Dating small fresh lunar craters with Mini-RF radar observations of ejecta blankets

摘要

The ages of large impact craters on the Moon can be estimated by measuring the size-frequency distribution of smaller craters superimposed on their ejecta, but applying this method to individual craters under ～3km in diameter remains difficult due to their limited areal extent. The ejecta blankets of fresh lunar impact craters are expressed as halos of optical and radar bright material that gradually fade with time. Although compositional differences make inferring crater ages from optical albedo problematic, radar is less sensitive to composition and thus may provide a more reliable means of estimating age. In this work, we use radar data from the Miniature Radio Frequency (Mini-RF) instrument on the Lunar Reconnaissance Orbiter (LRO) to characterize the radar backscatter of large numbers of ejecta blankets. By analyzing the size-frequency distribution of these craters, we show that the lifetime of the radar-bright discontinuous ejecta blanket varies with crater diameter in a predictable way for craters with diameters between 0.45 and 5km. Absolute ages of individual craters can then be estimated by combining this empirically derived model with estimates of the relative degradation state of each individual crater. The lifetimes of radar-bright discontinuous ejecta blankets are significantly shorter in the highlands than the maria, although this is likely due to local topography, creating difficulties in applying the method to the highlands. The cosmic ray exposure age of South Ray Crater, a fresh crater visited by Apollo 16, provides confirmation of these Results. This method therefore provides a new way to accurately date small, fresh craters using the Mini-RF data set.