Why does one side of Mars look more catastrophic than the other?
If one looks at recent photos of Mars even using false colors, it is apparent that one side is more catastrophic looking than the other, as if one side was showered in the past with a massive one time meteor event. One would ordinarily expect a more homogenous look, as our moon does.
Could this event be connected to a massive cataclysmic breakup of a nearby planet? Would the asteroid belt just outside the orbit of Mars qualify as remnants of a prior existing planet?
A small point here: You are right that Mars has a clear dichotomy with southern highlands and northern lowlands. But the surface of the Moon is not a homogeneous region. There are several maria on the near side (the side that we always see) of the Moon and there are almost none on the far side.
The leading theory now is that the northern lowlands were a result of either one giant impact or multiple impacts. The fewer craters in the northern lowlands is due to resurfacing from volcanism. There may be other motions like polar drifting involved in the complete story. The main evidence for this is that the underlying surface in the northern lowlands is very old so that the impacts occurred quite early in the planet's history. This is consistent with theories of solar system formation where the impact rate in the early stages was very high.
But this high impact rate is not due to a breakup of any planet. It is just a natural consequence of the way in which the planets form. Planets are formed by accretion of planetesimals and so planet formation is a violent process. In the early solar system, there were a lot of planetesimals which finally formed four inner planets.
Beyond Mars, the formation of Jupiter inhibited the formation of any planet between Mars and Jupiter due to the strong gravitational perturbations of Jupiter, which is why the asteroid belt is as it is. It is not a result of a breakup of a planet; it is rather the planetesimals that failed to form a planet due to Jupiter's influence. However, the asteroid belt is not pristine planetesimals of the solar system as there have been impacts which shattered moderate sized planetesimals into several smaller bodies.
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