The most significant crustal feature on Mars is the hemispheric dichotomy–a physiographic divide between the southern hemisphere (heavily cratered highlands) and the northern hemisphere (relatively smooth plains).
The southern highlands cover ~2/3 of Mars and are ~2–4 km higher than the northern lowlands. This area is composed of heavily cratered, relatively old geologic units mainly Noachian in age [4.1–3.7 Ga]. The northern lowlands occupy ~1/3 of Mars and are composed of lightly cratered, younger geologic units mainly Amazonian in age [3.0 Ga to present], including plains, volcanic edifices, and volcanic calderas.
There are mainly two different categories for hypotheses on how the dichotomy may have formed: endogenic (from within the planet itself) and exogenic (from outside the planet).
Endogenic hypothesis include plate tectonics, degree-1 mantle convection, and mantleoverturn to explain the formation of the dichotomy. Exogenic hypotheses suggest
either a giant meteorite impact or multiple impacts to explain its formation.
However, there is also the possibility that these two distinct categories
aren’t mutually exclusive, as there may be a hybrid model to incorporate both
endogenic and exogenic factors. [All of these hypotheses could merit their own post, so let me know if you’d like to learn more about any of them!]
There is still little consensus as to the formation mechanism, but it is agreed that the dichotomy formed early in martian history. Age estimates place its time of formation from as early as 50 million years within the solar system formation to around 3.7 Ga. The majority of the
age estimates situate the formation of the dichotomy during the period of the
Late Heavy Bombardment (4–3.8 Ga), the timing of which may support an impact
dichotomy has been able to persist for billions of years, and its boundary has
been subject to subsequent erosion and glacial modifications. Understanding the origin of the dichotomy is critical in understanding
ancient crustal evolution, especially if supporting an endogenic hypothesis.
Please contact me if you’d like to know any of the papers that were used as a source!