"Global Classification of Mercury's Impact Craters based on Spectral Reflectance: Implications for Crustal Stratigraphy"

Mercury's surface displays a range of spectral properties from low-reflectance material (LRM), which has a relatively shallow spectral slope, to terrain with higher reflectance and a steeper (redder) spectral slope. LRM is often exposed from depth by impact events, and while high-reflectance red material is also exposed in impact deposits, it is most widely associated with smooth plains thought to be of volcanic origin. Using mosaics obtained by MESSENGER's Mercury Dual Imaging System, we inspected all craters ≥ 20 km in diameter for the presence of LRM and high-reflectance red material. Of the 7,450 previously mapped craters in this size range, 1,230 display spectrally distinct material in their ejecta and/or central peaks based on a manual classification utilizing a color composite designed to highlight spectral differences in composition (the second and first principal components and the 430/1000 nm ratio displayed in red, green, and blue, respectively). For each crater, the depth of origin of spectrally distinct material was estimated by calculating the maximum depth of excavation for ejecta and the maximum depth of impact melting (representing the minimum depth of origin) for central peaks. Our results confirm that LRM typically originates from deeper in the crust than other materials. The global median depth of origin for LRM is 3.8 km, compared to 3.0 km for high-reflectance red material. We also examined the origin depth for "pure" LRM, defined as material with a reflectance < 0.048 at 560 nm and value < 0.033 in the second principal component, and find that its median depth of origin is 6.1 km. The intercrater and smooth plains are stratigraphically similar to first order, with LRM found below high-reflectance red material, though the median depth of origin for both is slightly larger in the intercrater plains. This observation supports the suggestion that much of the intercrater plains share a common origin with the smooth plains as effusive volcanic deposits. Several regions of intercrater plains depart from this global trend with either a substantially higher concentration of craters that expose LRM, or a dearth of craters that expose LRM at all diameters. We are examining the geologic context of these areas, along with compositional information, to explore Mercury's stratigraphy at the regional level.