Isotope Ratios Clarify Moon’s Origin

The making of our moon, according to a widely accepted theory, took place billions of years ago when a giant planetary body called Theia collided with a nascent Earth, producing debris that eventually became our planet’s satellite. Although scientists think the crash certainly occurred, they still debate the details: For example, is the moon primarily made of debris from Theia or from a mix of the two planetary bodies? One way to resolve these and other debates is to compare isotope ratios of moon and Earth rock samples. At the time of the collision, Theia and Earth would have had different isotope ratios, a characteristic of all planets. So if today’s moon and Earth samples have different isotopic ratios, then the moon likely originated from Theia’s debris only. A new study in Science finds the opposite—at least according to oxygen isotope ratios. A team of researchers led by Edward D. Young and Issaku E. Kohl of UCLA report that Earth and the moon have “indistinguishable oxygen isotope ratios” (2016, DOI: 10.1126/science.aad0525). The scientists propose that during the collision, there was substantial mixing of water and rock between both planetary bodies, leading to common, averaged oxygen isotope ratios on Earth and the moon-forming debris.