Editing Giant-impact hypothesis (section)

== Composition ==
In 2001, a team at the [[Carnegie Institution for Science|Carnegie Institution of Washington]] reported that the rocks from the [[Apollo program]] carried an [[Isotopic signature#Solar system origins|isotopic signature]] that was identical with rocks from Earth, and were different from almost all other bodies in the Solar System.<ref name=wiechert/>

In 2014, a team in Germany reported that the Apollo samples had a slightly different isotopic signature from Earth rocks.<ref name="Herwartz2014">{{Cite journal | doi = 10.1126/science.1251117| title = Identification of the giant impactor Theia in lunar rocks| journal = Science| volume = 344| issue = 6188| pages = 1146–1150| year = 2014| last1 = Herwartz | first1 = D.| last2 = Pack | first2 = A.| last3 = Friedrichs | first3 = B.| last4 = Bischoff | first4 = A.|bibcode = 2014Sci...344.1146H | pmid=24904162| s2cid = 30903580}}</ref> The difference was slight, but statistically significant. One possible explanation is that Theia formed near Earth.<ref name="Herwartz2014_BBC">{{cite news|url=https://www.bbc.com/news/science-environment-27688511|work=BBC News |date=2014-06-06|title=Traces of another world found on the Moon}}</ref>

This empirical data showing close similarity of composition can be explained only by the standard giant-impact hypothesis, as it is extremely unlikely that two bodies prior to collision had such similar composition.

=== Equilibration hypothesis ===
In 2007, researchers from the California Institute of Technology showed that the likelihood of Theia having an identical isotopic signature as Earth was very small (less than 1 percent).<ref name=ps2007/> They proposed that in the aftermath of the giant impact, while Earth and the proto-lunar disc were molten and vaporised, the two reservoirs were connected by a common silicate vapor atmosphere and that the Earth–Moon system became homogenised by convective stirring while the system existed in the form of a continuous fluid. Such an "equilibration" between the post-impact Earth and the proto-lunar disc is the only proposed scenario that explains the isotopic similarities of the Apollo rocks with rocks from Earth's interior. For this scenario to be viable, however, the proto-lunar disc would have to endure for about 100 years. Work is ongoing{{when|date=September 2020}} to determine whether or not this is possible.

===Direct collision hypothesis===
According to research (2012) to explain similar compositions of the Earth and the Moon based on simulations at the [[University of Bern]] by physicist Andreas Reufer and his colleagues, Theia collided directly with Earth instead of barely swiping it. The collision speed may have been higher than originally assumed, and this higher velocity may have totally destroyed Theia. According to this modification, the composition of Theia is not so restricted, making a composition of up to 50% water ice possible.<ref>{{cite web|url=http://www.nzz.ch/|title=Retuschen an der Entstehungsgeschichte des Erdtrabanten|first=Thorsten|last=Dambeck|date=11 September 2012|language=de|trans-title=Retouches on the genesis of Earth's moon|archive-url=https://web.archive.org/web/20120923014135/http://www.nzz.ch/|archive-date=23 September 2012|access-date=23 September 2012|url-status=bot: unknown}}</ref>

=== Synestia hypothesis ===
One effort, in 2018, to homogenise the products of the collision was to energise the primary body by way of a greater pre-collision rotational speed.  This way, more material from the primary body would be spun off to form the Moon.  Further computer modelling determined that the observed result could be obtained by having the pre-Earth body spinning very rapidly, so much so that it formed a new celestial object which was given the name '[[synestia]]'.  This is an unstable state that could have been generated by yet another collision to get the rotation spinning fast enough.  Further modelling of this transient structure has shown that the primary body spinning as a doughnut-shaped object (the synestia) existed for about a century (a very short time){{Citation needed|date=July 2019}} before it cooled down and gave birth to Earth and the Moon.<ref name="Boyle_2017">{{cite web|last1=Boyle|first1=Rebecca|title=Huge impact could have smashed early Earth into a doughnut shape|url=https://www.newscientist.com/article/2132763-huge-impact-could-have-smashed-early-earth-into-a-doughnut-shape|website=New Scientist|date=25 May 2017|access-date=7 June 2017}}</ref><ref name="Lock_etal_2018">{{cite journal|last1=Lock|first1=Simon J.|last2=Stewart|first2=Sarah T.|last3=Petaev|first3=Michail I.|last4=Leinhardt|first4=Zoe M.|last5=Mace|first5=Mia T.|last6=Jacobsen|first6=Stein B.|last7=Ćuk|first7=Matija|title=The origin of the Moon within a terrestrial synestia|journal=Journal of Geophysical Research|volume=123|issue=4|pages=910|date=2018|doi=10.1002/2017JE005333|arxiv=1802.10223|bibcode=2018JGRE..123..910L|s2cid=119184520}}</ref>

=== Terrestrial magma ocean hypothesis ===

Another model, in 2019, to explain the similarity of Earth and the Moon's compositions posits that shortly after Earth formed, it was covered by a [[Magma ocean|sea of hot magma]], while the impacting object was likely made of solid material. Modelling suggests that this would lead to the impact heating the magma much more than solids from the impacting object, leading to more material being ejected from the proto-Earth, so that about 80% of the Moon-forming debris originated from the proto-Earth. Many prior models had suggested 80% of the Moon coming from the impactor.<ref>{{cite web |last1=Puiu |first1=Tibi |title=Ocean of magma blasted into space may explain how the moon formed |url=https://www.zmescience.com/space/ocean-magma-moon-formation-04232/ |website=ZME Science |access-date=12 May 2019|date=2019-04-30 }}</ref><ref>{{cite journal |last1=Hosono |first1=Natsuki |last2=Karato |first2=Shun-ichiro |last3=Makino |first3=Junichiro |last4=Saitoh |first4=Takayuki R. |title=Terrestrial magma ocean origin of the Moon |journal=Nature Geoscience |volume=12 |issue=6 |pages=418–423 |date=29 Apr 2019 |doi=10.1038/s41561-019-0354-2 |bibcode=2019NatGe..12..418H |s2cid=155215215 }}</ref>