Copernican and Eratosthenian craters represent the cratering record over the two most recent periods in the lunar geologic timescale. Copernican and Eratosthenian craters both exhibit crisp morphology but only the former have high reflectance rays. Clarifying their classification and abundance is essential for understanding the cratering rate in the solar system over the last 3 G.y. Wilhelm’s global catalog (1987) identified craters from Apollo and Lunar Orbiter images and classified ~250 (Copernican and Eratosthenian) craters of diameter (D) 30 km and larger. Grier et al. (2001) identified ~50 Copernican craters (D > 20 km) globally using the maturity index of Lucey et al. (2000). We extend the previous efforts by expanding the crater size range down to D >= 10 km using a monochrome mosaic and normalized reflectance basemap from the Lunar Reconnaissance Orbiter Camera (LROC) and also the optical maturity parameter index (OMAT) derived from Clementine images (Lucey et al, 2000)
Currently, we have classified all (total number of craters = 3998) nearside craters (D >= 10 km, 60° S to 60° N) and have identified ~550 near side craters as either Copernican or Eratosthenian. Craters were classified based on Wilhelm’s (1987) classification scheme – craters with high reflectance ejecta rays and crisp morphology were classified as Copernican and craters with only the latter characteristic (i.e., crisp morphology, but no rays) were classified as Eratosthenian. Degraded craters were classified as older.
On the far side, we have classified more than 30% of all craters (D >= 10 km, 60° S to 60° N; 2470 craters of a total of 7891 craters) and have identified 308 craters as Eratosthenian or younger. Based on our preliminary classification of the nearside craters, the count ratio of small (10 km 20 km) Copernican craters is ~2.5, and the corresponding ratio for Eratosthenian craters is ~ 1.24. Similar (small to large) count ratio statistics from the current preliminary classification for the farside are 1.63 for Copernican and 0.97 for Eratosthenian. The ratios from both nearside and farside broadly show that larger impacts were less frequent compared to smaller impacts in both Copernican and Eratosthenian periods, as the current understanding of the meteorite flux would suggest. Further, from our preliminary classification, the ratio of total counts of Eratosthenian to Copernican nearside craters is ~ 6.5, and the corresponding ratio for the far side craters is ~ 13 implying more impacts (~ 6 times more at least, D > 10 km) during the Eratosthenian period compared to the Copernican period.