The role of target strength on the ejection of martian meteorites

Our collection of meteorites recovered here on Earth contain over 200 samples believed to have originated on Mars. It is commonly accepted that these rocks were launched as ejecta during the formation of impact craters. The majority of the meteorites are young and basaltic representing only a small fraction of the martian surface. Previous studies have only examined impacts into basalt or other competent material, but the material bias remains unexplained. In this study we define fragmentation parameters for dry tuff, a proxy for the weakly cemented regolith and sedimentary material covering much of Mars. We simulate hypervelocity impacts into the martian surface using a joint Eulerian-Lagrangian shock physics code. A wide parameter search is conducted over impactor sizes ranging from 10 meter to 10 kilometers in radius. We find that dry tuff produces significantly smaller fragments than basalt for all impactor sizes. Maximum fragment size and amount of material ejected scale with impactor size. We also examine impacts into mixed targets, mainly simple layers and embedded boulders. We find that a layer of basalt overtop dry tuff inhibits ejection, while a layer of tuff overtop basalt enhances ejection and increases dry tuff fragment sizes. Boulders create a complex pressure field in the nearsurface region, but fragment sizes are relatively unchanged.