Giant impact hypothesis

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 protoplanet. This is the favoured scientific hypothesis for the formation of the Moon.^[1]

Evidence for this hypothesis comes from Moon samples which show that:

1. the surface of the Moon was once molten
2. the Moon's apparently relatively small iron core and a lower density than the Earth, and
3. evidence of similar collisions in other star systems (which result in 'debris disks')

The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.^[2][3]

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.^[4] Also, lunar samples do not have expected ratios of volatile elements, iron oxide, or siderophilic elements (chemical elements which bond with iron), and there is no evidence that the Earth ever had the magma ocean implied by the hypothesis.

1. ↑ Belbruno, Edward; Gott Iii, J. Richard (2005). "Where did the Moon come from?". The Astronomical Journal. 129 (3): 1724–1745. arXiv:astro-ph/0405372. Bibcode:2005AJ....129.1724B. doi:10.1086/427539. S2CID 12983980.
2. ↑ 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.
3. ↑ 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.
4. ↑ "Moonwalk". Geological Society of London. September 2009. Archived from the original (PDF) on 2012-02-29. Retrieved 2010-03-01.