Assessing the Formation and Implications of Self-Secondary Craters on Copernican Impact Ejecta

Impact melt ponds, flows, and veneers are found around dozens of Copernican-aged impact structures on the Moon. Melt and ejecta are emplaced contemporaneously, and should record the primary flux of inner Solar System projectiles, but recent observations
have shown that: (1) Small-diameter (<500 m) crater size-frequency distribution (CSFDs) differ between melt features and adjacent continuous ejecta associated with the same impact cratering event; (2)Ghost craters buried by impact melt suggest that ejecta blankets are cratered immediately following emplacement, and some craters formed on melt units possess unusual shapes, including irregular, circular craters with splash-like morphologies (possibly formed by secondary impacts into viscous melt ponds). Both observations favor the hypothesis that small-diameter craters are formed on continuous ejecta deposits by late-arriving ejecta fragments from the parent crater and are emplaced penecontemporaneously with the impact melt facies. These craters are referred to as self-secondary craters (SSCs). Detailed study of the abundance, distribution, and morphology, of SSCs may help to improve our understanding, first, of late-stage ejecta processes and the timing of events shaping the continuous ejecta blanket and, second, of crater size-frequency discrepancies between melt and ejecta facies and their possible effect on the lunar crater chronology. Although SSC existence has been widely proposed recently not only on the Moon, but also on Mercury, Vesta, and icy satellites, the mechanism of their formation is still not clear; existing hypotheses have yet to be shown feasible by means of numerical models. Emplacement of impact melts has been a subject of long-standing debate, and detailed analysis of impact craters on melt units gives us a new instrument which may help to resolve issues of timing and emplacement.

The two main goals of this study are: (1) To better understand the population and distribution of small craters (<500 m diameter) on Copernican crater ejecta blankets and impact melts using crater counts and numerical modeling; (2) To investigate plausible formation mechanisms for SSCs and anomalous melt craters using numerical modelling. To fulfill our goals we will complete the following four tasks: Task 1. Count craters and measure CSFDs and population density within areas of the continuous ejecta blanket of Copernican-age, melt-rich complex craters (Jackson, Copernicus), transitional craters (Giordano Bruno, Necho), and simple craters (North Ray, Cone, Mandel'shtam F). Task 2. Identify and map the occurrence of impact melt morphologies (i.e., melt facies, ponds, and veneers), and characterize and catalog enigmatic melt features (e.g., ghost craters and irregular splash-like craters) associated with the seven primary craters listed in Task 1. Task 3. Model the parent impact craters from Task 1 with special attention to melt production and ejecta fragment characteristics (i.e., ejection angles, velocities, fragment sizes, timing and duration) and their dependence on impact scenario. Task 4. Model the processes involved in the formation of splash-like craters (inferred to form when SSFs impact partially molten impact melt ponds); compare typical characteristics of modelled craters with observations from Task 2 to characterize melt units and timing of SSF deposition.

In this study we will use Lunar Reconnaissance Orbiter (LRO) data products. Two hydrocodes, SOVA and iSALE, will be used to model high-velocity parent impact crater formation and the emplacement of low-velocity ejecta.As part of our data management plan (DMP), all crater counting and mapping products will be made publicly available at open source repositories in accordance with NASA recommendations for analysis by the impact crater community.