Legend : Martian fluvial valleys and channels are ancient features. The peak of activity was about 3.5 billion years ago. At that time, many valleys formed. After this period, fluvial activity became localized and episodic. Cataclysmic discharges of ground water formed the huge outflow channels during this time. This water would have ponded in the northern plains of Mars, as shown in the figure below. During the recent Amazonian period, only modest fluvial activity is observed. We conclude that the water that remains on Mars today is trapped, probably as permafrost and ice beneath the martian surface.

Today, Mars is cold and dry. Valleys could not form. So how did valleys form in the past? At least two models of early Mars are under investigation:

1. Mars was wet and warm. In this model, carbon dioxide released by volcanism early in Mars' history produced a greenhouse effect. Under a thick, warm atmosphere, water could flow on the surface as a liquid. An atmospheric hydrologic cycle would be possible and valley networks would form by rainfall much as they do on Earth today.

2. Mars was already cold but wet. Even under very cold conditions, water released at springs would still have been able to flow for vast distances under an ice covering. The water would pond in low areas and freeze or infiltrate back into the subsurface. Much of the ponded and frozen flood water might be protected almost indefinitely by a covering of red soil. In this case the valley networks would not represent erosion by rainfall.