Effects of rocket exhaust on lunar soil reflectance properties

High-resolution images of the Surveyor, Luna, and Apollo landing sites obtained by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) show regions around the landers where reflectivity of the surface was modified. We interpret the change in reflectance properties of these regions mainly as disturbance of the regolith by rocket exhaust during descent of the spacecraft and we refer to these areas herein as "blast zones" (BZs). The BZs consist of an area of lower reflectance (LR-BZ) compared to the surroundings that extends up to a few meters out from the landers, as well as a broader halo of higher reflectance (HR-BZ) that extends tens to hundreds of meters away from the landers. When approximated as an ellipse, the average Apollo BZ area is ~29,000m² (~175±60m by 200±27m) which is 10× larger than the average Luna BZ, and over 100× larger than the average Surveyor BZ. The LR-BZs are most evident at the Apollo sites, especially where astronaut activity disturbed the soil, leading to a 15-30% (relative to background undisturbed areas) reduction in reflectance at ~30° phase angle. The LR-BZs at the Surveyor and Luna sites are less evident and are unresolvable with NAC images. The average reflectance in the HR-BZs as determined for 30° phase angle is 3-12% higher than in the undisturbed surrounding areas; this magnitude is the same, within uncertainty, for all sites, indicating a common process or combination of processes causing differences in reflectance properties of the regolith. Phase-ratio images and photometric data collected over a range of illumination geometries show that a greater separation in reflectance occurs between the HR-BZs and undisturbed areas at phase angles between 0° and 70° and indicates that the HR-BZs are less backscattering than undisturbed areas. The LR-BZs are affected by macroscopic disruption of the surface and astronaut activity (at the Apollo sites). For the HR-BZ areas, reflectance has likely been affected by scouring from particles entrained by exhaust gases with low-angle trajectories. Regolith particle interactions with surface soil within HR-BZs may destroy fine-scale surface structure (e.g., "fairy-castle") and decrease macroscopic roughness, contributing to a decrease in backscattering character within the HR-BZs and an increase in backscattering character within the LR-BZs. Redistribution of fine particles from the LR-BZ to the HR-BZ may have also contributed to the changed reflectance. Photometric modeling is consistent with one or a combination of these processes.