Valley network

Several types of valley system have been found on the plains of Venus. They show none of the forms made by running water, however, such as traces of a channel.

Three main types of valley network have been described from Magellan radar images (Komatsu et al. 2001). Labyrinthine valleys are the most common. They are hundreds of kilometres long, and they usually occur near tectonic deformation or next to coronae. The labyrinth pattern forms under the deforming forces that act on the area, and the valleys are fairly wide (see plate 2). No liquid flowed in them. The course of the valleys has no link to the slope, and the valleys do not widen where tributaries join them.

The branches of rectangular valleys (see plate 1) join one another almost at right angles. These are the most continuous valley networks: the valleys have no vanishing reaches. They are less than 1 km wide. They are usually found on radar-dark smooth plains. No faults or troughs can be seen around them. So their straight, angular layout suggests that a buried fracture system, covered by lava, controls their course.

The third type is irregular, “beaded” valley networks, which include pits. They are merging collapse pits or depressions with scalloped margins (see plate 3).

A fourth type of valley network is only hypothetical. The interior plains on tesserae have a pattern suggesting that they may once have been valleys shaped by water (Khawja et al. 2020).

The first three types are explained by tectonic fractures and by erosion. The erosion would come from volcanic material flowing on the surface or below it, in the thinner fractures. Some labyrinthine valleys are wide and have rounded, “amphitheatre” valley heads. These are interpreted by analogy with fluvial landforms on Mars and Earth. In this view, fluid lava seeps slowly out of the layers below the surface and undercuts the valley head, so the valley cuts back. The seeping material may be silicate, sulphur or carbonatite lava (Kargel et al. 1994).

A curious fact: the idea of an (invisible) labyrinth on Venus appeared in science fiction as early as the 1930s (H. P. Lovecraft: In the Walls of Eryx). This is as much a coincidence as the discovery of real channels and valleys on Mars just when the illusion of the Martian canals was shattered.

Research on Martian river valleys is a very active field. No water flows in the valleys today. Researchers explain the traces by the action of water or ice, which may have shaped different branches of the same valley network at different times (Steinmann et al. 2020).