re: You still don't get it: there are no big secondary craters
Ahah! Good question, as a matter of fact a great one...
You have to remember that even the trans-planetary escape velocity for Mars is roughly an order of magnitude lower than the slowest speeds for asteroids and comets...
If I remember, the ejection studies for the Martian meteorites also showed that there were severe size constraints on the ejecta - most was simply pulverized, with little retainting integrity, and then only very small pieces - which were preserved in shock "shadows" within the blast craters as they were being formed...
ep