Oxygen isotopic evidence for accretion of Earth's water before a high-energy moon-forming giant impact
Based on an extensive collection of lunar and terrestrial samples, a new study probing the elusive origins of the Moon - now typically thought to have formed from a collision between a proto-Earth and a solid impactor -- supports theories of a collision with extremely high energy. So high, in fact, that it resulted in nearly complete mixing of materials between the impactor and proto-Earth. Critically, the study further suggests that most of Earth's water was delivered before the Moon-forming impact, and not later, as often proposed. A collision between two large planetary objects with unique isotopic compositions is thought to have created the Earth-Moon system. However, explaining why the Earth and Moon don't then themselves have unique isotopic characteristics, as most planets in the Solar System do, has been challenging. To resolve this, some have proposed a high-energy collision model in which isotopes between the two were nearly equally mixed upon impact; any differences, then, may have resulted from subsequent impacts to the rocky planets, later in time. To better understand the likelihood of such a scenario in the Earth-Moon system's origin, Richard C. Greenwood and colleagues analyzed the oxygen isotopic compositions of a large set of lunar and terrestrial samples. Their analysis showed a 3- to 4-ppm (parts per million) difference between the oxygen isotopic concentrations of the lunar rocks and the terrestrial basalts, but no significant difference between the lunar samples and terrestrial olivine, a common mineral in Earth's subsurface. According to the authors, these findings are consistent with high-energy impact simulations that suggest near-complete mixing. Greenwood and colleagues suggest that the 3- to 4-ppm difference they did uncover can be explained by a "late veneer," or input of stony meteorite material to Earth in an impact event subsequent to the Moon-forming impact. Their results further imply, say the authors, that a large portion of the Earth's water was present earlier than the giant impact event that formed the Moon. In fact, no more than 5-30% of water was contributed to Earth from the late veneer process, Greenwood et al. say. The retention of Earth's ocean, despite a high-energy impact, can potentially have implications for exoplanet habitability more broadly.