For 30 years, the prevailing model for the Moon’s formation has included a major collision. When a Mars-sized object, referred to as Theia, slammed into Earth’s surface in an unexpectedly special way it liquefied Earth many miles deep, vaporized Theia, and ejected enough material to form the Moon. Three recent discoveries affirm the vast superiority of this giant impact model compared to any other Moon formation scenario.
Previous research indicates that most of the material in the Moon came from Theia. Since Theia and Earth formed in different regions of the solar nebula, they should have noticeably different compositions. However, scientists had not yet found the expected variations in specific elements. More recent results have now revealed key differences between Earth and Moon oxygen isotopes.1 The new research also indicates that scientists might be able to identify Theia’s original composition (and, thus, location of formation).
The giant impact model also explains why the dark side of the Moon looks so different from the side we see (the near side). Tidal forces have locked the Moon into a situation where the same side always faces Earth. Not until satellites first orbited the Moon 55 years ago did scientists see the mismatch between the two sides. In the giant impact model both Earth and Theia started out molten, but the Moon’s smaller size meant it solidified more rapidly. With the Moon tidally locked, heat from the molten Earth caused its near side to cool more slowly, while the dark side cooled more quickly, concentrating a greater amount of aluminum, calcium and other hard-to-vaporize elements. Thus, the dark side formed a much thicker crust. Meteoroids striking the near side of the Moon would occasionally punch through the crust, but those that hit the dark side never penetrated the thicker crust.
Additional confirmation of the giant impact model came from research by Harvard scientists. Though an impact large enough to form the Moon would have deposited sufficient energy to liquefy Earth completely, that energy might not have distributed evenly through the planet. So some parts of the earth never liquefied. The Harvard team found isotopic evidence indicative of incomplete melting. If the result holds under further scrutiny, these chemical signatures would enable scientists to understand Earth’s primordial composition prior to contamination from the impact.
The giant impact model continues to provide a compelling explanation for how Earth hosts such a large satellite. Not only does the Moon serve an important role in keeping Earth habitable, but its formation via giant collision also demonstrates remarkable fine-tuning.
- Daniel Herwartz et al., “Identification of the Giant Impactor Theia in Lunar Rocks,” Science 344 (June 6, 2014): 1146–50.