Until now, scientists have thought that the origin of Earth’s moon was a glancing blow between Earth and a rock the size of Mars named Theia that occurred 100 million years after the formation of the solar system. New evidence is leading scientists to the theory that “the collision that formed the moon was a high-energy impact,” according to cosmochemist Edward Young, from the University of California, Los Angeles, and lead study author. This means a head-on collision that thoroughly mixed together the rocks that went on to form to the two bodies prior to their separation.
Young said in a story in Forbes that a comparison of rocks found here on earth and those found on the moon show they both contain identical oxygen isotopes, meaning they were well-mixed, indicative of a giant impact. The samples used in the study were collected during Apollo 12, 15 and 17 moon missions. Young said the oxygen isotopes from the two sources are “indistinguishable.”
The head-on collision would have occurred about 4.5 billion years ago. If it were a glancing blow, or merely a sideswipe of planet earth, scientists would expect to find isotopes thatt were not identical.
The researchers say that at the time the impact occurred, the newly-formed Earth would have had a large amount of volcanic activity and a thin outer crust. They think that the impacting planet was probably incorporated into the Earth and moon after the collision. Young said, “Theia was thoroughly mixed into both the Earth and moon, and evenly dispersed between them. This explains why we don’t see a different signature of Theia in the moon versus the Earth.”
Although previous researchers in Germany found slight differences in oxygen isotope ratios between the moon and Earth, suggesting that less mixing occurred before the split than the new theory, Young and his colleagues say their analysis is more precise. The new analysis was able to identify and overcome potential major causes of error, such as water contamination. The findings suggest that both Theia and the newborn Earth contained a substantial amount of water bearing oxygen-17, which may have come in the form of water- or ice-laden minerals.
The new theory as to the moon’s origin is detailed in the Jan. 29 issue of the journal Science.