Summary
Harold Urey's 1966 book "The Origin of the Moon" proposes the fission hypothesis, asserting that the Moon was ejected from a rapidly spinning early Earth. Urey argues that Earth's rotation, accelerated by impacts, could have spun off a mass that subsequently formed the Moon. This theory aimed to explain the Moon's relatively low density and lack of volatile elements compared to Earth.
The book elaborates on the conditions necessary for fission, including sufficient angular momentum and the physical properties of the early Earth. Urey discusses how the debris from such an event would coalesce into a single body. Readers gain an understanding of a significant early scientific proposal for lunar formation, its supporting arguments, and its limitations.
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Key concepts
- Fission Hypothesis — The theory that the Moon was formed by a large chunk of Earth being spun off due to rapid rotation.
- Earth's Angular Momentum — The rotational inertia of the early Earth, crucial for the proposed fission mechanism.
- Volatile Depletion — The observation that the Moon has significantly fewer volatile elements than Earth, which the fission hypothesis attempts to explain.
- Giant Impact Hypothesis — (Note: Urey proposed fission, but this concept is a key later development in lunar origin theories that superseded fission.) The theory that the Moon formed from debris after a Mars-sized object collided with Earth.