Giant impact theory

The Big Splash. View from the south pole.

The giant impact hypothesis (sometimes referred to as the big whack, or, less frequently, the big splash) 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 that is sometimes called Theia^[1] or, on rare occasion, Orpheus. The name of Theia (IPA: ['θei.a]) is derived from Greek mythology, as Theia was the Titan who gave birth to the Moon goddess Selene. The hypothesis was first proposed at a conference on satellites in 1974 and then published in Icarus in 1975 by Drs. Bill K. Hartmann and Donald R. Davis.

Origins

One hypothesis is that Theia formed at a Lagrangian point relative to Earth, that is, in about the same orbit and about 60° ahead or behind.^[2] When the protoplanet Theia had grown to about the size of Mars, it became too massive to reside stably in a Trojan orbit. As a result, its angular distance from Earth fluctuated, with the fluctuations growing larger until it hit the Earth. This is calculated to have occurred 4.533 billion years ago (4.533 Ga); Theia is thought to have struck the Earth at an oblique angle, destroying Theia and ejecting most of Theia's mantle and a significant portion of the Earth's mantle into space, while Theia's core sank into Earth's core. Current estimates based on computer simulations of such an event suggest that some two percent of the original mass of Theia ended up as an orbiting ring of debris, about half of which coalesced into the Moon between one and 100 years after the impact. Regardless of the rotation and inclination the Earth had before the impact, after the impact it would have had a day some five hours long, and the Earth's equator would have shifted closer to the plane of the Moon's orbit.

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 needed to form such a magma ocean. Several lines of evidence show that, if the Moon has an iron-rich core, it must be small. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response all suggest that the radius of 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.^[3]

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

Difficulties

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

• Ratios of the Moon's volatile elements are not consistent with the giant impact hypothesis.^[4]
• There is no evidence that the Earth ever had a magma ocean (an implied result of the giant impact hypothesis), and some material was found which may never have been in a magma ocean. ^[4]
• 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.^[5]
• 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.^[6]
• Certain simulations of the formation of the Moon require about twice the amount of angular momentum that the Earth-Moon system has now. However, these simulations do not take into consideration Earth's rotation before impact. Some researchers consider this as insufficient evidence for disregarding the giant impactor theory.^[7][8]

See also

• Moon
• Geology of the Moon
• KREEP
• Roche radius

References

Cited references

Scientific references

General references

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
• [http://www.swri.org/press/impact.htm SwRI giant impact hypothesis simulation] (.wmv and .mov)
• How Did The Moon Get Into Orbit?
• Origin of the Moon - computer model of accretion
• Discovery Channel DVD covering theories of moon formation

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