 SURFACE AND SUBSURFACE CHARACTERISTICS OF WESTERN ELYSIUM PLANITIA, MARS.  M. Golombek1, N. Warner2, I. J. Daubar1, D. Kipp1, R. Fergason3, R. Kirk3, A. Huertas1, R. Beyer4, S. Piqueux1, N. E. Putzig5, F. Calef1, and W. B. Banerdt1, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, 2State University of New York, Geneseo, NY, 14454, 3U.S. Geological Survey, Flagstaff AZ 86001, 4NASA Ames Research Center, Moffett Field, CA, 94035, 5Southwest Research Institute, Boulder, CO 80302.  Introduction: The InSight lander was planned to launch in 2016 and land in western Elysium Planitia. The surface and subsurface of this region have been characterized and compared to engineering constraints [1] in a landing site selection effort lasting almost four years. This effort concluded with the selection and certification of ellipse E9 after it was shown to meet all of the engineering constraints for landing and simulations indicated a >98% probability of success. This abstract provides a summary of the surface and subsurface characteristics, their comparison to landing site engineering constraints and a brief summary of the selection of ellipse E9 after the downselection to four possible ellipses [2]. Although the 2016 launch has been suspended, two of the three main engineering constraints (latitude, ellipse size and elevation) that drove the landing site selection to western Elysium Planitia would likely remain unchanged if InSight were to be launched at a future opportunity. As a result, InSight would likely land very near the E9 ellipse. Regional Setting: The InSight landing ellipse E9 is located in western Elysium Planitia on Hesperian plains just north of the dichotomy boundary. The center of the 130 km by 27 km ellipse is located at 4.4°N, 135.8°E (Fig. 1) on a surfaec mapped as Early Hesperian transition unit (eHt) [3], which could be sedimentary or volcanic. A volcanic interpretation of the plains here is supported by: 1) the presence of rocks in the ejecta of fresh craters with diameters ~0.04-20 km that argues for a strong competent layer at ~4-200 m depth and weak material beneath [4], 2) exposures of strong, jointed bedrock overlain by ~10 m of fine grained regolith in nearby Hephaestus Fossae in southern Utopia Planitia at 21.9°N, 122.0°E [4], 3) platy and smooth lava flows mapped in 6 m/pixel CTX images south of the landing site [5], and 4) the presence of wrinkle ridges, which have been interpreted to be faultpropagation folds, wherein slip on thrust faults at depth is accommodated by asymmetric folding in strong, but weakly bonded layered material (likely basalt flows) near the surface [6] (Fig. 1). Surface Terrains:  Mapping in CTX images along with verification in HiRISE images allowed the definition of terrains based on their morphology and potential hazards [7]. The main terrains mapped in the area of the ellipse are Smooth, Etched, Dark, Gradational Etched, Ridged, Crater Rim, Highland Scarp and Ejecta (Fig. 1). The E9 ellipse is located dominantly on the Smooth terrain, which is the most benign for landing. The Etched Terrain appears much rougher with lighttoned eolian beforms and high rock abundance in depressions up to ~5 m deep commonly surrounded by remnants of Smooth Terrain. The Gradational Etched and Dark Terrains appear gradational between the Smooth and Etched Terrains and are consistent with eolian erosional stripping of fines from the Smooth Terrain that would eventually lead to depressions with a rocky lag and residual bedforms. The Ridged Terrain has lobate flow fronts and sub-parallel (pressure) ridges and a younger Amazonian age, suggesting it is younger volcanics from Elysium. The Crater Rim and Highland Scarp Terrains have slopes in MOLA elevation tracks that exceed 15°. Ejecta Terrain surrounds craters and is rough with high rock densities for craters 0.4-2 km diameter. Selection of Ellipse E9: Incorporating 75 HiRISE images and 10 stereo pairs of the final four ellipses, the rock abundance, slopes, and thermal inertia of the terrains were compared to the engineering requirements [1]. Average terrain slopes and rock abundance from the 10 Digital Elevation Models (DEMs) were extended throughout the ellipses over the same terrain types, and hazard maps were made based on the modeled failure of the spacecraft on touchdown. Finally, landing points for landing ellipses were moved systematically across the hazard map to yield contour maps of the probability of success and the demarcation of the safest ellipses. Ellipse E9 exceeded 98% probability of success for all likely ellipse sizes and azimuths (that rotate clockwise with 