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"The Big Splash" redirects here. For the book, see The Big Splash (book).
The giant impact hypothesis (sometimes referred to as the big whack, or, less frequently, the big splash) is the now-dominant 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[1] or, on rare occasion, Orpheus or Hephaestus. The name of Theia (IPA: /ˈθiːə/) 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. William K. Hartmann and Donald R. Davis.
[edit] OriginsOne 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. [edit] EvidenceIndirect 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 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] 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.[4] This is similar to another belt of warm dust detected around the star BD+20 307 (HIP 8920, SAO 75016).[5] [edit] DifficultiesEven the dominant lunar origin theory has some difficulties which have yet to be explained. These difficulties include:
[edit] See also[edit] ReferencesCited references
Scientific references
General references
[edit] External links
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