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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. According to the hypothesis, 4.533 Billion Years (4.533 Ga ) ago, shortly after the formation of the Earth, a Mars-sized Planetesimal hit the Earth at an oblique angle, destroying the impactor and ejecting most of that body along with 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 impactors ended up as an orbiting ring of debris, and about half of this material coalesced into the Moon between one and one hundred years after the impact. Whatever original rotation and inclination the proto-Earth had prior to 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 after the impact. Evidence for this impact comes from rocks collected during the Apollo Moon Landing s, which show an Oxygen Isotope composition that is nearly identical to 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. Seismometer s 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 tandem formation with the Earth. 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 impacting body and not from the core of the impacting body (it is thought that the core of the impactor sank and merged with the Earth's core). The exact impact scenerio 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 the planet, 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 L4 or L5, 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 the planet 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. REFERENCES
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