RECENT LUNAR IMPACTS AND EVIDENCE OF REGOLITH GARDENING (Invited Presentation)

Before and after temporal image pairs acquired by the Lunar Reconnaissance Orbiter Camera (LROC) allow for the identification of newly formed impact craters and other surface changes. Using this dataset collected over the past decade, we have identified over 500 newly formed impact craters and thousands of other surface changes. We can estimate the contemporary cratering rate on the Moon by tracking the spatial coverage, the time between each observation, and the number of craters found of various diameters (70 meters down to the resolution limit of the LROC Narrow Angle Camera - NAC). These new impact events also provide insight into the crater formation process. For example, one newly formed 70-meter crater has distal rays that span over 100 km away from the impact site. These disturbances are much broader than previous models predicted and can only be accurately identified in temporal images.

In addition to newly formed impacts, temporal images have resolved over 120,000 other surface changes called splotches. Robinson et al. (2015) first identified splotches around an 18-meter impact crater that formed on 17 March 2013. Follow-on analysis (Speyerer et al. 2016) of thousands of temporal images revealed over 40,000 new splotches across the Moon and additional searches have identified another 80,000. Splotches are interpreted as small areas of reflectance change with no morphologic signatures such as a rim structure (at the scale of the NAC). While most splotches result in a local decrease in the surface reflectance, a small population of splotches (< 10%) are brighter and thought to be the result of the exposure of immature regolith churned from depth. By mapping the distribution of splotches and their reflectance differences, we can identify regions on the Moon where the upper regolith is just starting to mature, which can help feed into models of regolith maturation. Continued observations during the next extended mission will help further constrain the cratering rate, understand the formation process, and evaluate the importance of regolith overturn in the maturation process.

References:

Robinson, M. S. et al. (2015) New crater on the Moon and a swarm of secondaries. Icarus 252, 229–235.

Speyerer, E. J. et al. (2016) Quantifying crater production and regolith overturn on the Moon with temporal imaging. Nature 538, 215-218.