Secondary craters reveal differences in neighboring geologic units: An example on Mars

Abstract

Secondary craters, formed from ejecta during impact events, complicate surface age estimation, as they violate key assumptions required for age inference from crater counts. However, the nearly common formation conditions of nearly co-located secondaries make them potentially useful for inferring target properties. We present a technique to estimate surface property differences between bordering geologic units using secondary crater characteristics. This process is straightforward if secondary crater clusters sourced from a known primary crater cross a geologic border and can be properly surveyed. We demonstrate the technique at the geologic contact between Amazonis Planitia, a volcanic plain, and the Olympus Mons aureole, a landslide deposit, on Mars. We located a secondary crater cluster sourced from the primary crater, Tooting, which impacted ∼165 km to the southwest in Amazonis Planitia. The Tooting-sourced secondary craters in Amazonis Planitia are ∼1.43 times larger than the craters found just across the geologic border in the Olympus Mons aureole. The smaller craters in the aureole suggest the aureole is either stronger and/or more porous. Based on the geology of those units, we prefer an explanation that attributes the difference to increased porosity; if so, the mean porosity in the aureole is >43 %. The technique we employ here can be applied to any solid surface in the solar system with young impact craters relative to the target surfaces and clear geologic contacts where the contact transition is shorter than the width of the secondary crater cluster. We encourage others to use this technique opportunistically when contact-crossing secondary clusters are discovered in their geologic analysis.