Editing Giant-impact hypothesis (section)

==Basic model==
[[File:Moon - Giant Impact Hypothesis - Simple model.png|alt=|thumb|449x449px|Simplistic representation of the giant-impact hypothesis.]]

Astronomers think the collision between Earth and Theia happened at about 4.4 to 4.45 billion years ago ([[bya]]); about 0.1 billion years after the [[Formation and evolution of the Solar System|Solar System began to form]].<ref>{{cite web |first=David |last=Freeman|url=http://www.huffingtonpost.com/2013/09/23/how-old-is-the-moon-younger-research_n_3975109.html?ir=Science |title=How Old Is The Moon? 100 Million Years Younger Than Once Thought, New Research Suggests |website=[[The Huffington Post]] |publisher=[[Huffington Post Media Group]]|location=New York City| date=September 23, 2013 |access-date=September 25, 2013}}</ref><ref name="Soderman_2016">{{cite web|last1=Soderman|title=Evidence for Moon-Forming Impact Found Inside Meteorites|url=http://sservi.nasa.gov/articles/evidence-for-moon-forming-impact-found-inside-meteorites/|publisher=NASA-SSERVI|access-date=7 July 2016}}</ref> In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck Earth at an [[oblique angle]] when Earth was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an initial impactor velocity below {{convert|4|km/s}} at "infinity" (far enough that gravitational attraction is not a factor), increasing as it approached to over {{convert|9.3|km/s|abbr=on}} at impact, and an impact angle of about 45°.<ref name=icarus168_2_433/> However, [[oxygen]] [[isotope]] abundance in [[lunar rock]] suggests "vigorous mixing" of Theia and Earth, indicating a steep impact angle.<ref name="YoungEtAl">{{Cite journal
 |last1=Young |first1=Edward D. |last2=Kohl |first2=Issaku E. |last3=Warren |first3=Paul H. |last4=Rubie |first4=David C. |last5=Jacobson |first5=Seth A. |last6=Morbidelli |first6=Alessandro
 |date=2016-01-29 |title=Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact
 |journal=[[Science (journal)|Science]]|publisher=[[American Association for the Advancement of Science]]|location=Washington DC|language=en
 |volume=351 |issue=6272 |pages=493–496 |doi=10.1126/science.aad0525 |issn=0036-8075 |pmid=26823426
|arxiv = 1603.04536 |bibcode = 2016Sci...351..493Y |s2cid=6548599 }}</ref><ref>{{Cite magazine
 |first=John|last=Wenz|url=http://www.popularmechanics.com/space/a19143/earth-moon-theia-collision/
 |title=The Earth and Moon Both Contain Equal Parts of an Ancient Planet
 |date=January 28, 2016|magazine=[[Popular Mechanics]]|publisher=[[Hearst Corporation]]|location=New York City|access-date=April 30, 2016
}}</ref> Theia's iron [[Core (geology)|core]] would have sunk into the young Earth's core, and most of Theia's [[Mantle (geology)|mantle]] accreted onto Earth's mantle. However, a significant portion of the mantle material from both Theia and Earth would have been [[ejecta|ejected]] into orbit around Earth (if ejected with velocities between [[orbital speed|orbital velocity]] and [[escape velocity]]) or into individual orbits around the Sun (if ejected at higher velocities).

Modelling<ref name=SalmonCanup2012/> has hypothesised that material in orbit around Earth may have accreted to form the Moon in three consecutive phases; accreting first from the bodies initially present outside Earth's [[Roche limit]], which acted to confine the inner disk material within the Roche limit. The inner disk slowly and viscously spread back out to Earth's Roche limit, pushing along outer bodies via resonant interactions. After several tens of years, the disk spread beyond the Roche limit, and started producing new objects that continued the growth of the Moon, until the inner disk was depleted in mass after several hundreds of years. Material in stable [[Kepler orbit]]s was thus likely [[Late Heavy Bombardment|to hit the Earth–Moon system]] sometime later (because the Earth–Moon system's Kepler orbit around the Sun also remains stable). Estimates based on [[computer simulation]]s of such an event suggest that some twenty percent of the original mass of Theia would have ended up as an orbiting ring of debris around Earth, and about half of this matter coalesced into the Moon. Earth would have gained significant amounts of [[angular momentum]] and [[mass]] from such a collision. Regardless of the speed and tilt of Earth's rotation before the impact, it would have experienced a day some five hours long after the impact, and Earth's equator and the Moon's orbit would have become [[Coplanarity|coplanar]].<ref name=areps15/>

Not all of the ring material need have been swept up right away: the thickened crust of the Moon's far side suggests the possibility that a second moon about {{convert|1,000|km|abbr=on}} in diameter formed in a [[Lagrange point]] of the Moon. The smaller moon may have remained in orbit for tens of millions of years.  As the two moons migrated outward from Earth, solar tidal effects would have made the Lagrange orbit unstable, resulting in a slow-velocity collision that "pancaked" the smaller moon onto what is now the far side of the Moon, adding material to its crust.<ref>{{cite news |first=Richard|last=Lovett
 |url=http://www.nature.com/news/2011/110803/full/news.2011.456.html#B1
 |title=Early Earth may have had two moons
 |publisher=Nature.com |date=2011-08-03 |access-date=2013-09-25}}</ref><ref>{{cite web|url=http://theconversation.edu.au/was-our-two-faced-moon-in-a-small-collision-2659
 |title=Was our two-faced moon in a small collision?
 |date=3 August 2011
 |publisher=Theconversation.edu.au |access-date=2013-09-25}}</ref>
Lunar magma cannot pierce through the thick crust of the far side, causing fewer [[lunar maria]], while the near side has a thin crust displaying the large maria visible from Earth.<ref>[http://www.slate.com/blogs/bad_astronomy/2014/07/01/the_moon_s_two_faces_why_are_they_so_different.html Phil Plait, ''Why Do We Have a Two-Faced Moon?'', Slate: Bad Astronomy blog, July 1, 2014]</ref>

[[File:ARC-20221004-AAV3443-MoonOrigin-Social-NASAWeb-1080p medium.oggtheora.ogg|right|thumb|Simulation of the formation of the moon caused by a giant impact.]]
Above a high resolution threshold for simulations, a study published in 2022 finds that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits. Furthermore, the outer layers of these directly formed satellites are molten over cooler interiors and are composed of around 60% proto-Earth material. This could alleviate the tension between the Moon's Earth-like isotopic composition and the different signature expected for the impactor. Immediate formation opens up new options for the Moon's early orbit and evolution, including the possibility of a highly tilted orbit to explain the lunar inclination, and offers a simpler, single-stage scenario for the origin of the Moon.<ref>{{cite journal |last1=Kegerreis |first1=J.A. |last2=Ruiz-Bonilla |first2=S. |last3=Eke |first3=V.R. |last4=Massey |first4=R.J. |last5=Sandnes |first5=T.D. |last6=Teodoro |first6=L.F.A.  |display-authors=1 |date=4 October 2022 |title=Immediate Origin of the Moon as a Post-impact Satellite |journal=The Astrophysical Journal Letters |volume=937 |issue=L40 |pages=L40 |doi=10.3847/2041-8213/ac8d96 |arxiv=2210.01814 |bibcode=2022ApJ...937L..40K |s2cid=249267497 |doi-access=free }}</ref>