Origin of the Moon

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Measuring the ages of lunar rocks has revealed that the Moon is about 4.6 billion years old, or about the same age as Earth and probably the rest of the solar system. Before the modern age of space exploration, scientists had three major models for the origin of the Moon. The fission from Earth model proposed that the young, molten Earth rotated so fast that it flung off some material that became the Moon. The formation in Earth orbit model claimed that the Moon formed independently, but close enough to Earth to orbit the planet. The formation far from Earth model proposed that the Moon formed independently in orbit around the Sun but was subsequently captured by Earth’s gravity when it passed close to the planet. None of these three models, however, is entirely consistent with current knowledge of the Moon. In 1975, having studied moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation: a giant, planetary impact.

The giant impact model proposes that early in Earth’s history, well over 4 billion years ago, Earth was struck by a large planet-sized body. Early estimates for the size of this object were comparable to the size of Mars, but a computer simulation by American scientists in 1997 suggested that the body would have to have been at least 2.5 to 3 times the size of Mars. The catastrophic impact blasted portions of Earth and the impacting body into Earth orbit, where debris from the impact eventually coalesced to form the Moon. After years of research on lunar rocks during the 1970s and 1980s, this model became the most widely accepted one for the Moon’s origin. The giant impact model seems to account for all of the available evidence: the similarity in composition between Earth and Moon indicated by analysis of lunar samples, the near-complete global melting of the Moon (and possibly Earth) in the distant past , and the simple fact that the other models are all inadequate to one degree or another. Research continues on the ramifications of such a violent lunar origin to the early history of Earth and the other planets.

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