50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 3100.pdf

SECONDARY ILLUMINATION CONDITIONS AT CABEUS CRATER T. J. Thompson1, P. Mahanti1, M. S. Robinson1 1Lunar Reconnaissance Orbiter Camera, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA([email protected]);

Introduction: The possibility of surface water-ice at the permanently shadowed regions (PSRs) of the lu- nar poles is intriguing and recent investigations [1] are directed towards providing quantitative evidence of sur- face water-ice. One such location is Cabeus crater, a 100 km diameter crater centered at (85◦S, 43◦W). Sci- ence investigations conducted at Cabeus reveal interest- ing results. Others have investigated the thermal envi- ronment based on illumination modeling [2] and shown areas where ice would not be stable at the surface but would be at shallow depth. Measurements by the Di- viner Lunar Radiometer [3] on LRO show areas near the poles are cold enough to trap volatiles, which is Figure 1: South Polar Stereographic projection of the Moon consistent with observations from the LCROSS impact showing the study area, Cabeus crater, in relation to other no- plume which contained a variety of volatile compounds table craters near the south pole. [4]. Neutron data from the Lunar Prospector mission [5] show enhanced epithermal neutrons but limited fast neu- the grid cell [9, 10, 11]. This calculation is executed as trons, implying burial beneath hydrogen poor overbur- a set of batch jobs on a computation cluster at LROC us- den in Cabeus. The Lunar Exploration Neutron Detec- ing Rector [12]. Lastly, the viewfactor map is multiplied tor (LEND) aboard LRO identified suppression of ep- by the primary illumination map to calculate how much ithermal neutrons from Cabeus [6]. Bistatic radar ob- wattage is cast from each cell a particular grid point. servations by Mini-RF aboard LRO show an opposition This is repeated for each grid point. Finally, a value of effect due to scatterers of cm to m scale proposed to be wattage cast toward each grid point is mapped. blocky ice beneath the surface [7]. The Lunar Crater Observation and Sensing Satellite (LCROSS) discov- Table 1: Cabeus study area NAC images for 2012-278, ered H O (absorption in near-infrared, ultraviolet emis- 2 and subsolar latitude(S lat) and longitude(S lon) sions from hydroxyl radicals) in the material thrown aloft by the impact of the Centaur upper stage [4]. In Image Exp(s) S lat S lon summary, Cabeus presents a strong case that urges fur- M1104032836R -1.12 305.62 ther investigation of the local radiation geometry to un- M1104004256L -1.13 309.64 derstand the conditions at the floor of Cabeus. In this M1104004256R -1.13 309.64 work, we focus on simulating the incident secondary M1104011401L -1.13 309.81 illumination of Cabeus crater floor using uniform sur- M1104011401R -1.13 308.63 face reflectance to compare with images acquired of the M1104018546L 0.02423 -1.12 308.64 same area. M1104018546R -1.12 307.63 Methods: To calculate wattage scattered into ar- M1104025691L -1.12 306.62 eas on the floor, we start with LOLA topography origi- M1104025691R -1.12 306.62 nally at 30 meters per pixel. The elevations were to 100 M1104032836L -1.12 305.62 meters per pixel. The average illumination conditions defined by the mean sub solar latitude and longitude Results & Discussion: for the set of NAC images (Table 1) collected in one Modelling efforts show that light gets scattered into day were used to simulate primary illumination at the every PSR, with Cabeus near the LCROSS impact be- pole using a multi-threaded lighting code developed by ing especially dark in terms of flux [13]. Long exposure LROC, which calculates incident wattage at each cell ( 24 ms) Narrow Angle Camera (NAC) images have also [8]. Then, a regular grid is cast over the region of in- been taken in the Cabeus crater PSRs to measure scat- terest (300 meters between grid points). For each grid tered light off the sun illuminated walls into the shad- point, a viewshed is calculated in GRASS GIS. Then, owed area and up to the instrument. While it is possible the topography and viewshed are used to calculate a to see brightness differences, the existence of multiple viewfactor map for each grid point which represents the light sources (lit wall facets in different orientations and fraction light entering each cell which will make it to positions) means the relative brightnesses of surfaces 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 3100.pdf

Figure 2: Calculation steps for a particular receiver(green star) A) LOLA elevation for the region B) The areas that are within the line of site of the green star (white pixels), the view- shed C) Viewfactor map, the fraction of incoming light which scatters to the green star D) Primary illumination multiplied by viewfactor map, the wattage each cell contributes to the Figure 3: The pink area shows a cropped region of high ex- green star posure NAC images all take on day 278 of 2012 within a per- manently shadowed region of Cabeus crater. Outside the pink apparent in the image could be the result of either sur- polygon is simulated primary illumination from the sun. The face properties (given all light sources are the same for NACs are in terms of digital number (DN). B) Modeled sec- all surfaces), or topographical relations between facets, ondary wattage intercepted by a grid of receivers. or a combination. Thus, our model shows how much light gets scattered to Cabeus crater where NAC images were taken to investigate whether brightness differences could be attributable to the facet relationships or surface reflectivity. Qualitatively, the general light and dark areas in the NAC images in Figure 3A also correspond to light and dark areas in Figure 3B from the modeled floor wattage interception. Since the NACs are shown uncalibrated (only decompanded) there is some differences between images. Quantitively, there is a positive linear trend with wattage (Figure 4) at the same geographic locations. At this pixel scale and under these conditions, it appears local slope and shadowing (surrounding illumination, relative plate slopes) are the primary control on NAC DN, evidenced by the correlation between this simpli- fied model and NAC DN. For future work we will apply Figure 4: For each grid point in the study area, the DN is NAC calibration, extend the DN range of comparison extracted from the underlying NAC, and the modelled wattage received at the grid point is plotted to reveal a positive rela- across multiple regions, and a photometric model. tionship. Courier Corporation. [9] M. F. Modest (2013) Radiative heat References: [1] S. Li, et al. (2018) Proceedings of the transfer Academic press. [10] A. R. Vasavada, et al. (1999) National Academy of Sciences 115(36):8907. [2] E. A. Icarus 141(2):179. [11] T. J. Thompson, et al. (2018) in Kozlova, et al. (2010) in LPSC vol. 41 1779. [3] D. A. Paige, Planetary Sci. Informatics and Data Analytics Conf. vol. et al. (2010) Science 330(6003):479. [4] A. Colaprete, et al. 2082 of LPI Contributions 6037. [12] N. M. Estes, et al. (2010) Science 330(6003):463. [5] R. S. Miller, et al. (2014) (2018) in Planetary Science Informatics and Data Analytics Icarus 233:229. [6] I. G. Mitrofanov, et al. (2010) Science Conference vol. 2082 of LPI Contributions 6039. [13] E. 330(6003):483. [7] G. W. Patterson, et al. (2017) Icarus Mazarico, et al. (2011) Icarus 211(2):1066. 283:2. [8] S. Chandrasekhar (2013) Radiative transfer