Formation age of the Rima Sharp sinuous rill on the Moon, source of the returned Chang’e-5 samples

Abstract

Context. In December 2020, China’s Chang’e-5 spacecraft successfully returned 1731 g of lunar samples to Earth. Rima Sharp, one of the longest sinuous rilles on the Moon, lies very near (<20 km) to the Chang’e-5 sampling site and is very likely to be the source of the returned samples. Constraining its formation age would provide key references for the ongoing sample analyses. Aims. Our intent is to obtain the accurate formation ages of Rima Sharp and related lunar rilles through crater population studies and to assess their implications for the Chang’e-5 sample analyses and interpretations. Methods. We utilized two separate techniques to measure and analyze craters that were formed after the formation of the studied sinuous rilles: the conventional crater statistics for the rille floors and the buffered crater statistics that are specialized in the dating of narrow linear features. We were able to carefully compare the results derived from the two methods and estimate the formation ages of these rilles. Results. We suggest that the buffered crater statistic method produces more reliable ages for the studies rilles than the conventional method. The formation sequence of sinuous rilles around the Chang’e-5 site is proposed as follows, in chronological order: Rimae Harpalus, Louville, Sharp, and Mairan. The basalt samples returned by Chang’e-5 are likely to have mainly been sourced from Rima Sharp, with an estimated age of ~1.7 Ga based on the crater N(1) value (spatial density of craters ±1 km in diameter) of (2.10 ± 0.20) × 10−3 km−2 and the new crater chronology function (CF). The other, younger rille, Rima Mairan, which was formed ~1.0 Ga (N(1) = (1.17 ± 0.20) × 10−3 km−2 and with the same CF), may also contribute to the basaltic deposits at the landing site. These crater count statistics and dating results can be coordinated with the radio-isotopic age of Chang’e-5 samples for a recalibration of lunar impact flux and the crater population dating method.