Could MArs have been a Moon around a large water world called Tiamat and when Tiamat came apart (for what reason I would only guess at using Sumerian mythology) The Moon was dashed with the debris and tilted 180 degrees, thus one hemisphere is scowered and the other cratered heavily. Some of the impact craters on our Moon might help with the issue, since debris from Tiamat would not just collect in an astoid belt but be tossed across the solar system.
Guessing is all any of us can do until soil samples are scrutinized from Mars.
I miss tiamat, “Just a Bronze-Age Gal, Trapped in a Techno-World!”.