How did valley networks in Mars’s southern highlands form?

The valley networks found in the southern highlands of Mars are ancient features that resemble terrestrial river systems, indicating they were formed by the slow erosion of running water. Because these networks are heavily cratered, scientists believe they formed early in the planet's history, likely more than 3.5 billion years ago, during a period when the climate was much warmer and wetter than it is today.123

Regarding the specific mechanism of their formation, planetary scientists have proposed two primary theories:

• Surface Runoff: The valleys may have been carved by the runoff of rainfall occurring on the Martian surface.2
• Groundwater Sapping: The networks may have formed through erosion caused by the outflow of groundwater that seeped onto the surface. Some researchers argue that a sapping mechanism—where groundwater undermines the surface material—best explains the steep-walled, theatre-like headward terminations observed in many of these valleys.23

Regardless of the specific process, the presence of these networks suggests that liquid water was once active on the surface, requiring climatic conditions significantly different from the cold, arid environment observed on Mars today.12