Crater Retention Ages at the InSight and Spirit Landing Sites

InSight landed in a ~25 m-diameter (D), quasi-circular depression informally named "Homestead hollow" in Elysium Planitia. Elysium is a basaltic lava plain estimated to be Early Hesperian (based on the size frequency distribution (SFD) of craters with D > 5 km) to Early Amazonian in age (based on the SFD of craters with D < 1 km). The morphology of impact craters in this region follow a degradational continuum from pristine, bowl-shaped craters to nearly completely filled, quasi-circular topographic depressions that include Homestead hollow. The cumulative SFD of small-diameter craters (D ~20-150 m) follow the -2 power law slope of Hartmann's equilibrium function, indicating the production of craters below D ~ 200 m is in equilibrium with geomorphic processes that are eroding them. Thus, craters with D < ~200 m provides a "crater retention" age of the surface, rather than the formation age of the landscape, which is reflective of surface processes and rates. By contrast, craters with D ~0.2-1 km follow a production curve, suggesting a regional resurfacing event in the Early Amazonian covered the kilometer-sized population of craters from the Early Hesperian.

The Mars Exploration Rover, Spirit, landed on Hesperian- to Amazonian-aged basaltic lava plains, where it investigated sediment-filled hollows that are comparable in size, yet many appear better-preserved than Homestead hollow. At the resolution of images from the Context Camera (CTX, ~6 m/pixel), morphologically distinct populations of craters are apparent on the Gusev plains, ranging from bowl shaped to quasi-circular depressions. In a ~2,500 km² area encompassing the volcanic plains in the vicinity of the Spirit landing site, a preliminary population of craters between D ~ 0.2-1 km is consistent with the expected population production function, yielding an Early Amazonian age (similar to the InSight landing site). We are refining this analysis with the goal of determining whether populations of degraded and quasi-circular depressions on the floor of Gusev follow the same degradation continuum observed at the InSight landing site. The comparison between Gusev and Elysium will help further our understanding of the degradation history of craters on Hesperian-to Early Amazonian-aged volcanic surfaces, and may provide insight into the timing and extent of burial and exhumation events.