Wikipedia currently presents the origin of the Moon as the result of a Mars-sized body striking the Earth, making a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternate explanations, and research into how the Moon came to be continues.[1][2] Other proposed scenarios include captured body, fission, formed together (condensation theory), planetesimal collisions (formed from asteroid-like bodies), and collision theories.[3]
The standard giant-impact hypothesis suggests the Mars-sized body, called Theia, impacted Earth, creating a large debris ring around Earth, which then accreted to form the Moon. This collision also resulted in the 23.5° tilted axis of the earth, thus causing the seasons.[1] The Moon's oxygen isotopic ratios seem to be essentially identical to Earth's.[4] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body.[5] If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature from Earth, as would the ejected mixed material.[6] Also, the Moon's titanium isotope ratio (50Ti/47Ti) appears so close to the Earth's (within 4 ppm) that little if any of the colliding body's mass could likely have been part of the Moon.[7]
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As an interesting sidenote, a natural nuclear fission reactor was discovered by the French in 1972 in central Africa (more). For forty years, France mined for uranium in Gabon. Once extracted, the uranium was used for electricity production in France and much of Europe. Today, however, the uranium deposits are exhausted, and the mine is no longer worked. Currently, mine reclamation work is ongoing in the region affected by the mine operations. Oklo in Gabon is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging probably less than 100 kW of thermal power during that time.
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