Topographic modeling of Phoebe using Cassini images

High-resolution Cassini stereo images of Saturn's moon Phoebe have been used to derive a regional Digital Terrain Model (DTM) and an orthoimage mosaic of the surface. For DTM-control, a network of 130 points measured in 14 images (70-390 m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at RA = 78.0°±0.1° and Dec = 356.6°±0.3°, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2 km with RMS residuals of 6.2 km demonstrating the irregular shape of this body. The DTM obtained is based on methods of digital image correlation. It has a horizontal resolution of 1-2 km and vertical accuracies in the range of 50-100 m. Although limited in coverage, it is higher in resolution than the previously derived global shape model of Phoebe (Porco et al., 2005) and allows us to study the morphology of the surface in more detail. The most important feature revealed by the DTM is the conical shape of craters which has not been reported on planetary bodies so far. The formation of such craters may hint at highly porous material on the surface of Phoebe