Cratering History and Lunar Chronology

The Moon is exceptional and important because it is the only planetary body besides the Earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units (Fig. 5.1⇓). The Moon has preserved much of its magmatic and impact record of at least the last 4 billion years. While its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces, the Moon has become a calibration plate for the cratering record of the Earth-Moon system, and by extrapolation, of the entire inner solar system if one assumes a heliocentric origin for impactor populations. These populations range from asteroids through long and short period comets to interplanetary dust, and cover a size range from hundreds of kilometers to micrometers. Figure 5.1. Telescopic view of the nearside of the Earth’s Moon with landing sites of the Apollo and Luna missions. This chapter reviews the presently available data sets in support of this paradigmatic assumption, as follows: (1) the phenomenology of lunar impact craters, (2) the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this “ground truth” is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks, (3) the theory and numerical simulation of the cratering process and the characteristics of the Earth-Moon crossing population of impactors (asteroids and comets), (4) the principles of relative age dating of lunar surface units and the general lunar stratigraphy, (5) the stratigraphic significance and ages of lunar samples (impactites and basalts) and, based on this data set, the absolute ages of lunar surface units, (6) the cratering rate of the Moon as a function of time, and (7) the time calibration of this cratering rate based on the …