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What role did the Mars-sized body Theia play in the giant-impact hypothesis?
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Moon
(Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.
The giant-impact hypothesis posits that shortly after the Earth formed, it was struck by a Mars-sized body named Theia. This collision is central to explaining the origin of the Moon through the following mechanisms:1
- Debris Formation: The titanic impact ejected a cloud of fragments from the mantles of both the proto-Earth and Theia. This material aggregated into a ring around Earth before coalescing into a proto-Moon.1
- Chemical Composition: Because the ejected matter consisted primarily of mantle material, the resulting Moon is relatively depleted in iron and volatile elements, which aligns with observed lunar properties.1
- Energy and Heating: The collision generated enormous heat, which contributed to the Moon's early geological state, potentially including a magma ocean.1
- Alternative Variations: Some models suggest the impact was so violent that it vaporized both bodies into a rotating, bagel-shaped cloud called a synestia, which acted as a mixing bowl to blend their chemical elements before the Moon coalesced. Other theories propose that the debris may have initially formed two moons that later collided, potentially explaining the differences between the Moon's near and far sides.23