Copernican and Eratosthenian craters represent the cratering record over the two most recent periods in the lunar geologic timescale. Craters from both periods exhibit crisp morphologies, 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 Gy. The global catalogue of Wilhelms (1987) is based on Apollo and Lunar Orbiter images, and includes ~250 (Copernican and Eratosthenian) craters of diameter (D) ≥30 km. 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; Lucey et al, 2000)
Currently, we classified all (n = 4066) nearside mid-latitude craters (D ≥10 km, 60° S to 60° N) and identified ~550 examples as either Copernican or Eratosthenian. Craters were classified based on the Wilhelms (1987) classification scheme – craters with high reflectance ejecta rays and crisp morphology were classified as Copernican and craters with only a crisp morphology (but no rays) were classified as Eratosthenian. Degraded craters were classified as older (pre-Eratosthenian). From a preliminary analysis of this new nearside database we find that the ratio of small (10 km 20 km) nearside Copernican craters is ~2.5, and the corresponding size ratio for Eratosthenian craters is ~ 1.24. The smaller diameter ratio for the Eratosthenian craters may indicate a more rapid transition from Eratosthenian to Imbrian age for smaller diameter craters. Currently we are classifying farside craters and have completed ~ 30% (2470 craters from a total of 7891 craters; D ≥ 10 km, 60° S to 60° N), final nearside and farside results will be reported at the SSERVI Forum.
References: Wilhelms, 1987., Grier et al., 2001, Lucey et al. 2000