YOUNG LUNAR MARE BASALTS IN THE LANDING REGION OF THE CHANG’E-5 SAMPLE RETURN MISSION, NORTHERN OCEANUS PROCELLARUM

Chang’e-5, China’s first lunar sample return mission, is targeted to land in Northern Oceanus Procellarum (OP), a region selected on the basis of 1) its location outside the Apollo-Luna sample return region, 2) occurrence in the anomalous Procellarum-KREEP Terrain (PKT), 3) containing one of the youngest lunar mare basalt units (Em4), and 4) its association with Rima Sharp, the longest lunar sinuous rille. OP is also a non-mascon mare, shows a diversity of smaller gravity anomalies attributed to lava-filled buried craters, is the site of the thinnest regional crust (typically <30 km), displays three major volcanic complexes (Marius Hills, Aristarchus Plateau, Rümker Hills), has the highest concentration of silica-rich red spots (e.g., Gruithuisen/Mairan domes), has the highest abundance, longest and largest sinuous rilles, is the region of two of the youngest impact basins (Imbrium, Iridium) and may have been the site of an extremely large, very ancient impact basin. In order to provide context for returned sample analyses, we conducted a comprehensive study of the geomorphology, composition, mineralogy, and chronology of the Em4 mare basalts. The unit is superposed on Imbrian-aged low-Ti basalts, covers 37,000 km² and is composed of Eratosthenian-aged (~1.46 Ga), high-Ti basalts with a mean thickness of ~51 m and a volume between ~1450 and 2350 km³. Thorium content is high (~6.7 wt%), typical of PKT mare basalt regolith. We document minor variations in TiO₂ and FeO abundance and we found no specific source vents (e.g., fissures, cones, domes) within the unit. Detailed analysis of Rima Sharp, however, revealed that it was composed of two major rilles whose source vents, located outside the Em4 unit, enter and merge in its eastern part, suggesting that they may be the Em4 sources. The small source vent sizes and lack of significantly associated pyroclastic deposits suggest that erupting lavas had low volatile content. Regolith is thin, consistent with the young age of the unit and there is abundant evidence for vertical/lateral mixing. Returned samples 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.