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fission hypothesis One of the three classical hypotheses for lunar origin. Proposed by George Darwin in 1879, it derives the Moon from the silicate mantle of the Earth, following core separation. Although this accounts for the low density of and paucity of metallic iron in the Moon, the process requires about four times the observed angular momentum of the present Earth—Moon system. Detailed compositional differences between the Moon and the terrestrial mantle, from the Apollo data, appear fatal to the hypothesis.