Absolute model ages of three craters in the vicinity of the Chang'E-5 landing site and their geologic implications

The Chang'E-5 (CE-5) mission has successfully collected lunar regolith samples from the northern Oceanus Procellarum. The Eratosthenian-aged mare units where the CE-5 landed on, might have been contaminated by distant ejecta delivered by several craters, such as Pythagoras, Sharp B, Harpalus, Copernicus, and other craters. Laboratory dating of distant ejecta in the CE-5 regolith can provide the formation ages of these craters. Therefore, these ejecta materials, the same as local young mare basalts, would serve as new calibration points for lunar chronology curve. Among above craters, the absolute model ages (AMAs) of Pythagoras, Sharp B and Harpalus are not well constrained. Here we used the Crater Size Frequency distribution (CSFD) method to yield their formation ages and the crater density values (N(1)/(km−2)). Using high resolution images, craters on the selected counting areas of their floors, ejecta blankets were identified. Our results indicate that Pythagoras is an Eratosthenian-age crater with an AMA of 2.13 ± 0.16 Ga and a N(1) =1.79 × 10−3 km−2. The Sharp B and Harpalus are both Copernican. The AMA of Sharp B crater is 1.01 ± 0.12 Ga with a N(1) of 8.46 × 10−4 km−2 while the AMA of Harpalus is 1.10 ± 0.04 Ga with a N(1) of 9.22 × 10−4 km−2. The ejecta from Sharp B and Harpalus as well as local mare basalt of the CE-5 site may be served as calibration points for lunar chronology curve, which will fill the age gaps of lunar chronology curve. Based on the estimated ages, we interpreted that Pythagoras ejecta might be beneath the young CE-5 mare unit, while the Sharp B and Harpalus ejecta should overlie the surface of the CE-5 site.