DEGRADATION OF ONE-HUNDRED-METER-SCALE ROCKY EJECTA CRATERS AT THE INSIGHT LANDING SITE ON MARS AND IMPLICATIONS FOR SURFACE PROCESSES AND EROSION RATES

The degradational sequence of simple and complex kilometer-sized craters on Mars, and the erosion rates that have been derived from their preservation, may not be appropriate for 100-m-scale craters. The initial morphology of 100-m-scale craters is varied and they are far more sensitive to target properties. Craters of this size also show evidence for relatively rapid degradation and are therefore ideal for evaluating local surface processes. During the landing site selection process for the InSight mission, the HiRISE camera (~0.25 m pixel^-1) targeted a geologically-uniform, Hesperian to Early Amazonian-age ridged plains unit in western Elysium Planitia. This study provides a detailed morphologic analysis of 100-m-scale rocky ejecta craters (RECs) across this unit to evaluate surface processes and rates with implications for mission science and the Hesperian to Amazonian climate. The RECs are between 10 m and 1.2 km in diameter and exhibit five classes of preservation. Class 1 represents pristine craters with sharp rims and abundant ejected rocks in a continuous ejecta blanket. From Class 2 to 5, rims become more-subdued, craters are infilled, and the ejecta become discontinuous. HiRISE DEMs 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 likely related to rapid early-stage eolian infill and the presence of a loosely consolidated, 2 to 5 m thick, fragmented regolith. Crater 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, indicate that different crater classes within 200 m to 1.2 km diameter are separated by ~100 to 200 Myr. Craters that are smaller than 200 m degrade faster and different classes are separated by <100 Myr. Rim erosion can be entirely modeled by non-linear diffusional processes using the calculated timescales and a constant diffusivity of 8 × 10^-7 m² yr^-1. However, diffusion models only partly capture depth-related degradation, which also requires eolian infill. Depth degradation and rim erosion rates are 10^-2 to 10^-3 m Myr^-1, respectively. The rates are consistent with relatively slow modification that is typical of the last two epochs of Martian history.