^1,2Yuqi Qian,^1,3Long Xiao,²James W.Head,⁴Carolyn H.van der Bogert,⁴Harald Hiesinger,⁵Lionel Wilson
Earth and Planetary Science Letters 555, 116702 Link to Article [https://doi.org/10.1016/j.epsl.2020.116702]
¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China
²Departmental of Earth, Environmental, and Planetary Sciences, Brown University, Providence, 02912, USA
³Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei, 230026, China
⁴Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, 48149, Germany
⁵Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK
Copyright Elsevier

Chang’e-5, China’s first lunar sample return mission, is targeted to land in northern Oceanus Procellarum, within a region selected on the basis of 1) its location away from the Apollo-Luna sampling region, 2) the presence of the Procellarum KREEP Terrane (PKT), 3) the occurrence of one of the youngest lunar mare basalts (Em4), and 4) its association with Rima Sharp. In order to provide context for returned sample analyses, we conducted a comprehensive study of the regional and global settings, geomorphology, composition, mineralogy, and chronology of the Em4 mare basalts. Superposed on Imbrian-aged low-Ti basalts, Em4 covers 37,000 km2 and is composed of Eratosthenian-aged (∼1.53 Ga), high-Ti basalts with a mean thickness of ∼51 m and a volume between ∼1450 and 2350 km3. Minor variations in TiO2 and FeO abundance occur within the unit and the thorium content averages ∼6.7 ppm, typical of PKT mare basaltic regolith. No specific source vents (e.g., fissures, cones, domes) were found within the unit. We show that Rima Sharp is actually composed of three major rilles, whose source vents are located outside of, and which flow into, and merge in Em4, suggesting that they may be among the sources for Em4. Regolith thickness averages ∼7 m and there is abundant evidence for vertical and lateral mixing; the most likely sources of distal ejecta are Aristarchus, Harpalus, and Sharp B craters. Returned samples from local and distant materials delivered by impact will thus provide significant new insights into lunar geochronology, inner Solar System impact fluxes, the age of very young mare basalts, the role of the PKT in the generation of mare basalts, the role of sinuous rilles in lava flow emplacement, and the thermal evolution of the Moon.

¹William K. Hartmann,²Alessandro Morbidelli
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13595]
¹Planetary Science Institute, Tucson, Arizona, 85719‐2395 USA
²Université Côte d’Azur, CNRS–Lagrange, Observatoiré de la Côte d’Azur, CS 34229, Nice Cedex 4, F 06304 France
Published by arrangement with John Wiley & Sons

Impact rates in the first 500 Myr of the solar system are critical to an understanding of lunar geological history, but they have been controversial. The widely accepted, post‐Apollo paradigm of early lunar impact cratering (about 1975–2014) proposed very low or negligible impact cratering in the period from accretion (>4.4 Ga) to ~4.0 Ga ago, followed by an ~170 million year long spike of cataclysmic cratering, during which most prominent multi‐ring impact basins formed at age ~3.9 Ga. More recent dynamical models suggest very early intense impact rates, declining throughout the period from accretion until an age of ~3.0 Ga. These models remove the basin‐forming spike. This shift has important consequences vis‐à‐vis megaregolith evolution and properties of rock samples that can be collected on the lunar surface today. We adopt the Morbidelli et al. (2018) “accretion tail” model of early intense bombardment, declining as a function of time. We find effects differing from the previous models: early crater saturation and supersaturation; disturbance of magma ocean solidification; deep early megaregolith; and erosive destruction of the earliest multi‐ring basins, their impact melts, and their ejecta blankets. Our results explain observations such as differences in numbers of early lunar impact melts versus numbers of early igneous crustal rocks, highland breccias containing impact melts as old as 4.35 Ga, absence of a 170 Myr long spike in impact melt ages at 3.9 Ga among lunar and asteroidal meteorites, and GRAIL observations of lunar crustal structure.