Giant impact hypothesis

One suggested pathway for 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 currently favored^[1] scientific hypothesis 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 (or Orpheus) for the mythical Greek Titan who ruled the Sun.^[2] Evidence for this hypothesis includes Moon samples which indicate the surface of the Moon was once molten, the Moon's apparently relatively small iron core, and evidence of similar collisions in other star systems.

There remain several unanswered issues surrounding this hypothesis. These include lunar samples which do not have expected ratios of volatile elements, iron oxide, or siderophilic elements, as well as a lack of evidence to suggest that the Earth ever had the magma ocean implied by this hypothesis.

Origins

In 1898, George Howard Darwin made an early suggestion the Moon and Earth had once been one body. Darwin's hypothesis was that a molten Moon had been spun from the Earth because of centrifugal forces, and this became the dominant academic explanation.^[3] Using Newtonian mechanics, he calculated that the Moon had actually orbited much closer in the past and was drifting away from the Earth. This drifting was later confirmed by American and Soviet experiments using laser ranging targets placed on the Moon. Darwin's calculations could not, however, resolve the mechanics required to trace the Moon backwards to the surface of the Earth. In 1946, Reginald Aldworth Daly of Harvard University challenged Darwin's explanation, adjusting it to postulate that the creation of the Moon was caused by an impact rather than centrifugal forces.^[4] Professor Daly's challenge was paid little attention until a conference on satellites in 1974 where it was reintroduced. It was then republished in Icarus in 1975 by Drs. William K. Hartmann and Donald R. Davis.

Theia

The name of the hypothesized protoplanet is derived from the mythical Greek goddess Theia, a Titan who gave birth to the Moon goddess Selene. According to the giant impact hypothesis, Theia formed along with the other planets of our solar system about 4.6 billion years ago, and was approximately the size of Mars. One formation theory is that it would have materialized at the L₄ or L₅ Lagrangian points relative to Earth (in about the same orbit and about 60° ahead or behind),^[1] similar to a trojan asteroid.^[5]

However, the stability of Theia's orbit was affected when its growing mass exceeded a threshold.^[quantify] At some point, greater gravitational interaction with the proto-Earth caused the two bodies to collide with each other. Astronomers think this collision happened about 4.53 billion years ago, about 30-50 million years after the rest of the Solar System formed.

Impact

In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle. Theia's iron core sank into the young Earth's core, as most of Theia's mantle and a significant portion of the Earth's mantle and crust were ejected into orbit around the Earth. This material quickly coalesced into the Moon (possibly within less than a month, but in no more than a century). 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, and about half of this matter coalesced into the Moon. The Earth would have gained significant amounts of angular momentum and mass from such a collision. Regardless of the rotation and inclination the Earth had before the impact, it would have had a day some five hours long after the impact, and the Earth's equator would have shifted closer to the plane of the Moon's orbit.^{[citation needed]}

It has been suggested that other significant objects may have been created by the impact, which could have remained in orbit between the Earth and Moon, stuck in Lagrangian points. Such objects may have stayed within the Earth-Moon system for up to 100 million years, until the gravitational tugs of other planets destabilized the system enough to free the objects.^[6]

Evidence

Animation of Theia possibly 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.

Indirect evidence for this impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios identical to those of 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 contrast 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.^[7]

A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades cluster appears similar to the predicted results of Theia's collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other.^[8] This is similar to another belt of warm dust detected around the star BD +20°307 (HIP 8920, SAO 75016).^[9]

Difficulties

This lunar origin hypothesis 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 (not resolved as of 1998^[update])^[10]
• There is no evidence that the Earth ever had a magma ocean (an implied result of the giant impact hypothesis), and it is likely there exists material which has never been processed by a magma ocean (not resolved as of 1998^[update])^[10]
• 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 (not resolved as of 1997^[update])^[11]
• 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 (not resolved as of 2005^[update])^[12]

Alternate hypotheses

Other mechanisms which have been suggested at various times for the Moon's origin are that the Moon was spun off of the Earth's surface by centrifugal force,^[3] that it was formed elsewhere and later captured by the Earth's gravitational field,^[13] and that the Moon formed at the same time and place as the Earth from the same accretion disk. Each of these hypotheses is claimed to lack a mechanism to account for the high angular momentum of the Earth–Moon system.^[14]

See also

Astronomy portal

Space portal

• Geology of the Moon
• History of Earth
• Roche limit

References

Further reading

Academic articles

Non-academic books

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)
• Origin of the Moon - computer model of accretion
• Moon Archive - Including articles about the giant impact hypothesis
• Planet Smash-Up Sends Vaporized Rock, Hot Lava Flying (2009-08-10 JPL News)

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