Giant impact hypothesis

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The giant impact hypothesis (or Big Splash or Big Whack; cf. Big Bang) is the now-dominant scientific theory for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body sometimes called Theia (/'θeɪ.a/: the name derives from Greek mythology, as Theia was the Titan who gave birth to the Moon goddess Selene) or Orpheus. The original hypothesis was first proposed in a paper published in Icarus in 1975 by Dr. William K. Hartmann and Dr. Donald R. Davis.

1 Origins
2 Geological evidence
3 Difficulties
4 See also
5 References
6 External links

[edit] Origins

One hypothesis posits that Theia formed at a Lagrangian point relative to Earth, i.e. in about the same orbit and about 60° ahead or behind.<ref>E. Belbruno and J. Richard Gott III, The Astronomical Journal, Volume 129, Issue 3, pp. 1724-1745 (2005) preprint</ref> When Theia had grown to about the size of Mars, its size made it too heavy for its status, comparable to the Trojan asteroids in the orbit of Jupiter, to be stable, and its angular distance from Earth varied more and more until it hit Earth.

According to the hypothesis, 4.533 billion years (4.533 Ga) ago, 34 million years after Earth formed, a Mars-sized planetesimal hit the Earth at an oblique angle, destroying the impactor and ejecting most of the impactor and a significant portion of the Earth's silicate mantle into space. Current estimates based on computer simulations of such an event suggest that some 2% of the original mass of the impactor ended up as an orbiting ring of debris, and about half coalesced into the Moon between 1 and 100 years after the impact. Whatever original rotation and inclination the proto-Earth had before the impact, after the impact it would have had a day about five hours long and its equator would have been close to the plane of the Moon's orbit.

[edit] Geological evidence

Indirect evidence for this impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope compositions that are nearly the same as the Earth. The highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich samples, gave rise to the idea that a large portion of the Moon was once molten, and a giant impact scenario could easily have supplied the energy required to form such a magma ocean. Several lines of evidence indicate that if the Moon possesses an iron-rich core, that it must be small. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response all suggest that the core is less than about 25% the radius of the Moon, in comparison to about 50% for most of the other terrestrial bodies. Impact conditions can be found that give rise to a Moon that formed mostly from the mantles of the Earth and impactor, with the core of the impactor accreting to the Earth, and which satisfy the angular momentum constraints of the Earth-Moon system<ref>R. Canup and E. Asphaug (2001). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature 412: 708-712.</ref>.

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[edit] Difficulties

Even the dominant lunar origin theory has some difficulties which have yet to be explained. These difficulties include:

Some of the Moon's volatile elements are not depleted as expected from the giant impact hypothesis.<ref name="4045.pdf">Tests of the Giant Impact Hypothesis, J. H. Jones, Lunar and Planetary Science, Origin of the Earth and Moon Conference, 1998 [1]</ref>
There is no evidence that the Earth ever had a magma ocean (an implied result of the giant impact hypothesis)<ref name="4045.pdf"/>
Iron oxide (FeO) content of 13% of the bulk Moon properties rule out the derivation of the proto-lunar material from any but a small fraction of Earth's mantle.<ref>The Bulk Composition of the Moon, Stuart R. Taylor, Lunar and Planetary Science, 1997, [2]</ref>
If the bulk of the proto-lunar material had come from the impactor, the Moon should be enriched in siderophilic elements, when it is actually deficient of those.<ref>http://www.ias.ac.in/jessci/dec2005/ilc-3.pdf</ref>

[edit] See also

[edit] References

Cited references <references /> Scientific references

William K. Hartmann and Donald R. Davis, Satellite-sized planetesimals and lunar origin, (International Astronomical Union, Colloquium on Planetary Satellites, Cornell University, Ithaca, N.Y., Aug. 18-21, 1974) Icarus, vol. 24, Apr. 1975, p. 504-515
Alastair G. W. Cameron and William R. Ward, The Origin of the Moon, Abstracts of the Lunar and Planetary Science Conference, volume 7, page 120, 1976
R. Canup and K. Righter, editors (2000). Origin of the Earth and Moon. University of Arizona Press, Tuscon, 555 pp.
Charles Shearer and 15 coauthors (2006). "Thermal and magmatic evolution of the Moon". Reviews in Mineralogy and Geochemistry 60: 365-518.

General references

Dana Mackenzie, The Big Splat, or How Our Moon Came to Be, 2003, John Wiley & Sons, ISBN 0-471-15057-6.
G. Jeffrey Taylor (December 31, 1998). Origin of the Earth and Moon.

[edit] External links

Planetary Science Institute: Giant Impact Hypothesis
Computer modelling of the Moon's creation (Space.com)
Origin of the Moon by Prof. AGW Cameron
Klemperer Rosette simulations using Java applets
SwRI giant impact hypothesis simulation (.wmv and .mov)

The Moon v • d • e</div>
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General: Phases of the Moon · Solar eclipse · Lunar eclipse · The Moon's orbit · Near and Far sides of the Moon · Tides
Lunar science: Moon rocks · Geology · Lunar mare · Lunar geologic timescale · Lunar meteorites · KREEP · Giant impact theory
Exploration: Exploration of the Moon · Manned exploration (20th Century)(21st Century) · Robotic exploration · Future missions
See also solar system, natural satellite, and impact craters.