KentuckyFC writes: The famous Oklo georeactor in Gabon is a natural nuclear fission reactor where some 1.5 billion years ago, the concentration of uranium was high enough to start a chain reaction which generated roughly 100 kW of power for several hundred thousand years. Now planetary geologists claim a similar nuclear chain reaction could have been responsible for the origin of the Moon. The thinking is that the Earth originally formed from a rapidly spinning blob of molten rock in which the force of gravity only just overcame the centrifugal forces at work. The spinning tended to concentrate heavy radioactive elements such as uranium and thorium near the surface near the equator. That led to a runaway chain reaction and a nuclear explosion that blasted a substantial chunk of material into orbit. The new theory explains why the isotopic content of the Moon and Earth rocks are almost identical. By contrast, the current leading theory of lunar formation is that the Moon formed after the young Earth was hit by another giant body, in which case the Moon's composition ought to be substantially different from Earth's. Interestingly, the idea that Earth and Moon originated from the same parent body was first put forward in 1879 by George Darwin, son of Charles.