Valley networks on Venus

Valley networks on Venus are classified as rectangular, labyrinthic and pit ted, or irregular. The venusian valley networks are structurally controlled , as indicated by the morphological patterns of valley branches, consistenc y between valley and fracture orientations, and associations with the defor med terrains. The morphologies resemble those of terrestrial and martian sa pping valleys. Valley networks on Venus probably formed initially from frac ture systems and became enlarged by low viscosity lava sapping processes. S ubsurface flow of lava may locally have been assisted by surface flows. The lavas probably moved through permeable media and fractures. Venusian valle y networks have a higher degree of network integration than do lunar sinuou s rilles, but they are less integrated than martian and terrestrial sapping valleys. The viscosity of valley-forming lavas must have been very low, bu t was not low enough to exploit the permeable media so extensively as to at tain a high degree of network integration. The compositions of these lavas may have been mafic to ultramafic or made alkaline. Alternatively, the lava s could have had more exotic compositions, such as carbonatite and sulfur. Valley networks are often associated with corona and corona-like features, which are hypothesized to be the surface expressions of mantle plumes. A pl ume association could mean that the lavas came from the mantle. (C) 2001 El sevier Science B.V. All rights reserved.