Constraints on the impactor flux to the Earth–Moon system from oxygen isotopes of the lunar regolith
Anthony M. Gargano, Justin I. Simon, Erick Cano, Karen Ziegler, Charles K. Shearer, James M. D. Day, and Zachary Sharp
PNAS: Earth, Atmospheric, and Planetary Sciences
January 20, 2026, 123 (4) e2531796123
“Significance: Lunar regolith is the longest continuously accessible record of impact debris, integrating bombardment products for nearly 4 billion years. Conventional reconstructions of impactor flux based on siderophile and highly siderophile elements are often overprinted by repeated impacts and crystallization of melt sheets, obscuring projectile type and mass. We instead use triple-oxygen-isotope compositions, which are largely resilient to these overprints, and find that the regolith contains ~1 wt% of partially evaporated, CM-chondrite-like material emplaced over the last 4 billion years. This flux implies a negligible water contribution to Earth, so a “late addition” cannot explain Earth’s present water budget. By contrast, the same delivery suffices to account for most water stored in lunar cold traps.”
“The impactor flux record to Earth has largely been erased by active tectonics, weathering, and continual reworking of the crust. Instead, a record of highly siderophile elements (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) in lunar impactites has been used as a proxy for the type of impactor material added to the Earth–Moon system. Quantifying impactor mass and flux with the HSE can potentially be complicated by numerous secondary processes, however, including silicate–metal segregation and multiple impact heritage. In contrast, because oxygen has an invariant geochemical affinity, triple oxygen isotope compositions have the potential to offer a robust long-term record of impactor fluxes in complex mixtures such as regolith. Here, we use high-precision triple oxygen isotopes to deconvolve the influences of meteorite addition and silicate vaporization and identify a ubiquitous impactor contaminant comprised of partially evaporated CM or ureilite-like material representing at least 1 wt% of the lunar regolith. Water delivered to Earth by meteorite material over 4 billion years therefore is only a fraction of an ocean’s worth of water but is a significant contributor to the ice reservoir of the lunar cold traps.”
