Reconstructing the late-accretion history of the Moon

• Published in: Nature (London) Vol. 571; no. 7764; pp. 226 - 229
• Main Authors: Zhu, Meng-Hua, Artemieva, Natalia, Morbidelli, Alessandro, Yin, Qing-Zhu, Becker, Harry, Wünnemann, Kai
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2019; Nature Publishing Group
• Subjects: 704/2151/209; 704/445/3928; Accretion; Astrophysics; Basins; Budgets; Computer simulation; Core loss; Deposition; Earth; Earth and Planetary Astrophysics; Earth gravitation; Geology; Gold; Gravity; Humanities and Social Sciences; Impactors; Iridium; Letter; Lunar core; Lunar crust; Lunar mantle; Magma; Monte Carlo simulation; Moon; multidisciplinary; Natural history; Osmium; Palladium; Planet formation; Platinum; Production functions; Ratios; Retention; Rhenium; Rhodium; Ruthenium; Science; Science (multidisciplinary); Sciences of the Universe; France; United States; China; Germany; Russia
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1359-0

The importance of highly siderophile elements (HSEs; namely, gold, iridium, osmium, palladium, platinum, rhenium, rhodium and ruthenium) in tracking the late accretion stages of planetary formation has long been recognized. However, the precise nature of the Moon’s accretional history remains enigmatic. There is a substantial mismatch in the HSE budgets of the Earth and the Moon, with the Earth seeming to have accreted disproportionally more HSEs than the Moon 1 . Several scenarios have been proposed to explain this conundrum, including the delivery of HSEs to the Earth by a few big impactors 1 , the accretion of pebble-sized objects on dynamically cold orbits that enhanced the Earth’s gravitational focusing factor 2 , and the ‘sawtooth’ impact model, with its much reduced impact flux before about 4.10 billion years ago 3 . However, most of these models assume a high impactor-retention ratio (the fraction of impactor mass retained on the target) for the Moon. Here we perform a series of impact simulations to quantify the impactor-retention ratio, followed by a Monte Carlo procedure considering a monotonically decaying impact flux 4 , to compute the impactor mass accreted into the lunar crust and mantle over their histories. We find that the average impactor-retention ratio for the Moon’s entire impact history is about three times lower than previously estimated 1 , 3 . Our results indicate that, to match the HSE budgets of the lunar crust and mantle 5 , 6 , the retention of HSEs should have started 4.35 billion years ago, when most of the lunar magma ocean was solidified 7 , 8 . Mass accreted before this time must have lost its HSEs to the lunar core, presumably during lunar mantle crystallization 9 . The combination of a low impactor-retention ratio and a late retention of HSEs in the lunar mantle provides a realistic explanation for the apparent deficit of the Moon’s late-accreted mass relative to that of the Earth. Lunar impact simulations find an impactor-retention ratio three times lower than previously thought and indicate that highly siderophile element retention began 4.35 billion years ago, resolving accretion mass discrepancies between Earth and the Moon.