What is the broad smooth plain covering much of Mars's northern lowlands?

What is the broad smooth plain covering much of Mars’s northern lowlands? It is Vastitas Borealis. The name means “Northern Vastness” and describes the immense, relatively flat region surrounding much of Mars’s north polar area.

Vastitas Borealis is not a single crater or volcano. It is a broad geological feature occupying the low-lying northern plains, which are smoother and less heavily cratered than Mars’s ancient southern highlands. The region includes several major basins and plains, and its low elevation has made it central to theories about ancient water and possible northern oceans.

The plain is often confused with Mars’s north polar cap, but they are not the same thing. The polar cap is a seasonal and permanent deposit of water ice and carbon-dioxide ice; Vastitas Borealis is the much larger underlying terrain. Its surface has been modified by wind, ice, volcanic material, impact processes, and possible sediment deposited by ancient water. Mars Global Surveyor’s topography helped establish how extensive and unusually flat these northern plains are.