VENUSIAN CHANNEL FORMATION AS A SUBSURFACE PROCESS

Based on the observation that some venusian channels locally cut topographic highs with no deflection in the channel course, we use full resolution (75 m/pixel) Magellan synthetic aperture radar (SAR) imagery to question the underlying assumption that venusian channels result from surface fluid flow. Because truncated topographic highs are seemingly more consistent with channel formation as a subsurface, rather than a surface, process, we geologically mapped three regions where channels cut topographic highs in order to ask the fundamental question: Do venusian channels result from surface or subsurface processes? Our mapping illustrates that channels (Lunang Vallis [68.5N, 308E], Sinann Vallis [49S, 268E], and Vesper and Citlalpul valles [58S, 173E]) in these three regions: 1) locally cut ridges and small volcanic edifices, or shields, with no deflection in the channel course or apparent presence of lag deposits, 2) occur in regions characterized by abundant shields that resemble shield terrain (Hansen, 2005; see also, Aubele, 1995), 3) are locally segmented, and 4) typically lack levees. In all localities, shield terrain is a mechanically thin (~m's to 10's m) layer and, in two localities, shield terrain resides unconformably on local basement materials. Based on these observations, we propose that some venusian channels result from fluid flow along a shallow crustal interface. Fluid movement along the crustal interface will erode the overlying material, or suprastrate, initially creating a subsurface conduit. Continual fluid movement and subsequent erosion will result in discontinuous channel segments exposed at the surface; further erosion will eventually result in the coalescence of the discontinuous segments creating a continuous channel. Erosion of the suprastrate may be largely mechanical, but thermal erosion may also play a role depending upon the properties of the fluid and suprastrate. It is possible that the interface reflects the boundary between shield terrain and local basement materials where shield terrain is the eroded suprastrate in each locality. The fluid responsible for channel formation is enigmatic. Although basalt is a likely candidate, a variety of fluids may have been responsible, including water.