The Moon’s Big Impact

For the first time, scientists have found physical evidence that supports the reigning hypothesis of how Earth’s moon was formed (Science 2014, DOI: 10.1126/science.1251117). A Mars-sized body, which scientists call Theia, is believed to have slammed into early Earth billions of years ago. In the aftermath, fragments, mostly of Theia, coalesced into the modern-day moon. However, no differences between moon and Earth rock samples had been found to show evidence that Theia came calling. A standard marker for such differences is the ¹⁷O/¹⁶O ratio, which has a characteristic value in most of our solar system’s bodies. Daniel Herwartz of Georg August University, in Göttingen, Germany, and colleagues took a closer look at freshly prepared lunar samples, using the latest in isotope ratio mass spectrometry techniques. They found that the ¹⁷O/¹⁶O ratio of lunar samples was shifted 12 ppm from the ratio in Earth samples. That difference is enough to validate the giant impact hypothesis, the researchers say. “The next goal is to find out how much material of Theia is in the moon,” Herwartz says.