Fragmentation and ejection of the martian clan meteorites

I have used the SALE2D hydrocode to study spall in impacts into layered terrains. Application of my results to the problem of martian meteorite provenance resolves two outstanding paradoxes. First, the minimum size crater previously thought to be required to eject martian meteorites is so large (12 km) that it is highly unlikely such an event occurred on shergottite age terrain in the last few million years. The geochemical evidence supports four launch events. This issue I have resolved by establishing a new lower limit to the minimum size crater of 3 km.;Second, the martian meteorites are dominated by shergottites (62%) which come from the youngest and apparently rarest martian terrains. The vast majority of Mars appears to be under represented. This paradox lies on the false premise that all terrains are equally efficient in launching material during an impact. I have found that the presence of a weak, low density layer suppresses spall velocity and increases shock pressures in an impact. Since the regolith on Mars can be expected to be largely impact-generated, the older terrains are covered by a greater depth of regolith. Qualitatively, older terrains are under represented in the martian meteorites because they require larger (rarer) impacts to launch material into space. I have shown this quantitatively for shergottites, nakhlites, and Chassigny. An extension of my work provides some constraints on the extent of martian ancient terrain.