Among the more surprising discoveries of Magellan was the identification of numerous channel and valley systems on the surface of Venus. While the presence of channelized lava flows was predictable, the abundance and extreme lengths of some of these features was unexpected. The most unusual of these features are the canali [ Baker et al., 1992], which are narrow channels that in some cases exceed 1000 km in length. Baker et al. [1992] describe four basic categories for channels and valleys, including simple, complex, compound, and integrated types. Simple channels do not branch and tend to be relatively long and sinuous. They are further subdivided into sinuous rilles and canali, to which we limit this discussion.
As implied by their name, sinuous rilles are morphologically similar to features seen on the Moon. They emanate from distinct source regions and form a deep, single channel that narrows toward its distal end. They tend to be 1--2 km wide and several tens to hundreds of kilometers in length, and are found most commonly in and around coronae and related features [ Komatsu et al., 1993]. Coronae are large (most commonly 200--500 km diameter) circular features defined by an annulus of tectonic features and the presence of volcanic flows and constructs. They are thought to result from the rise of a high-temperature mantle plume or diapir [ Stofan et al., 1991]
With regard to possible global resurfacing, it is the canali that provoke the most interest. Baker et al. [1992] describe their basic characteristics. Canali are commonly more than 500 km in length; the longest one (Hildr Fossa, partially mapped from Venera 15/16 data) is nearly 7000 km in length, longer than the entire Nile River. Canali are remarkably uniform in width at 3--5 km. No levees are visible in radar images, and a measurement of depth using Magellan images from different mapping cycles suggested no more than a few tens of meters of relief. Canali generally emanate from indistinct or unidentifiable sources and display characteristics more commonly associated with terrestrial fluvial features, such as cutoff meanders, cutbank relationships, and abandoned paleochannels. They are found only in plains regions.
Because of these characteristics, the formation of canali presents a challenge to models of lava flow cooling and dynamics. Their lack of levees and shallow depths are suggestive of an erosional mechanism. However, conventional silicate magmas (e.g., tholeiite, komatiite) appear to cool too quickly to sustain the turbulent and superheated conditions necessary for channel erosion, requiring that eroding magmas be composed of exotic lavas such as sulfur or carbonitite [ Komatsu et al., 1992]. If, on the other hand, the canali are formed by construction, a crust must form on top of a flow in order to insulate it and keep it from solidifying near its source. Despite its higher temperature, the thick Venus atmosphere forms a convectively-cooled crust more quickly than Earth's atmosphere [ Gregg and Greeley, 1993]. This insulation effect potentially allows for longer flow lengths on Venus. It is not clear that a constructional model, which appears to require the formation of large levees [ Komatsu et al., 1992] can be reconciled with morphological observations.
Whether the canali are due to thermal and mechanical erosion of plains by an exotic lava or to construction, they clearly require eruption of large volumes of magma over relatively short periods of time. Given their association with plains regions, any constraints that can be placed on the composition of magmas that once flowed through the canali would add considerably to understanding of plains composition and formation.