PHOTOMETRIC AND COLOR PROPERTIES OF CONTEMPORARY LUNAR IMPACTS

The Lunar Reconnaissance Orbiter Camera (LROC) consists of two Narrow Angle Cameras (NACs) providing meter scale imaging and a single Wide Angle Camera (WAC) delivering broad image coverage at seven spectral band passes (321 to 689 nm). Through a systematic image search, the LROC science team has identified over 500 newly formed impact craters that have formed in the past decade. As new impacts are discovered, follow-up NAC observations are acquired over a range of lighting and viewing geometries. Photometric image sequences can be constructed from these observations spanning many years to study how the surface reflectance varies spatially and temporally and infer properties of the regolith. For one newly formed 50 m crater, we have acquired twenty NAC observations with phase angles ranging from 1.5 to 88 degrees. Based on analysis of these image sequences, we have inferred that the proximal high reflectance ejecta layer closest to the crater rim has lower porosity possibly due to the emplacement of small rocks, increase in the relative grain size, and/or compaction of the surface. Meanwhile, we infer that the adjacent proximal low reflectance zone has higher porosity (than the background regolith) possibly due to less rocks being present, decrease in the relative grain size, and/or dilation of the surface.

In addition to analyzing the photometry of the site, we have also identified color variations with the LROC WAC. While almost all the impact craters found to date are less than the pixel scale of the WAC, composite maps created with hundreds of individual photometrically normalized WAC observations can be combined to boost the signal to noise and resolve subtle features, including surface disturbances associated with all craters > 30 meters in diameter. By calculating band ratios, we can analyze the spectral slope of the crater and surrounding area. We find that the immature regolith exposed by the impact event has a blue slope compared to the same region before the impact occurred and the surrounding regolith. Meanwhile, we do not see any spectral changes corresponding to the distal zones, which is consistent with the churning of local material thought to be caused by small distal secondaries and/or impact induced jetting.