Radar Polarization Properties And Lunar Secondary Cratering

Abstract

Age dating of planetary surfaces relies on an accurate correlation between lunar crater size-frequency distributions and radiometric ages of samples returned from the Moon. For decades, it has been assumed that cratering records are dominated by "primary" impacts of interplanetary bolides [McEwen et al., 2005]. Unlike primary craters, secondary craters, which originate as ejecta from large primary events, occur in large clusters in both space and time. It was long believed that the majority of secondary craters formed at low velocities near their parent crater, resulting in a class of craters with morphologies which are easily distinguished from primary craters of a similar size [McEwen et al., 2005]. However, recent work by Bierhaus et al. (2005), McEwen et al. (2005) argues that cratering records in the Solar System may be strongly contaminated by hard-to-identify secondary craters. They advise caution when relying on counts at small diameters [McEwen et al., 2005; Bierhaus et al., 2005]. Despite the difficulties, something must be done to improve the accuracy of age dates derived from size-frequency distributions of small craters. In this thesis, a method of secondary crater identification based on radar circular polarization properties is presented. The radar polarization and photographic studies of lunar secondary craters in this thesis reveal that secondary cratering is a widespread phenomenon on the lunar surface. Tycho and Copernicus secondary craters occur at non-negligible densities in and outside of obvious albedo rays and clusters for distances of many parent crater radii. Extreme caution should be used when counting small diameter lunar craters, as the level of secondary contamination can lead to overestimates in age of lunar surfaces with true ages of less than 1 Ga. Impactinitiated debris flows highlighted by the radar CPR reveal the significant role that secondary craters play in global surface modification and transport of local materials. The same is likely true on other airless bodies with substantial regoliths, such as Mercury. More work is needed to address these phenomena on bodies such as Mars, where impact cratering is not the only active geologic process.