Apollo Impact Melts Record a Rapidly Declining Impact Rate in the Late Imbrian

Abstract

Crater chronology functions are used to estimate absolute surface ages using the number density of observed craters. The calibration of chronology functions is done using crater counts on regions with radiometrically dated samples. Both the Neukum Production Function (NPF) and Robbins Production Function (RPF) contain lunar chronology functions anchored by Apollo and Luna samples that have measured absolute ages and are known to be from surfaces with measured crater densities. However, these functions were constructed using different data and assumptions for their respective crater counts, and their functions differ in the rate of decline during the Imbrian period (3–3.9 Ga). In particular, the RPF suggests that the decline in impact rate was much more rapid than the decline suggested by the NPF. We use a numerical impact bombardment model called CTEM to track the production and transportation of impact melts. Using CTEM, we simulated the bombardment history of the Moon under each chronology function, and calculated the age distribution of impact melts mixed in the top meter of regolith at locations corresponding to the Apollo 14–17 landing sites. These results were then compared to the age distribution of Apollo impact melts. We find the rapid decline suggested by the RPF to be a better match to the age distribution of Apollo impact melts than the gradual decline of the NPF.