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B. Giese, R. Wagner, G. Neukum (DLR), R. Sullivan (Cornell Univ.), Galileo SSI Team
Galileo images show that in many places the surface of Europa is dominated by plains of densely criss-crossing doublet ridges. In most doublet ridges a central V-shaped trough divides the two opposing ridge flanks, which are roughly bilaterally symmetric. Several formational mechanisms for doublet ridges have been proposed. Some models involve new material ejected or squeezed through cracks to the surface, forming twin debris piles on either side of the crack. These models generally require liquid water very near or at the surface at the time of doublet ridge formation. Other models attribute doublet ridge relief to the upwarping of pre-exisiting terrain on either side of a crack. These models require solid-state deformation of the surface and vertical movement of solid-state material in the near-subsurface, but do not require the presence of liquid water as close to the surface as in the first group of models. Thus doublet ridge formation has important implications for whether substantial liquid water influenced the formation of surface features on Europa. So far, this question remains controversial and more evidence is required. Additional evidence was provided by Galileo stereo observations (56m/pxl, 29m/pxl) of a doublet ridge located at (195W, 16S). The stereo images allowed to reconstruct the doublet ridge topography at a resolution of about 200m. Based on the topographic data, there was clear evidence that this doublet ridge was formed by upwarping of terrain rather than piling up of debris. Specifically, we found preexisting terrain features on one of the doublet ridge flanks and a pronounced asymmetry in slopes between the trough and the flanks. The upwarping angle is about 20deg.