Giant impact hypothesis

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The Big Splash. View from the south pole.

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. The original hypothesis was first proposed in a paper published in Icarus in 1975 by Dr. William K. Hartmann and Dr. Donald R. Davis.

Theia is thought to have formed Trojan to Earth, i.e. in about the same orbit and about 60° ahead or behind. When Theia had grown to about the size of Mars, its size made it too heavy for its Trojan status 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 the 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 felsic-rich 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.

Simulation of impact and formation of the moon

Evidence for this impact comes from rocks collected during the Apollo Moon landings, which show an oxygen isotope composition nearly the same as the Earth's mantle. Chemical inspection of those rocks found them to be nearly devoid of volatile and lighter elements, leading to the inference that they formed from an unusually extreme amount of heating that boiled them off. Seismometers on the Moon have measured the size of its nickel-iron core and have found that it is much smaller than predicted under other formation scenarios, such as forming as the Earth formed. A smaller core is consistent with the impact hypothesis because it predicts that the Moon was formed mostly from the mantle of the Earth and partly from the mantle of the impactor and not from the core of the impactor. It is thought that the impactor's core sank and merged with the Earth's core.

Animation of Theia forming in Earth's L₅ point and then drifting into impact. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.

The exact impact scenario which would produce the moon seems to be improbable: a Mars-sized body hit the Earth at exactly the correct angle to avoid completely destroying Earth, and produced a moon of suitable size and orbit to stabilize the precession of the Earth's axis. This moderates Earth's climate and makes Earth more hospitable to life. The apparent rarity of such impact/formation events has been put forward by some to explain the apparent rarity of life in the universe (the Fermi Paradox): the improbability of such events occurring means that formation of Earth-like climates is rare, and thus Earth-like life is also rare.

This idea is one of the arguments supporting the Rare Earth hypothesis. However, in [a recent article] Edward Belbruno and Richard Gott III argue that an impact body could have formed at the Earth-Sun Lagrangian point L₄ or L₅ (in the same orbit and 60° ahead or behind), and then drifted into a chaotic orbit that would impact the Earth with a suitably low velocity; this mechanism would allow for such impact events with a significantly increased probability.

Simulation work published in 2005 by Robin Canup suggested that Pluto's largest moon Charon could also have formed by a giant impact around 4.5 billion years ago, in this case by another Kuiper belt object between 1600 and 2000 kilometres in diameter that struck Pluto at a speed of 1 kilometre per second. Canup speculated that this process of moon formation could have been common in the early solar system.

Difficulties

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

Seismic velocity structures of the Moon's middle and lower mantle are consistent with melting of the upper mantle only. [link][link]
The Moon's volatile elements are not depleted as expected from the giant impact hypothesis. [link]
There is no evidence that the Earth ever had a magma ocean (an implied result of the giant impact hypothesis) [link]
The pre-impact Earth would need to be only ~50% of the Earth's final mass for tungsten and lead isotope compositions of the earth and moon to be consistent with measured values and for reasonable final angular momentum values. [link][link]
Heterogeneous isotope abundances in lunar mare basalts rule out vigorous convection of the early lunar magma ocean. [link]
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 the Earth's mantle. [link]
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. [link]

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
Alfred 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
Dana Mackenzie, The Big Splat, or How Our Moon Came to Be, 2003, John Wiley & Sons, ISBN 0-471-15057-6.

External links

[Planetary Science Institute page] on the Giant Impact Hypothesis. Hartmann and Davis belonged to the PSI. This page also contains several paintings of the impact by Hartmann himself.
[Southwest Research Institute, UCSC researchers identify the Moon-forming impact]
[Computer modelling of the Moon's creation]
[Southwest Research Institute press release] on the impact hypothesis of the origin of Charon.
[Klemperer Rosette simulations using Java applets]: While mostly interested in Klemperer rosettes, some of these applets simulate multiple body problems with bodies in the Lagrangian points, and demostrate chaotic variations in the orbit similar to the proposed chaotic instability in Theia's orbit which lead to the collision.
[giant impact hypothesis simulation] (.wmv and .mov)

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