NEWLY FORMED IMPACTS CHURNING THE LUNAR REGOLITH (Invited Presentation)

We identified new impact craters, recent mass wasting events, and secondary surface changes by analyzing thousands of LROC Narrow Angle Camera (NAC) observations collected of the same area under nearly identical lighting conditions. These recent changes are evidence of rapid churning and gardening of the upper few cm of lunar regolith. Our initial investigation of these NAC temporal observations led to the discovery of over 200 newly formed impact craters and 47,000 other surface disturbances [1]. Additional observations collected during the LRO Cornerstone Mission have increased our count to nearly 500 resolved impact craters and 123,000 other surface changes we describe as “splotches.” Splotches were first seen in temporal images collected around an 18 m impact crater that formed on 17 March 2013 [2]. These splotches lack a resolved rim structure at the NAC pixel scale or any other morphologic signature and instead are noted as a change in the local surface reflectance. While some splotches may be small primary impact craters (diameter < 3 pixels), many are secondary surface disturbances that are the result of distal ejecta impacting from nearby larger impact events.

Almost 90% of all splotches are low reflectance, meaning the surface reflectance is lower (average = -4%) in the later NAC observation, likely due to an increase in surface roughness from local, impact-induced churning [1]. The remaining splotches exhibit high or two-toned reflectance. The observed increase in surface reflectance (average = 10%) is thought to be due to the exposure of immature regolith churned from depth. The distribution of low-reflectance splotches is mostly uniform across the Moon, with the exception of dense clusters around larger (>20 m diameter) of the newly formed craters. Conversely, the density of high reflectance splotches is non-uniform and instead they are clustered near recent Copernican-era craters (i.e. Giordano Bruno, Tycho and Necho). We infer that high reflectance splotches form where the top mature regolith layer is thinnest (mm to cm) and their occurrence provides a bound on the rate at which the mature layer forms and is reworked at shallow depths.

References:[1] Speyerer et al. (2016) Nature, 258, 215-218. [2] Robinson et al. (2015) Icarus, 252, 229-235.