CRATER DEGRADATION AND SURFACE EROSION RATES AT THE INSIGHT LANDING SITE, WESTERN ELYSIUM PLANITIA, MARS

Erosion rate estimates for Martian terrains are key indicators of past surface processes. Here, we utilize new data over the Hesperian ridged plains at the InSight landing region in Western Elysium Planitia (1) to quantify the degradation of 100-meter-scale impact craters and (2) to estimate local erosion rates. From this work, we gain not only a better understanding of how relatively small craters erode on Mars but of surface processes that have operated at the InSight landing site. For this analysis we used rocks in the ejecta of craters as a first proxy for classifying relative degradation state. All rocky ejecta craters (n = 597) were mapped on three High Resolution Imaging Science Experiment (HiRISE) images (25 cm pixel^-1). The craters were then visually classified by morphology from class 1 craters (ideal pristine crater morphology) to class 5 craters (the most degraded craters that still exhibit rocks). Because only three class 1 craters were mapped, class 2 craters - which show a near pristine morphology - are used as our pristine crater dataset. Using 1 m HiRISE digital terrain models we measured the depth (d), diameter (D), and rim height (R) of all rocky ejecta craters. From a comparison of the mean depths of 100-m-scale class 2 craters with that of similar sized class 5 craters we estimate 18 m of total vertical degradation between the two end members. Comparing rim heights, we estimate 5 m of rim erosion. A power law fit to the d/D data for class 2 craters gives the relationship: d = 0.05D^1.13 (R² = 0.9324). The rim height relationship of class 2 craters is R = 0.01D^1.15 (R² = 0.7961). To estimate timescales of degradation we plotted the cumulative size frequency distribution (SFD) of class 1-2 craters against the SFD of class 1-5 craters. Using standard chronology functions for Mars, we estimate that modification from a class 2 to a class 5 crater requires ~600 Ma +/- 90 Ma. Using these ages with the depth data, we calculate a 0.03 m/Myr crater degradation rate. This rate accounts for both rim erosion and the filling of the crater floor by aeolian bedforms and airfall dust. The rim height data provides a lower rate of 0.009 m/Myr. The factor of 3 difference between the depth-dependent degradation rate and the rim erosion rate suggests that infill plays a more significant role in the modification of craters in this region.