It would explain alot of things.
A 1600 kilometre diameter protoplanet [half the diameter of our moon] striking Mars long ago
would not only have blasted away a large part of the martian crust
but...most of the early martian atmosphere as well, leaving
an atmosphere of less than 10 millibars.
It would have also vaporized and blasted away most of the H20
in any martian ocean, and wiped out any DNA lifeforms in that
large, martian body of water.
Finally, that impact may have imparted enough heat energy in
to the martian crust and mantle to trigger centuries of volcanic
activity.
A lot would be explained by such a huge impact.