Introduction: The Lunar Reconnaissance Orbiter Camera (LROC) team digitized rims of a total of 22,746 craters 5 to 20 km in diameter. A global crater density map using this dataset was created and compared to a ≥20 km diameter crater density map produced from Lunar Orbiter Laser Altimeter data . The resulting difference map revealed several regions of crater density differences.
Mapping: All craters between ~4 km and ~21 km in diameter (to ensure completeness) were digitized between 1:250,000 and 1:500,000 scale in ArcGIS. Our basemaps included: 1) a 100 m/pixel scale LROC Wide Angle Camera (WAC) monochrome (643 nm) mosaic with an average solar incidence of 60°, and 2) a 100 m/pixel LROC WAC Digital Elevation Model mosaic to demarcate craters in shadowed regions at the poles and/or subdued craters. Craters outside the 5 to 20 km diameter range were not used in the generation of the global crater density map.
Crater Density: We determined areal crater density for the two diameter ranges (5-20 km and ≥20 km) independently using a moving neighborhood method with a radius of 500 km and an output cell size 15 km. Density magnitude values for each map were divided into 10 equal-interval bins and reclassified with a ranking of 1-10 (1 being lowest density and 10 being highest). The resulting 5 to 20 km density map was subtracted from the ≥20 km density map to produce a crater density difference map. Output cell values of the difference map range from -4 to +5 with positive values representing a high density of ≥20 km craters relative to 5-20 km craters within the search area of the output cell and negative values representing low density.
Discussion: The difference map shows a high density difference west of the Mare Australe region (50°S to 70°S, 15°E to 45°E) with a value of +5, potentially indicative of resurfacing. A larger region extending from ~40°S to ~80°S and ~22°W to ~100°E had a difference value of +4. Overlaying the LROC WAC albedo map shows ejecta from Tycho crater extending into the search area of this region and thereby might have been a source of resurfacing material.
Two regions just south of Mare Moscoviense exhibit a negative crater density difference value of -4, indicating evidence for a high secondary crater population. Density differences in these areas are attributed to the combination of a high 5 to 20 km crater density region (13°N to 35°N, 123°E to 131°E) along the western side of the search area together with the relatively low number of ≥20 km in and around Moscoviense. Another area with a negative crater density difference south of Jackson crater extending from ~2°N to ~17°N and ~162°W to ~171°W is surrounded by a high number of 5 to 20 km craters. Few secondary impacts were found in this area, indicating that the density difference may be an “accident” of cratering statistics.
References:  Head et al. (2010) Science, 329, 1504-1507.