Bob, the impact of an asteroid into the martian surface would have been substantial, ejecting material off the martian surface into space.
See the moon? It is believed by most scientist and astronomers today that a comet or large asteroid had a cataclysmic impact with earth, and that material eventually formed our moon.
Oddly enough, there is little doubt some of that material eventually made it's way to the martian surface.
Where do you get this scientists “believe”. Don’t you know that we aren’t supposed to believe in things nowadays. To believe smacks of religion and we can’t have that now can we?