The martian hemispheric dichotomy may be due to a giant impact

Of the two approximately hemispherical, fundamentally different geological provinces into which Mars is divided, the more southerly province is heavily cratered, while the northern is lightly cratered and contains younger geological units. Lowlands occupy about one-third of Mars, and these are separated from the highlands by a distinct scarp or by a sloping transitional zone which is characterized by a variety of landforms that are unknown on other planets. It is presently proposed that the largest expanse of lowlands is substantially due to the formation of a large impact basin early in the planet's history, which has markedly influenced the character of the Martian surface.