A lunar time scale from the perspective of the Moon’s dynamic evolution

A geologic time scale is a chronological system that separates the geological strata of a planetary body into different units in temporal sequence and shows its progressive evolution. The time scale of the Moon was established a half-century ago during the telescopic-early Apollo exploration era, using data with limited spatial coverage and resolution. The past decades have seen a wide array of studies, which have significantly extended our understanding of global lunar geologic evolution. Based on a comprehensive review of lunar evolution with respect to the dynamical changes, we propose two major updates to the current lunar time scale paradigm to include the evolution of both endogenic and exogenic dynamic forces now known to have influenced early lunar history. Firstly, based on the temporal interplay of exogenic and endogenic processes in altering the Moon, we defined three Eon/Eonothem-level units to represent three dynamical evolutionary phases. Secondly, the pre-Nectarian System is redefined and divided as the magma ocean-era Magma-oceanian System and the following Aitkenian System beginning with the South Pole-Aitken basin. The ejecta of this basin, Das Formation, was deposited on the primordial lunar crust as the oldest stratum produced from exogenic processes. The updated lunar time scale, facilitated by the post-Apollo exploration and research advances, provides an integrated framework to depict the evolution of the Moon and has important implications for the geologic study of other terrestrial planets.