Editing Giant-impact hypothesis (section)

== Difficulties ==
This lunar origin hypothesis has some difficulties that have yet to be resolved. For example, the giant-impact hypothesis implies that a surface magma ocean would have formed following the impact. Yet there is no evidence that Earth ever had such a magma ocean and it is likely there exists material that has never been processed in a magma ocean.<ref name="4045.pdf"/>

=== Composition ===
A number of compositional inconsistencies need to be addressed.

* The ratios of the Moon's volatile elements are not explained by the giant-impact hypothesis. If the giant-impact hypothesis is correct, these ratios must be due to some other cause.<ref name="4045.pdf"/>
* The presence of volatiles such as water trapped in lunar [[basalt]]s and carbon emissions from the lunar surface is more difficult to explain if the Moon was caused by a high-temperature impact.<ref name=nature454/><ref>{{cite journal | title = KAGUYA observation of global emissions of indigenous carbon ions from the Moon | journal = Science Advances | date =  6 May 2020 | first = Shoichiro | last = Yokota |author2=Kentaro Terada |author3=Yoshifumi Saito |author4=Daiba Kato |author5=Kazushi Asamura|author6=Masaki N. Nishino |author7=Hisayoshi Shimizu|author8=Futoshi Takahashi|author9=Hidetoshi Shibuya|author10=Masaki Matsushima|author11=Hideo Tsunakawa| volume = 6 | issue = 19 | pages = eaba1050 | doi=10.1126/sciadv.aba1050 | pmid = 32494721 | pmc = 7202878 | bibcode = 2020SciA....6.1050Y |issn = 2375-2548 |doi-access=free }}</ref>
* The iron oxide (FeO) content (13%) of the Moon, intermediate between that of Mars (18%) and the terrestrial mantle (8%), rules out most of the source of the proto-lunar material from Earth's mantle.<ref name=taylor07/>
* If the bulk of the proto-lunar material had come from an impactor, the Moon should be enriched in [[Siderophile element|siderophilic]] elements, when, in fact, it is deficient in them.<ref name=jess114/>
* The Moon's oxygen isotopic ratios are essentially identical to those of Earth.<ref name=wiechert/> Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body.<ref name=scott010312/> If a separate proto-planet [[Theia (planet)|Theia]] had existed, it probably would have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.<ref name=moonwalk/>
* The Moon's [[titanium isotope]] ratio (<sup>50</sup>Ti/<sup>47</sup>Ti) appears so close to Earth's (within 4 ppm), that little if any of the colliding body's mass could likely have been part of the Moon.<ref>{{cite journal | title = The proto-Earth as a significant source of lunar material | journal = Nature Geoscience | date = 25 March 2012 | first = Junjun | last = Zhang |author2=Nicolas Dauphas |author3=Andrew M. Davis |author4=Ingo Leya |author5=Alexei Fedkin | volume = 5 | issue = 4 | pages = 251–255| doi= 10.1038/ngeo1429 |bibcode = 2012NatGe...5..251Z }}</ref><ref>{{cite web | url=https://news.uchicago.edu/article/2012/03/28/titanium-paternity-test-fingers-earth-moon-s-sole-parent | title=Titanium paternity test fingers Earth as moon's sole parent | work=UChicagoNews | date=March 28, 2012 | access-date=August 13, 2012 | last=Koppes |first=Steve}}</ref>

=== Lack of a Venusian moon ===
If the Moon was formed by such an impact, it is possible that other inner planets also may have been subjected to comparable impacts. A moon that formed around [[Venus]] by this process would have been unlikely to escape. If such a moon-forming event had occurred there, a possible explanation of why the planet does not have such a moon might be that a second collision occurred that countered the [[angular momentum]] from the first impact.<ref name=baas38_491/> Another possibility is that the strong tidal forces from the Sun would tend to destabilise the orbits of moons around close-in planets. For this reason, if Venus's slow rotation rate began early in its history, any satellites larger than a few kilometers in diameter would likely have spiraled inwards and collided with Venus.<ref name=icarus202_1_12/>

Simulations of the chaotic period of terrestrial planet formation suggest that impacts like those hypothesised to have formed the Moon were common. For typical terrestrial planets with a mass of 0.5 to 1 Earth masses, such an impact typically results in a single moon containing 4% of the host planet's mass. The inclination of the resulting moon's orbit is random, but this tilt affects the subsequent dynamic evolution of the system. For example, some orbits may cause the moon to spiral back into the planet. Likewise, the proximity of the planet to the star will also affect the orbital evolution. The net effect is that it is more likely for impact-generated moons to survive when they orbit more distant terrestrial planets and are aligned with the planetary orbit.<ref name=lewis2011/>