STATISTICS OF SMALL LUNAR CRATERS: ORIGIN OF DIFFERENCES BETWEEN IMPACT MELT AND EJECTA UNITS

Recent work on the lunar chronology using Lunar Reconnaissance Orbiter Camera data has reencountered a curious discrepancy in crater size-frequency distribution (CSFD) measurements between impact units that was observed, but not solved, during the Apollo era. For example, at Tycho, Copernicus, and Aristarchus, CSFDs of impact melt pools give significantly younger relative (Strom and Fielder, 1968a, b; Hartmann, 1968) and absolute model ages (Hiesinger et al., 2012; Zanetti et al., 2012) than the impact ejecta blankets, although these two units formed simultaneously. This effect has also been observed at the craters Jackson (van der Bogert et al., 2010) and King (Schultz and Spencer, 1979; Ashley et al., 2012). We investigated possible reasons for these discrepancies including differing illumination angles, poor statistics due to small count areas (<1 km²) and difficulty measuring small and/or degraded crater diameters, pollution of the primary crater population by secondary and self-secondary craters, occurrence of subsequent volcanism and/or the formation of endogenic craters, and the effects of differing target properties on the size-distribution of the small craters (<~500 m diameter). In particular, it has been suggested that self-secondary craters may cause an excess of craters on the impact ejecta blanket versus the melt pools resulting in a higher apparent age (Shoemaker, 1968; Plescia et al., 2010; Plescia and Robinson, 2011; Zanetti et al., 2012). However, based on an updated analysis of the King crater impact melt pond (after Schultz and Spencer, 1979), and our previous work, we find that target property differences play a more significant role in causing apparent age differences between the two unit types particularly for craters <~300 m diameter (Schultz et al., 1977; Schultz and Spencer, 1979; van der Bogert et al., 2010; Dundas et al., 2010; Wünnemann et al., 2012; Hiesinger et al., 2012). Understanding the causes of the discrepancies in CSFDs of small craters on contemporaneous units is important for ensuring the appropriate use of CSFDs for the derivation of absolute model ages and understanding their limitations, particularly for young and spatially limited geological units.