.gov/topics/moonmars/features/lunar_core.html |title=NASA Research Team Reveals Moon Has Earth-Like Core |date=January 6, 2011 |url-status=live |archive-url=https://web.archive.org/web/20120111112210/http://www.nasa.gov/topics/moonmars/features/lunar_core.html |archive-date=January 11, 2012}}</ref><ref name="Weber2011">{{cite journal |last1=Weber |first1=R.C. |last2=Lin |first2=P.-Y. |last3=Garnero |first3=E.J. |last4=Williams |first4=Q. |last5=Lognonne |first5=P. |title=Seismic Detection of the Lunar Core |journal=Science |volume=331 |issue=6015 |date=January 21, 2011 |pages=309–312 |url=http://www.earth.northwestern.edu/people/seth/351/lunarcore.2011.pdf |doi=10.1126/science.1199375 |pmid=21212323 |url-status=dead |archive-url=https://web.archive.org/web/20151015035756/http://www.earth.northwestern.edu/people/seth/351/lunarcore.2011.pdf |archive-date=October 15, 2015 |bibcode=2011Sci...331..309W |s2cid=206530647 |access-date=April 10, 2017}}</ref> This structure is thought to have developed through the [[Fractional crystallization (geology)|fractional crystallization]] of a global magma ocean shortly after the Moon's formation 4.5&nbsp;billion years ago.<ref>{{cite journal |doi=10.1038/ngeo417 |title=Timing of crystallization of the lunar magma ocean constrained by the oldest zircon |date=2009 |last1=Nemchin |first1=A. |last2=Timms |first2=N. |last3=Pidgeon |first3=R. |last4=Geisler |first4=T. |last5=Reddy |first5=S. |last6=Meyer |first6=C. |journal=[[Nature Geoscience]] |volume=2 |issue=2 |pages=133–136 |bibcode=2009NatGe...2..133N |hdl=20.500.11937/44375 |hdl-access=free}}</ref>

Crystallization of this magma ocean would have created a [[mafic]] mantle from the [[Precipitation (chemistry)|precipitation]] and sinking of the minerals [[olivine]], [[clinopyroxene]], and [[orthopyroxene]]; after about three-quarters of the magma ocean had crystallized, lower-density [[plagioclase]] minerals could form and float into a crust atop.<ref name="S06" /> The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of [[Compatibility (geochemistry)|incompatible]] and heat-producing elements.<ref name="W06" /> Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly [[anorthosite]].<ref name="L06" /> The [[Moon rock]] samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.<ref name="W06" /> The crust is on average about {{convert|50|km}} thick.<ref name="W06" />

The Moon is the second-densest satellite in the Solar System, after [[Io (moon)|Io]].<ref name="Schubert2004" /> However, the inner core of the Moon is small, with a radius of about {{convert|350|km}} or less,<ref name="W06" /> around 20% of the radius of the Moon. Its composition is not well understood but is probably metallic iron alloyed with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotation suggest that it is at least partly molten.<ref>{{cite journal |last1=Williams |first1=J.G. |last2=Turyshev |first2=S.G. |last3=Boggs |first3=D.H. |last4=Ratcliff |first4=J.T. |title=Lunar laser ranging science: Gravitational physics and lunar interior and geodesy |journal=[[Advances in Space Research]] |date=2006 |volume=37 |issue=1 |pages=67–71 |bibcode=2006AdSpR..37...67W |doi=10.1016/j.asr.2005.05.013 |arxiv=gr-qc/0412049 |s2cid=14801321}}</ref> The pressure at the lunar core is estimated to be {{cvt|5|GPa|atm}}.<ref>{{cite journal |title=The Case Against an Early Lunar Dynamo Powered by Core Convection |last1=Evans |first1=Alexander J. |last2=Tikoo |first2=Sonia M. |first3=Andrews-Hanna |last3=Jeffrey C. |journal=Geophysical Research Letters |date=January 2018 |volume=45 |issue=1 |pages=98–107 |doi=10.1002/2017GL075441 |bibcode=2018GeoRL..45...98E |doi-access=free}}</ref>