Giant impact hypothesis

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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.

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 out into space. Some of this material then coalesced into the Moon from an orbiting ring of debris. Current estimates based on computer simulations of such an event suggest the spherical shape of the moon was attained between only one and one hundred years after the impact.

Evidence for this impact comes from rocks collected during the Apollo Moon landings, 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. Seismometers 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).

Other than the existence of the Moon itself, the primary legacy of this event, say researchers, is the fact that the Earth does not have enough of the lighter-colored felsic and intermediate rock-types to completely cover its entire surface. Thus we have continents made from felsic rocks and ocean basins which are made of the darker-colored, heavier and more metal-rich mafic rock types. This difference in composition along with the presence of water allows for an extensively active system of plate tectonics on the Earth. Others have postulated that the axial-tilt and initial rotation of the Earth had their origin at this time.

Image:Big Slash.gif The apparent improbability of a Mars-sized body hitting the Earth at exactly the correct angle to avoid completely destroying the planet, combined with the fortunate degree of axial-tilt that this event set up (thus allowing for seasons), and for making possible vigorous plate tectonics on the Earth (which is vital to the carbon cycle) has been put forward by some to explain the apparent rarity of life in the universe (the Fermi Paradox). This idea is called the Rare Earth hypothesis. However, in a recent article Edward Belbruno and Richard Gott III argue that an impact body could have formed at the Lagrangian point 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.

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de:Entstehung des Mondes gl:Big Splash ja:ジャイアント・インパクト説 pt:Big Splash ru:Теория Гигантского столкновения

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