Why are Mars’s southern highlands covered with many craters?

The southern highlands of Mars are covered with many craters because they represent an ancient surface that has survived from more than 3.5 billion years ago.1

In the early history of the solar system, the rate of large asteroid impacts was extremely high. Planetary scientists have determined that surfaces formed before the decline of these impacts—which occurred between 3.8 billion and 3.5 billion years ago—are heavily cratered, while younger surfaces are not. Because the southern highlands possess such a high density of these features, they are considered to be among the oldest terrains on the planet.1

The presence of these craters serves as a record of the intense bombardment that occurred early in the planet's history, a period that also affected the Moon and other solid bodies in the solar system. Unlike younger regions that may have been resurfaced by volcanism or other geologic activity, the southern highlands have remained largely preserved, allowing them to retain this history of ancient impacts.12