Valley Networks

Abstract The valley networks are the most common drainage feature on Mars and the feature most commonly cited as evidence for former warm and wet climatic conditions. Because of their resemblance to river valleys, they have generally been assumed to have formed by fluvial erosion. If formed by fluvial erosion, they imply warm, wet climatic conditions, for under present conditions, small streams would rapidly freeze and prevent development of a network. Since most of the valley networks are old, the former warm and wet conditions are proposed mainly for early in the planet’s history. These interpretations are, however, almost certainly too simplistic and the view of a warm, wet early Mars on which fluvial activity was common, changing early to the cold planet we know today, is likely to be only partly true, if true at all. The resemblance of the valley networks to terrestrial valleys is not exact. River channels, for example, are not observed within the valleys, as would be expected if they were fluvial valleys and other seemingly non-fluvial characteristics must be reconciled with a fluvial origin. In addition, while most of the networks appear to be very ancient, some, cut in young units, must have formed late. Thus, there are reasons to question both a purely fluvial origin and an exclusively old age for the networks. We are on questionable grounds if we invoke warm, wet terrestrial climatic conditions to explain the martian valleys without understanding the reasons for the differences between the terrestrial and martian examples, and also if we invoke wet conditions for the early period of intense network development, while calling upon some other circumstances to explain the younger networks. In this chapter, we examine the characteristics, distribution, ages, and mode of origin of the valley networks and attempt to draw some implications with respect to climate.