Dating individual several-km lunar impact craters from the rim annulus in region of planned Chang'E-5 landing: Poisson age-likelihood calculation for a buffered crater counting area

The CE-5 lander was designed to retrieve a 2 m core of regolith for return to Earth. We attempt to date the most significant impact craters in the region of its landing using the statistics of superposed craters, anticipating the possibility of the presence of impact ejecta in the soil sample. The craters formed after the mare surface and would have made significant mass contributions across the region so that, if ejecta components can be recognised and dated by radioisotope methods, there is the potential to establish links to source craters. The seven largest craters in the region range in size from 3-6 km, and we attempt to date them using the annulus method first proposed by Baldwin (1985). A unit covering most of the vicinity is estimated to be 3.3 Ga old, so that we expect the seven craters to be randomly spaced over this time interval. Superposed craters in the size range 20-80 m within an annulus around the craters' rims suggest ages of <50 Ma in all cases. We interpret that these times reflect the lifetime only of structures in the upper 20 m of regolith. The population of superposed craters >250 m in diameter, however, is shown to be consistent with the age of the unit, and is thus also able to provide information on the age of individual several-km craters. The age likelihood method based on Poisson statistics, which maximises the timing information that can be obtained from small-area crater populations, is extended to cover so-called buffered crater counts. Although, in this study, we are only able to constrain the ages of the selected craters to a limited degree, the method is generally applicable to buffered counts and should substantially improve their precision elsewhere. Crown Copyright (C) 2021 Published by Elsevier B.V. All rights reserved.