2016 launch date). Subsequent simulations with over 70% coverage by HiRISE images, six HiRISE DEMs, 31 individually tuned photoclinometry slope maps, and 35 rock abundance maps of ellipse E9 (Fig. 1), show that it meets all of the engineering constraints and was selected and certified by the project, received concurrence from an Independent Peer Review, and passed a Planetary Protection Review in October 2015. Surface and Subsurface Characteristics:  Comparison of the landing site surface with engineering requirements shows that the Smooth Terrain is the most benign. Rock abundance determined from shadows in HiRISE images and fits to exponential models [8] in the Smooth Terrain averaged ~2.5% (well below the 10% requirement), increasing in other terrains to about 35% in the Crater Rim, and Ejecta Terrains. Rock maps of ellipse E9 have an even lower average rock abundance of 1.5%.  At length scales of ~100 m, MOLA data (extrapolated RMS slopes, relief, and pulse spread), SHARAD roughness, and CTX and HiRISE DEMs indicate very smooth and flat surfaces that would not violate the radar tracking constraint of <1% area >15° at 84 m length scale.  Slope statistics at 1-5 m length scale from HiRISE DEMs [9] and photoclinometry [10] show that the Smooth Terrain has less that 0.5% area that exceeds the 15° slope (below the engineering constraint of <1%), and is smoother at this scale than previous landing sites with the possible exception of Opportunity and Phoenix. The mean 1 m slope for E9 (~3.1°) makes it the smoothest part of the Smooth Terrain with the least area (0.3%) that exceeds 15° slope. Sampling of Corinto secondaries in DEMs and photoclinometry slope maps show much shallower depth/diameter ratios (~0.04-0.06) than expected for secondaries (0.1) and interior shapes with slopes rarely approach the 15° limit, indicating that they are not a hazard [11]. The area covered by secondaries in E9 is only 1.5% and slope distributions of the secondaries show 0-0.4% area with slopes >15°, confirming that they don't contribute significantly to the average slope distributions of the ellipses.  Analysis of TES and THEMIS data shows an average thermal inertia of ~200 J m-2 K-1 s-1/2 that is consistent with cohesionless fine sand, no dust layers >2 mm thick, very weak cohesion (<3 kPa), and no outcrops. The low level of seasonal variations in apparent thermal inertia indicates that these properties persist to a depth of 0.5-1 m beneath the surface.   Arecibo backscatter maps show a radar reflective surface [12], and when considered together with the thermal inertia, they indicate a loadbearing surface without thick deposits of fine-grained dust. SHARAD observations reveal no subsurface reflectors, water, or ice, except for isolated weak returns south of ellipse E9 that may be related to an interface at 20-43 m depth, perhaps at the basement or between basalt flows [12]. Study of rocky ejecta craters separate the effects of their degradation due to eolian activity and subsequent cratering [13] from the influence of the regolith. Results indicate that the regolith thickness varies from 3 to 6 m across the area [14], but that it might be slightly thinner near the center of ellipse E9. References: [1] Golombek, M. et al. (2013) 44th LPS Abs. #1691. [2] Golombek, M. et al. (2014) 45th LPS Abs. #1499. [3] Tanaka, K. et al. (2014) USGS Sci. Inv. Map 3292. [4] Golombek, M. et al. (2013) 44th LPS Abs. #1696. [5] Ansan, V., writ. com. [6] Mueller, K. & M. Golombek (2004) Ann. Rev. Earth Plan. Sci. 32, 435-464. [7] Wigton, N. et al. (2014) 45th LPS Abs. #1234. [8] Golombek, M. et al. (2012) Mars 7, 1-22. [9] Howington-Kraus, E. et al. (2015) 46th LPS, Abs. #2435. [10] Beyer, R. & R. Kirk (2012) Space Sci. Rev. 170, 775-791. [11] Daubar, I. et al. (2016) this issue. [12] Putzig, N. et al. (2016) this issue. [13] Sweeney, J. et al. (2016) this issue. [14] Warner, N. et al. (2016) this issue.  Fig. 1. InSight landing ellipse E9 in western Elysium Planitia showing terrain types in color and outlines of HiRISE images in black (thick outlines are stereo pairs). White is open, blue middle and orange close ellipses for the 2016 launch period. Background is THEMIS daytime thermal mosaic. Craters larger than ~40 m but smaller than ~2 km have rocky ejecta that is dark, indicating colder daytime temperatures and higher thermal inertia. These craters excavated strong coherent rock (likely basalt) from depths of ~4-200 m, with a broken up regolith on top and weaker sediments beneath. The uniform light tone of the smooth plains suggests fairly uniform thermophysical properties.        
