Martian dichotomy

The Martian dichotomy is a feature of Mars, the red planet. It means that Mars has two very different sides. On one side, the low-lying northern hemisphere is filled with flat plains and low hills, while the southern highlands are covered with volcanoes, craters, and mountains. So the dichotomy means that the planet has two very different kinds of surfaces. It's a bit like one side of an apple being really smooth and the other really bumpy.