The Local Topography of Uruk Sulcus and Galileo Regio Obtained from Stereo Images

Galilean satellite. To support the geologic analysis topographic data of high resolution are needed. However, such High resolution (,100 m/pixel) stereo images obtained by data has not been available from the Voyager missions. the Galileo SSI camera are analyzed using photogrammetric techniques to derive local Digital Terrain Models (DTMs) of Low-resolution (p600 m/pixel) topographic profiles Ganymede’s surface. The models cover areas of 22 3 56 km near the south pole of Ganymede were derived from Voyin Uruk Sulcus and 63 3 102 km in Galileo Regio and have ager images using photoclinometric (shape-from-shading) spatial resolutions of 500 m/pixel and 1000 m/pixel, respectechniques (Squyres 1981). More recently, this technique tively. For Uruk Sulcus, the DTM features a wave-like topograwas used to study the morphology of craters (Schenk 1991) phy with wavelengths of 2–6 km and amplitudes of up to 500 m. and volcanic features (Schenk and Moore 1995). PhotocliIndividual topographic highs show asymmetric shapes with nometry requires that surface albedo be constant along slopes of up to 208 and terracing. The terrain model in Galileo these profiles and that changes in surface brightness be Regio has isolated knobs, furrows of up to 10 km in width, and due to photometric shading effects only. Thus, the analyses ridges of up to 1 km in height. Surface roughness in Galileo were restricted to areas in which these assumptions were Regio at DTM scales is markedly higher than in Uruk Sulcus. believed to hold. To overcome these limitations, Schenk Bowl-shaped craters are identified in the models with depth to and Moore (1995) applied stereo image analysis techniques diameter ratios of about 1 : 12 (Uruk Sulcus) and 1 : 9 (Galileo Regio). This is in approximate agreement with data for simple for morphological studies. About 20% of the surface of craters measured in Voyager images. The large complex 20-km Ganymede was covered by stereo images (Schenk 1995). crater Ea is found to have a central dome 300 m above the However, the full photogrammetric analysis of the images crater floor, reaching more than half of the crater rim’s height. is difficult due to substantial differences in image resolution These unique topographic data, at far higher resolution than and viewing geometry for individual stereo pairs. In addiwhat could be derived from Voyager images, will provide new tion, the Voyager vidicon cameras are limited in their ininsights into the formation of Ganymede’s surface.  1998 Acatrinsic geometric precision. Thus, only selected areas could demic Press be studied and the resulting terrain models showed poor