Young Mare Basalts in the Chang’e-5 Landing Region, Northern Oceanus Procellarum

Abstract:

Chang’e-5 (CE-5) is China’s first lunar sample return mission, scheduled at the end of 2020 and plans to return ~ 2 kg of lunar samples. Northern Oceanus Procellarum was chosen as the landing region to sample some of the youngest mare basalts on the Moon. Sampling these young mare basalts could profoundly improve our understandings of lunar thermal and impact history. A complete understanding of the landing site geological context is crucial to properly interpret the laboratory analysis of the returned samples. Thus, we report on a study of the young mare basalts in the anticipated CE-5 landing region.

The young mare basalt unit, designated Em4, is an extensive lava plain covering an area of ~ 37000 km². Rima Sharp (described as the longest sinuous rille on the Moon) extends across the area, and three source vents closely associated with the rille have been identified. We find evidence that Rima Sharp is composed of two independent sinuous rilles; their channels are combined by rille capture, and we are investigating the two sinuous rilles as possible sources of the unit. Em4 is a distinctive mare basalt type on the Moon, with high TiO₂ (6 wt. %) and FeO (17 wt. %). The MGM model has been used to quantify the mineral abundance using M³ data. The Em4 basalts are richest in CPX (47 wt. %), followed by OPX (32 wt. %) and OLV (21 wt. %), assuming CPX + OPX + OLV = 100 wt. %. The Th content of the regolith suggests that the underlying lava flow may have been generated by preferential melting in the Th-rich Procellarum-KREEP (PKT) terrane, a hypothesis testable by analysis of the returned samples.

Em4 is the youngest unit in the region. It covers all preexisting units, including Em3, and Imbrian-aged low-Ti mare basalts, and embays preexisting hills, domes, and crater rims. Based on the crater excavation technique, the average thickness of Em4 is estimated to be 39 - 63 m, and its total volume to be between 1450 - 2350 km³. A systematic crater-count chronology study of Em4 young mare basalts reveals an average age of ~ 1.46 Ga, with some age variations. The locations of the youngest basalts within Em4 has been determined. Returned regolith samples will likely include a spectrum of young ages. CE-5 will thus address a number of important questions in the geological and thermal evolution of the Moon.