Giant-impact hypothesis

The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Proto-Earth collided with a Mars-sized co-orbital protoplanet likely from the L4 or L5 Lagrange points of the Earth's orbit approximately 4.5 billion years ago in the early Hadean eon (about 20 to 100 million years after the Solar System formed), and some of the ejected debris from the impact event later re-accreted to form the Moon. The impactor planet is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks published in a 2016 report suggests that the impact might have been a direct hit, causing a fragmentation and thorough mixing of both parent bodies. The giant-impact hypothesis is currently the favored hypothesis for lunar formation among astronomers. Evidence that supports this hypothesis includes: The Moon's orbit has a similar orientation to Earth's rotation, both of which are at a similar angle to the ecliptic plane of the Solar System. The stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin. The Earth–Moon system contains an anomalously high angular momentum, meaning the momentum contained in Earth's rotation, the Moon's rotation and the Moon's orbit around Earth is significantly higher than those of the other terrestrial planets. A giant impact could supply this excess momentum. Moon samples indicate that the Moon was once molten to a substantial, but unknown, depth. This may have required much more energy than predicted to have been available from the accretion of a celestial body of the Moon's size and mass. An extremely energetic process, such as a giant impact, could provide this energy. The Moon...