AN IMPACT ORIGIN FOR HOMESTEAD HOLLOW, THE LANDING LOCATION OF THE INSIGHT LANDER ON MARS

The InSight spacecraft landed on Hesperian-age lava plains at Elysium Planitia, Mars. Orbital images of the landing site show wrinkle ridges, degraded flow margins, and craters. InSight rests on the NW edge of a 20-30 m diameter quasi-circular depression informally named “Homestead hollow” that may represent a degraded crater. We quantified the morphometry of all craters > 20 m in diameter in a several km² region surrounding the lander to evaluate whether Homestead hollow, and other hollow-like landforms, are morphologically consistent with an impact origin. This work has implications for near-surface stratigraphy, the degradation history of lava plains, and crater retention timescales. Craters were mapped, classified based on their morphologic characteristics as observed in HiRISE images (25 m/pixel), and then measured using a HiRISE DEM (1 m postings with 10 to 20 cm vertical precision). Our classification builds on pre-landing work that established a Class 1 to 5 scheme for relatively fresh, rocky ejecta craters. A new scheme was developed that includes more degraded, non-rocky craters, expanding the classification to Class 6 to 8 (Class 8 = hollows). The measured morphometric parameters include crater depth, rim height, rim slope, floor slope, rim curvature, floor curvature, ejecta roughness, and floor roughness. The data indicate that the Class 8 hollows fall along a morphometric continuum that is consistent with an impact origin and the image-based observations of degradation. Depth, rim height, and slopes indicate progressive degradation of the rim and infilling from fresher classes to Class 8. Rim curvature values range from positive (convex-up slopes) for fresher craters to near zero for Class 7 and 8 craters, consistent with slope decline by diffusional processes. Floor curvature values are negative (concave-up) for the freshest examples and approach zero values for Class 7 and 8, a result of infilling. Ejecta roughness is similar for all craters, suggesting no variation or that roughness elements are not detectable in the DEM. Floor roughness decreases with increasing class, consistent with observations of m-size bedforms that degrade to a smooth surface from Class 1 to 8. The cumulative frequency for all Homestead hollow-size craters, including Class 8, suggest retention timescales of ~500 Myr.