RADAR PROPERTIES OF IMPACT EJECTA ON THE LUNAR MARIA: A MODEL FOR DEGRADATION AND AGE

Fresh impact craters on the Moon are surrounded by “halos” of optically bright and radar-bright ejecta; both the optical and radar signatures are known to fade with time. Previous work has shown that fragments within the ejecta are gradually comminuted and eroded, and corresponding radar circular polarization ratios (CPRs) decrease. Here, we focus on constraining the rate of ejecta degradation on the lunar maria by studying the radar signatures of craters 0.8 to 5.0 km in diameter with a range of ages. The data used in this study are ~15 m/pixel S-band (12.6 cm) level 1 radar images from the Miniature Radio Frequency (Mini-RF) instrument aboard the Lunar Reconnaissance Orbiter (LRO). The use of higher resolution level 1 data, coupled with median radial profiles of the CPR of craters of interest, helps to mitigate speckle noise and produce clearer CPR profiles. Our findings indicate CPR variability among craters of similar size and varying age, with the highest CPR values in all sampled craters are associated with crater walls and rims. Moreover, CPR values decrease with outward distance from the crater rim, and the ejecta of more degraded craters exhibit lower CPR values, while the ejecta of fresh, less degraded craters exhibit higher CPR values. Based on our initial observations of the relationship between CPR and degradation state of impact craters on the lunar maria, we see evidence for a quantifiable rate of halo fading, and we anticipate that future work on this subject will help to facilitate a better understanding and quantification of the rate at which ejecta halos fade. In continuing work, we are assessing a sample set of approximately 7,000 craters [King et al., 2017 LPSC] using the methods of this preliminary study. We anticipate that a larger sample set of craters, which spans the entire lunar maria, will provide a more precise model of crater ejecta radar halo degradation as a function of age.