How the Moon Formed: Violent Cosmic Crash Theory Gets Double Boost

The formation of the moon has long remained a mystery, but new studies support the theory that the moon was formed from debris left from a collision between the newborn Earth and a Mars-size rock, with a veneer of meteorites coating both afterward.

Earth was born about 4.5 billion years ago, and scientists think the moon arose a short time later. The leading explanation for the moon’s origin, known as the Giant Impact Hypothesis, was first proposed in the 1970s. It suggests the moon resulted from the collision of two protoplanets, or embryonic worlds. One of those was the just-forming Earth, and the other was a Mars-size object called Theia. The moon then coalesced from the debris.

The long-standing challenges this scenario faces are rooted in the chemistry of the moon. Most of the models of the giant-impact theory often say that more than 60 percent of the moon should be made of material from Theia. The problem is that most bodies in the solar system have unique chemical makeups, and Earth, Theia and therefore the moon should as well. However, rock samples from the moon reveal that it is puzzlingly more similar to Earth than such models would predict when it comes to versions of elements called isotopes. (Each isotope of an element has different numbers of neutrons.) [Evolution of the Moon: A Visual Timeline (Gallery)]

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