Posted by Dr. Goro Komatsu, IRSPS, Univ. G.d’Annunzio, Italy.
(Re-posted from IAG Image of the Month, December, 2007)
“…excitement and pleasure in science derive not so much from achieving the final explanation as from discovering the fascinating range of new phenomena to be explained” (Baker and Komatsu, 1999).”
Valley networks on Venus were discovered by the Magellan mission in the early 1990’s (Baker et al., 1992). This discovery was really unexpected on a dry planet with extremely high ambient temperatures (~500°C). The venusian valley networks are structurally controlled, as indicated by the morphological patterns of valley branches, consistency between valley and fracture orientations, and associations with the deformed terrains. However, the morphologies resemble those of terrestrial and martian sapping valleys.
Valleys are manifestations of conveyance of fluids on planetary bodies. They are widely distributed on Earth and Mars, where liquid water is considered to be the dominant agent in forming these landforms. How then, did valley networks on dry Venus form?
A possible origin of valley networks on Venus involves volcanism (Komatsu et al., 2001). They probably formed initially from fracture systems and became enlarged by low viscosity lava flows – a processes named “lava sapping.” Subsurface flow of lava may have locally been assisted by surface flows. The lavas probably moved through permeable media and fractures. Venusian valley networks have a higher degree of network integration than do lunar sinuous rilles, but they are less integrated than martian and terrestrial sapping valleys. The viscosity of valley-forming lavas must have been very low, but was not low enough to exploit the permeable media so extensively as to attain a high degree of network integration.
A lot more work has to be done to understand the origin(s) of these valley networks on Venus. But their discovery itself has already excited the soul of geomorphologists.
Baker, V.R. and G. Komatsu, (1999). Extraterrestrial fluvial forms, In: Miller, A., editor, Varieties of fluvial forms, Wiley, New York, 11-30.
Baker, V.R., G. Komatsu, T.J. Parker, V.C. Gulick, J.S. Kargel, and J.S. Lewis, (1992). Channels and valleys on Venus: Preliminary analysis of Magellan data. J. Geophys. Res., 97, no. E8, 13,421-13,444.
Komatsu, G., V.C. Gulick, and V.R. Baker, (2001). Valley networks on Venus. Geomorphology, 37/3-4, 225-240.