Giant impact hypothesis

Artist's picture of the giant impact that is thought to have formed the Moon

The giant impact hypothesis is a scientific idea about how the Moon formed. It says that around 4.5 billion years ago, the young Earth was hit by an early planet (called a proto-planet) about the size of Mars. The Moon formed out of the debris left behind after the crash. Many scientists think this is the most likely scientific hypothesis for how the Moon formed.

The object that hit Earth is sometimes called Theia. It is named after a Titan from Greek Mythology. In the stories, Theia was the mother of Selene, the moon goddess.

Evidence for this idea comes from samples collected from the Moon. These samples show that the Moon's surface was once molten (melted by heat). They also show that the Moon has a small iron core and is less dense than Earth. Scientists have also found evidence that similar crashes have happened in other star systems. These crashes created rings of rock and dust called debris disks.

However, some questions are still unanswered. Oxygen isotope ratios on the Moon are almost exactly the same as those on Earth. There is also no clear sign that another object added material to the Moon. Moon samples do not contain the ratios of some elements that scientists expected to find. These include things like volatile elements, iron oxide, or siderophilic elements (chemical elements which bond with iron). In addition, there is no evidence that Earth once had the huge ocean of magma that this hypothesis suggests.

Context

Age

The Earth's relatively large natural satellite, the Moon, is unique. Astronauts in the Apollo program brought rocks from the Moon's surface to Earth. Radiometric dating of these rocks has shown the Moon to be 4527 ±10 million years old: about 30 to 55 million years younger than other bodies in the Solar System.

New evidence suggests the Moon formed even later, around 4.48 ±0.02 Ga (billion years ago), or around 70–110 Ma (million years ago) after the start of the Solar System.

Low density

Another notable feature is the Moon's relatively low density, which must mean it does not have a large metallic core like other terrestrial bodies in the solar system do. The Moon has a bulk composition closely resembling the Earth's mantle and crust together, without the Earth's core. This has led to the giant impact hypothesis: the idea that the Moon was formed during a giant impact of the proto-Earth with another proto-planet.

Theia

The impactor, sometimes called Theia, is thought to have been a little smaller than the planet Mars. It collided with Earth about 4.533 Ga. Models show that if an impactor this size struck the proto-Earth at a low angle and relatively low speed (8–20 km/s or 5.0–12.4 mi/s), much material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space. Much of it stayed in orbit around the Earth. This material would eventually form the Moon.

However, the metallic cores of the impactor would have sunk through the Earth's mantle to fuse with the Earth's core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon's abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.

The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth's primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth's core of heavy elements untouched.

Consequences

The impact had some important consequences for the young Earth. It released an enormous amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth's mantle was vigorously convecting, the surface was a large magma ocean. The planet's first atmosphere must have been completely blown away by the enormous amount of energy released. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). The impact may also have sped up Earth’s rotation.

Recent evidence

Analysis of lunar rock brought back by Apollo astronauts appears to show traces of Theia. The researchers claim this confirms the theory that the Moon was created by a cataclysmic collision. Some scientists are surprised that the difference between the Theian material found in the Moon rock and the Earth is so small.

References

  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. ↑ 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.
  3. ↑ 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.
  4. ↑ 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.
  5. ↑ "Definition of MOLTEN". www.merriam-webster.com. 2025-04-22. Retrieved 2025-04-30.
  6. ↑ "Moonwalk". Geological Society of London. September 2009. Archived from the original (PDF) on 2012-02-29. Retrieved 2010-03-01.
  7. ↑ The Earth's Moon is larger relative to its planet than any other satellite in the solar system. Pluto's satellite Charon is relatively larger, but Pluto is considered a dwarf planet.
  8. ↑ Kleine et al. 2005. Hf-W chronometry of lunar metals and the age and early differentiation of the Moon. Science 310, 1671–1674.
  9. ↑ Halliday A.N. 2006. The origin of the Earth; what's new? Elements 2 205-210.
  10. ↑ Halliday, Alex N 2008. A young Moon-forming giant impact at 70–110 million years accompanied by late-stage mixing, core formation and degassing of the Earth. Philosophical Transactions of the Royal Society. 366, 4163-4181. doi:10.1098/rsta.2008.0209. http://rsta.royalsocietypublishing.org/content/366/1883/4163.full.
  11. ↑ Ida S; Canup R.M. & Stewart G.M. 1997. Lunar accretion from an impact-generated disk, Nature 389, 353-357.
  12. ↑ Münker, Carsten et al. 2003. Evolution of planetary cores and the Earth-Moon system from Nb/Ta systematics. Science 301 (5629): 84–87. doi:10.1126/science.1084662. PMID 12843390. http://sciencemag.org/cgi/content/abstract/301/5629/84 Archived 2005-09-13 at the Wayback Machine.
  13. ↑ Liu L-G. 1992. Chemical composition of the Earth after the giant impact. Earth, Moon, and Planets 57, 85-97.
  14. ↑ Newsom H.E. & Taylor S.R. 1989. Geochemical implications of the formation of the Moon by a single giant impact. Nature 338, 29-34.
  15. ↑ Benz W. & Cameron A.G.W. 1990. Terrestrial effects of the giant impact. LPI Conference on the Origin of the Earth. 61-67.
  16. 1 2 Ghosh, Pallab 2014. Traces of another world found on the Moon. BBC News Science & Environment.