What is Giant impact hypothesis?
[[File:Artist's concept of collision at HD 172555.jpg|thumb|300 px|Artist's picture of the giant impact that is thought to have formed the Moon|alt=]] The '''giant impact hypothesis''' is that the [[Moon]] was created out of the debris from a collision between the young [[Earth]] and a [[Mars]]-sized proto[[planet]]. This is the favoured scientific [[hypothesis]] for the formation of the Moon.<ref name=belbruno-2005>{{cite journal | year=2005 | title=Where did the Moon come from? | journal=The Astronomical Journal | volume=129 | issue=3 | pages=1724–1745 | doi=10.1086/427539 | s2cid=12983980 | last1=Belbruno | first1=Edward | last2=Gott Iii | first2=J. Richard | arxiv=astro-ph/0405372 | bibcode=2005AJ....129.1724B }}</ref> Evidence for this hypothesis comes from Moon samples which show that: #the surface of the Moon was once molten #the Moon's apparently relatively small iron [[core (geology)|core]] and a lower density than the Earth, and #evidence of similar collisions in other star systems (which result in 'debris disks') The colliding body is sometimes called [[Theia]] for the mythical [[Greece|Greek]] [[Titan (mythology)|Titan]] who was the mother of [[Selene]], the goddess of the moon.<ref name=eapsl176>Halliday, Alex N. 2000. Terrestrial accretion rates and the origin of the Moon. ''Earth and Planetary Science Letters'' '''176''' (1): 17–30. doi:10.1016/S0012-821X(99)00317-9.</ref><ref name=wiechert>Wiechert U; Halliday A.N; Lee D-C; Snyder G.A; Taylor LA; Rumble D. 2001. Science. ''Science'' '''294''' (12): 345–348. doi:10.1126/science.1063037. PMID 11598294. http://www.sciencemag.org/cgi/content/abstract/294/5541/345. Retrieved 2009-07-05.</ref> There are several unanswered issues with this hypothesis. Lunar oxygen isotopic ratios are essentially identical to Earth, with no evidence of a contribution from another solar body.<ref name=moonwalk>{{Cite web | url=http://www.geolsoc.org.uk/webdav/site/GSL/shared/pdfs/Geoscientist/Download%20PDF%20copy%20of%20Geoscientist%2019.9%20September%202009.pdf | t