Degradation of 100-m-Scale Rocky Ejecta Craters at the InSight Landing Site on Mars and Implications for Surface Processes and Erosion Rates in the Hesperian and Amazonian

Plain Language Summary Rocky ejecta craters (RECs) at the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) landing site on Elysium Planitia, Mars, provide constraints on crater modification and rates for the Hesperian and Amazonian. The RECs are between 10m and 1.2km in diameter and exhibit five classes of preservation. Class 1 represents pristine craters with sharp rims and abundant ejected rocks. From Classes 2 to 5, rims become more subdued, craters are infilled, and the ejecta become discontinuously distributed. High-Resolution Imaging Science Experiment digital elevation models indicate a maximum depth to diameter ratio of 0.15, which is lower than pristine models for craters of similar size. The low ratio is related to the presence of a loosely consolidated regolith and early-stage eolian infill. Rim heights have an average height to diameter ratio of 0.03 for the most pristine class. The size-frequency distribution of RECs, plotted using cumulative and differential methods, indicates that crater classes within the diameter range of 200m to 1.2km are separated by 100 to 200Myr. Smaller craters degrade faster, with classes separated by <100Myr. Rim erosion can be entirely modeled by nonlinear diffusional processes using the calculated timescales and a constant diffusivity of 8x10(-7)m(2)/year for craters 200 to 500m in diameter. Diffusion models only partly capture depth-related degradation, which requires eolian infill. Depth degradation and rim erosion rates are 10(-2) to 10(-3)m/Myr, respectively. The rates are consistent with relatively slow modification that is typical of the last two epochs of Martian history. The shape and form of impact craters on Mars can tell us something about processes that operate on the surface and therefore inform our understanding of the planet's climate. Small, 100-m-scale craters have a known bowl-shaped form in their pristine state. Over time, wind, gravity, and, in some cases, ice and water, degrade them to nearly flat landforms. This study analyzes the degradational sequence of craters at the InSight landing site to infer something about surface processes and climate history at this location. InSight is a geophysical mission that is set to land on Elysium Planitia in November 2018. The craters show a broad range of morphologies that suggest slow, long-term degradation. The data indicate that over time, ejected rocks and crater rims disappear at rates that suggest degradation by dry processes like gravity failure and wind erosion. Furthermore, the craters also show evidence of being filled, likely by wind-driven sand. The calculated rim erosion rates and infill rates are similar to other landing sites on Mars and suggest a cold, hyperarid climate. This is consistent with the age and equatorial location of the terrain, which formed well after the warmer and wetter period of Mars history.