The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the Earth and a Mars-sized body, sometime around 4 Ga (four billion years ago). The colliding body is sometimes called Theia, for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.[1][2] The moon has also been called Orpheus.
The giant impact hypothesis is the currently favoured scientific hypothesis for the formation of the Moon.[3] Supporting evidence includes the same direction of motion of the Earth's spin and the Moon's orbit,[4] Moon samples which indicate the surface of the Moon was once molten, the Moon's relatively small iron core and lower density than the Earth, and evidence of similar collisions in other star systems (which result in debris disks). Further, giant collisions are consistent with the leading computer models of the formation of the solar system.
There remain several questions concerning the best current models of the giant impact hypothesis. The energy of such a giant impact is predicted to heat Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle. Further, at present, there is no self-consistent model that starts with the giant impact event and follows the evolution of the debris into a single Moon. Other remaining questions include: when did the Moon lose its share of volatile elements and why does Venus, which also experienced giant impacts during its formation, not host a similar Moon?
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In 1898, George Darwin made the suggestion that the Earth and Moon 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.[5] 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.
However, Darwin's calculations could not 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.[6] Little attention was paid to Professor Daly's challenge until a conference on satellites in 1974 where the idea was reintroduced and later published and discussed in Icarus in 1975 by Drs. William K. Hartmann and Donald R. Davis. Their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of these objects may have collided with the Earth, ejecting refractory, volatile-poor dust that could coalesce to form the Moon. This collision could help explain the unique geological properties of the Moon.[7]
A similar approach was taken by Canadian astronomer Alastair G. W. Cameron and American astronomer William R. Ward, who suggested that the Moon was formed by the tangential impact of a body the size of Mars. The outer silicates of the colliding body would mostly be vaporized, whereas a metallic core would not. Hence, most of the collisional material sent into orbit would consist of silicates, leaving the coalescing Moon deficient in iron. The more volatile materials that were emitted during the collision would probably escape the Solar System, whereas silicates would tend to coalesce.[8]
The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia, who gave birth to the Moon goddess Selene. This designation was first proposed by the English geochemist Alex N. Halliday in 2000 and has since become accepted in the scientific community.[1][9] According to the giant impact hypothesis, Theia formed alongside the other planet-sized bodies in the Solar System about 4.6 Ga, and was approximately the size of Mars. One of the attractive features of the giant impact hypothesis is that the formation of the Moon fits into the context of the formation of the Earth itself: during the course of its formation, the Earth is thought to have experienced dozens of collisions with planet-sized bodies. The Moon-forming collision was only one such "giant impact."
Astronomers think the collision between Earth and Theia happened about 4.53 Ga; about 30-50 million years after the Solar System began to form. In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck the Earth at an oblique angle when the latter was nearly fully formed. Computer simulations of this "late-impact" scenario suggest an impact angle of about 45° and an initial impactor velocity below 4 km/s.[10] 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.
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.[3]
Warm silica-rich dust and abundant SiO gas, products of high velocity (> 10 km/s) impacts between rocky bodies has been detected around the nearby (29 pc distant) young (~12 My old) Beta Pic Moving Group star HD172555 by the Spitzer Space Telescope.[11] 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.[12] This is similar to another belt of warm dust detected around the star BD +20°307 (HIP 8920, SAO 75016).[13]
This lunar origin hypothesis has some difficulties that have yet to be fully resolved. These difficulties include:
Given that Earth's moon formed by such an impact, it is likely that other inner planets would also been subject to comparable impacts. A moon that formed around Venus by this process would have been unlikely to escape, so an explanation is needed to show why the planet does not have such a moon. One possibility is that a second collision occurred that countered the angular momentum from the first impact.[20] Another is that the strong tidal forces from the Sun would tend to destabilize the orbits of moons around close-in planets. For this reason, if Venus' slow rotation rate began early in its history, any satellites larger than a few kilometres in size would likely have spiraled into the planet.[21]
Simulations of the chaotic period of terrestrial planet formation suggest that impacts, such as those hypothesized to form the Moon, are common. For typical terrestrial planets with a mass of 0.5–1 Earth masses, such an impact typically result in a single moon containing 4% of the host planet's mass. The inclination of this moon's orbit is random, but this tilt affects the subsequent dynamic evolution of the system. For example, some orbits may cause the moon to spiral back into the planet. Likewise, the proximity of the planet to the star will also affect the orbital evolution. The net effect is that it is more likely for impact-generated moons to survive when they orbit more distant terrestrial planets and to be aligned with the planetary orbit.[22]
In 2004, Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III proposed that Theia coalesced at the L4 or L5 Lagrangian point relative to Earth (in about the same orbit and about 60° ahead or behind),[23][24] similar to a trojan asteroid.[4] Two-dimensional computer models suggest that the stability of Theia's proposed trojan orbit would have been affected when its growing mass exceeded a threshold of about 10% of the Earth's mass.[23] In this scenario, gravitational perturbations by planetesimals caused Theia to depart from its stable Lagrangian location, and subsequent interactions with proto-Earth led to a collision between the two bodies.[23]
In 2008, evidence was presented that suggests that the collision may have occurred later than the accepted value of 4.53 Ga, at about 4.48 Ga.[25]
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.[26] A study published in 2011 suggested that a subsequent collision between the Moon and one of these smaller bodies caused the notable differences in physical characteristics between the two hemispheres of the Moon.[27] This collision, simulations have supported, would have been at a low enough velocity so as not to form a crater; the material from the smaller body would instead have spread out across the Moon (in what would become its far side), adding a thick layer of highlands crust.[28] The resulting mass irregularities would subsequently produce a gravity gradient that helped to rotationally lock the Moon so that today only the near side remains visible from Earth.
Other mechanisms which have been suggested at various times for the Moon's origin are that the Moon was spun off from the Earth's molten surface by centrifugal force,[5] that it was formed elsewhere and later captured by the Earth's gravitational field,[29] or 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.[30]
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