Continuous Bombardment: Effect of Small Primary and Secondary Impacts on the Lunar Regolith

Abstract:

The Lunar Reconnaissance Orbiter Camera (LROC) began systematically mapping the Moon in the summer of 2009 with the goal of acquiring an image dataset to facilitate future exploration [1]. With the benefit of the extended science missions, we have acquired and systematically examined over 14,000 before and after Narrow Angle Camera (NAC) observations, called NAC temporal pairs. From this growing dataset, we have discovered a dynamic Moon with over a hundred new resolved craters and over 26,000 smaller features we define as splotches. These splotches lack a distinguishable crater rim, but alter the observed reflectance of the surface by a few percent. Some of these splotches contain very small primary or secondary craters, but the diameter is too small (< 3 pixels) to be clearly identified and measured in NAC images. However, many of the splotches are directly linked to recent impact events and are thought to be secondary disturbances of the regolith. Robinson et al. (2014) identified nearly 250 splotches around an 18 m impact crater that formed on 17 March 2013 [2]. The shape of some of the splotches indicated a direction of emplacement, which pointed back to the primary crater. In addition, other large populations of splotches exist around other new impacts as well, which support the hypothesis that the emplacement of distal ejecta from nearby craters may form them.

Using statistics gathered from these NAC temporal pairs and Monte Carlo simulations, we can estimate the timespan in which the entire lunar surface is affected by these small surface changes. This is important, in order to estimate the rate and understand the risk imposed by these small primary and secondary surface changes.

References: [1] Robinson et al. (2010) Space Sci. Rev., 150, 1-4, 81-124. [2] Robinson et al. (2015) Icarus, 252, pp. 229–235.