Why are the northern and southern hemispheres of Mars so different?

The majority of theories center on a massive asteroid or numerous smaller ones striking Mars during its formation, resulting in a massive lava flow and volcanic activity in the Southern Hemisphere.

The northern hemisphere of Mars is largely flat and low-lying, while the southern hemisphere has highlands and large volcanoes. This stark difference is primarily due to the varying geological processes that have shaped each hemisphere over time. The northern hemisphere experienced significant volcanic activity early in Mars' history, leading to the formation of vast plains. In contrast, the southern hemisphere has more ancient, heavily cratered terrain, indicating a lack of recent volcanic activity and a longer period of erosion and weathering. Additionally, differences in crustal thickness and composition between the two hemispheres have influenced their geological evolution.