Editing Giant-impact hypothesis (section)

== Evidence ==
Indirect evidence for the giant impact scenario comes from rocks collected during the [[Apollo program|Apollo Moon landings]], which show [[oxygen]] [[isotope]] ratios nearly identical to those of Earth. The highly [[anorthositic]] composition of the lunar crust, as well as the existence of [[KREEP]]-rich samples, suggest that a large portion of the Moon once was molten; and a giant impact scenario could easily have supplied the energy needed to form such a [[lunar magma ocean|magma ocean]]. Several lines of evidence show that if the Moon has an [[iron]]-rich core, it must be a small one. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response of the Moon all suggest that the radius of its core is less than about 25% the radius of the Moon, in contrast to about 50% for most of the other [[terrestrial planet|terrestrial]] bodies. Appropriate impact conditions satisfying the angular momentum constraints of the Earth–Moon system yield a Moon formed mostly from the [[mantle (geology)|mantle]]s of Earth and the impactor, while the core of the impactor accretes to Earth.<ref name=nature412/> Earth has the highest density of all the planets in the Solar System;<ref>{{cite web |last1=Williams |first1=David R. |title=Dr |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/ |website=NASA Space Science Data Coordinated Archive |publisher=NSSDCA |access-date=15 December 2020}}</ref> the absorption of the core of the impactor body explains this observation, given the proposed properties of the early Earth and Theia.

Comparison of the [[zinc]] isotopic composition of lunar samples with that of Earth and [[Mars]] rocks provides further evidence for the impact hypothesis.<ref name="Paniello2012">{{Cite journal | last1 = Paniello | first1 = R. C. | last2 = Day | first2 = J. M. D. | last3 = Moynier | first3 = F.| doi = 10.1038/nature11507 | title = Zinc isotopic evidence for the origin of the Moon | journal = Nature | volume = 490 | issue = 7420 | pages = 376–379 | year = 2012 | pmid = 23075987|bibcode = 2012Natur.490..376P | s2cid = 4422136 }}</ref> [[Zinc]] is strongly [[Isotope fractionation|fractionated]] when [[volatilization|volatilised]] in planetary rocks,<ref name="Moynier2006">{{Cite journal | last1 = Moynier | first1 = F. | last2 = Albarède | first2 = F. | last3 = Herzog | first3 = G. F. | doi = 10.1016/j.gca.2006.02.030 | title = Isotopic composition of zinc, copper, and iron in lunar samples | journal = Geochimica et Cosmochimica Acta | volume = 70 | issue = 24 | pages = 6103 | year = 2006 |bibcode = 2006GeCoA..70.6103M }}</ref><ref name="Moynier2009">{{Cite journal | last1 = Moynier | first1 = F. | last2 = Beck | first2 = P. | last3 = Jourdan | first3 = F. | last4 = Yin | first4 = Q. Z. | last5 = Reimold | first5 = U. | last6 = Koeberl | first6 = C. | doi = 10.1016/j.epsl.2008.11.020 | title = Isotopic fractionation of zinc in tektites | journal = Earth and Planetary Science Letters | volume = 277 | issue = 3–4 | pages = 482 | year = 2009 |bibcode = 2009E&PSL.277..482M | hdl = 20.500.11937/39896 | url = https://espace.curtin.edu.au/bitstream/20.500.11937/39896/2/120258_120258Moynier-EPSL2009_Zntektite%2520ORD.pdf | hdl-access = free }}</ref> but not during normal [[igneous]] processes,<ref name="Othman2006">{{Cite journal | last1 = Ben Othman | first1 = D. | last2 = Luck | first2 = J. M. | last3 = Bodinier | first3 = J. L. | last4 = Arndt | first4 = N. T. | last5 = Albarède | first5 = F. | title = Cu–Zn isotopic variations in the Earth's mantle | doi = 10.1016/j.gca.2006.06.201 | journal = Geochimica et Cosmochimica Acta | volume = 70 | issue = 18 | pages = A46 | year = 2006 |bibcode = 2006GeCAS..70...46B }}</ref> so zinc abundance and isotopic composition can distinguish the two geological processes. Moon rocks contain more heavy isotopes of zinc, and overall less zinc, than corresponding igneous Earth or Mars rocks, which is consistent with zinc being depleted from the Moon through evaporation, as expected for the giant impact origin.<ref name="Paniello2012"/>

Collisions between ejecta escaping Earth's gravity and asteroids would have left impact heating signatures in stony meteorites; analysis based on assuming the existence of this effect has been used to date the impact event to 4.47 billion years ago, in agreement with the date obtained by other means.<ref name = Bottke>{{Cite journal | doi = 10.1126/science.aaa0602| title = Dating the Moon-forming impact event with asteroidal meteorites| journal = Science| volume = 348| issue = 6232| pages = 321–323| year = 2015| last1 = Bottke | first1 = W. F.| last2 = Vokrouhlicky | first2 = D.| last3 = Marchi | first3 = S.| last4 = Swindle | first4 = T.| last5 = Scott | first5 = E. R. D.| last6 = Weirich | first6 = J. R.| last7 = Levison | first7 = H.|bibcode = 2015Sci...348..321B | pmid=25883354| doi-access = free}}</ref>

Warm silica-rich dust and abundant SiO gas, products of high velocity impacts{{snd}}over {{convert|10|km/s|abbr=on}}{{snd}}between rocky bodies, have been detected by the [[Spitzer Space Telescope]] around the nearby (29 [[parsec|pc]] distant) young (~12 My old) star [[HD 172555]] in the [[Beta Pictoris moving group]].<ref name = apj701/> A belt of warm dust in a zone between 0.25AU and 2AU from the young star [[HD 23514]] in the [[Pleiades]] cluster appears similar to the predicted results of Theia's collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other.<ref name=apj675/> A similar belt of warm dust was detected around the star [[BD+20°307]] (HIP 8920, SAO 75016).<ref name=nature436/>

On 1 November 2023, scientists reported that, according to computer simulations, remnants of Theia could be still visible inside the Earth as [[large low-shear-velocity provinces|two giant anomalies]] of the [[Earth's mantle]].<ref name="NYT-20231101">{{cite news |last=Chang |first=Kenneth |title=A 'Big Whack' Formed the Moon and Left Traces Deep in Earth, a Study Suggests - Two enormous blobs deep inside Earth could be remnants of the birth of the moon. |url=https://www.nytimes.com/2023/11/01/science/moon-formation-theia.html |date=1 November 2023 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20231101232849/https://www.nytimes.com/2023/11/01/science/moon-formation-theia.html |archivedate=1 November 2023 |accessdate=2 November 2023 }}</ref><ref name="NAT-20231101">{{cite journal |author=Yuan, Qian |display-authors=et al.|title=Moon-forming impactor as a source of Earth's basal mantle anomalies |url=https://www.nature.com/articles/s41586-023-06589-1 |date=1 November 2023 |journal=[[Nature (journal)|Nature]] |volume=623 |issue=7985 |pages=95–99 |doi=10.1038/s41586-023-06589-1 |pmid=37914947 |bibcode=2023Natur.623...95Y |url-status=live |archiveurl=https://archive.today/20231102061800/https://www.nature.com/articles/s41586-023-06589-1 |archivedate=2 November 2023 |accessdate=2 November 2023 }}</ref>