How old are young lunar craters?

Abstract

The accurate definition of the lunar cratering chronology is important for deriving absolute model ages across the lunar surface and throughout the Solar System. Images from the Lunar Reconnaissance Orbiter Narrow Angle Cameras and Wide‐Angle Camera and the SELENE/Kaguya Terrain Camera provide new opportunities to investigate crater size‐frequency distributions (CSFDs) on individual geological units at lunar impact craters. We report new CSFD measurements for the Copernican‐aged craters North Ray, Tycho, and Copernicus, which are crucial anchor points for the lunar cratering chronology. We also discuss possible reasons for an age discrepancy observed between the impact melt and ejecta units. Our CSFDs for North Ray and Tycho crater ejecta deposits are consistent with earlier measurements. However, for Copernicus crater and one of its rays, we find significantly lower cumulative crater frequencies than previous studies. Our new results for Copernicus crater fit the existing lunar absolute chronologies significantly better than the previous counts. Our derived model ages of the ejecta blankets of North Ray, Tycho, and Copernicus agree well with radiometric and exposure ages of the Apollo 16, 17, and 12 landing sites, respectively, and are generally consistent with a constant impact rate over the last 3 Ga. However, small variations of the impact rate cannot be resolved in our data and require further investigations.