|giant impact theory (jī'ənt) Pronunciation Key
A theory that explains the origins of Earth's moon, postulating that an asteroid roughly the size of Mars impacted the Earth during its formation. This impact resulted in rocky surface material being blown outward from the Earth, some of which accreted to form the Moon. This theory explains the similar oxygen isotope ratios between the Earth and the Moon as well as the Moon's lack of an iron core. It also accounts for the angular momentum that was necessary for the Moon to reach its current position.