1 Photometric Analysis of the Apollo Landing Sites Ryan N. Clegg 1 , Bradley L. Jolliff 1 , Philip T. Metzger 2 1 Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Dr, St. Louis, MO 63130: PH (314) 935-5189: email: rclegg@levee.wustl.edu or blj@wustl.edu 2 Granular Mechanics and Regolith Operations Laboratory, NE-S-1, Kennedy Space Center, FL 32899: PH (321) 867-6052: email: Philip.T.Metzger@nasa.gov ABSTRACT Disturbed lunar regolith has distinctly different photometric properties than undisturbed regolith. Properties such as composition, grain size and size distribution, grain shapes, glass and Fe 0 contents, and structure (fine-scale layering or “fairy-castle” forms) determine how light is reflected from the surface. Photometry can be used to extract this information from regions with different photometric behavior or to compare disturbed and undisturbed areas. Of interest here are regions around the Apollo landers that were disturbed during descent, which we refer to as blast zones. Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images show that the blast zones are brighter and have distinctly different photometric properties than surrounding regolith, presumably owing to smoothing of the surface and/or removal/redistribution of fine particles. We use phase-ratio images for each landing site to determine the size of the blast zone around each lander and to quantify the differences in reflectance within the blast zones in comparison to nearby undisturbed regolith. Most of the blast zones are elliptical in shape, but some are irregular. The average area of regolith brightening from preliminary measurements for the blast zones is ~115,000 m 2 , with values ranging from as low as 75,000 m 2 for Apollo 15 (~150 m average diameter) to as high as 215,000 m 2 (~260 m average diameter) for Apollo 12, which has an enlarged disturbed region extending in the direction of Surveyor Crater. The average normalized reflectance is 0.0480 for the blast zone, 0.0400 for the area under the LM, and 0.0443 for areas outside the blast zone, indicating some process or combination of processes are causing differences in surface properties of the regolith. Whereas experimental work and dynamical modeling indicate that soil grains excavated from beneath the landers would have been transported and deposited many kilometers away, the images provide visible evidence for effects in close proximity to the landers. We are investigating what part of the excavated particle flow regime corresponds to these areas of visible alteration using Hapke modeling. Our working hypothesis is that surface brightening results from scouring and smoothing by exhaust gases, and exposure of fresh regolith material. INTRODUCTION The photometric properties of lunar regolith can be used to extract information about the physical, mechanical, and compositional properties of the lunar surface (e.g., Kaydash et al., 218 Earth and Space 2012 © ASCE 2012 Earth and Space 2012 Downloaded from ascelibrary.org by WASHINGTON UNIV IN ST LOUIS on 01/20/14. Copyright ASCE. For personal use only; all rights reserved.