Lunar stratigraphy

Abstract

The lunar scene is a continuous panorama of ancient impact physiography. Multi-ringed circular basins and smaller craters scar the Moon’s highlands and provide evidence of a violent early history. Basin formation, the major material-transporting mechanism on the Moon, produces a deep inner depression, one or more benches, a basin rim, and radially lineated ejecta. Study of lunar photographs indicates that, on a relative age scale, subdued basin and crater features are older representations of younger, well-preserved forms. Absolute age dating of returned samples makes it feasible to calibrate this relative age scale. All the larger basins were formed during pre-Nectarian, Nectarian and Imbrian times, i.e. 4.6- 3.9 Ga ago. Following this major sculpturing episode, and during the Imbrian and Eratosthenian times, mare volcanism became the most important mode of deposition of lunar surface materials. Basaltic lavas from deep-seated sources flowed to partially fill the impact basins and cover their peripheral troughs and surrounding lowlands between 3.8 and 3.2 Ga ago. This occurred more frequently on the near side than on the far side, probably because the farside crust is thicker. During the past 1 Ga, i.e. Copemican time, only a small number of craters were formed in both highland and mare rocks. Successes and failures of photogeologists in studying lunar stratigraphy provide the necessary lessons for understanding the geological history of the terrestrial planets. This is particularly true since both Mars and Mercury display many types of features in common with the Moon.