Large Impact Features on Saturn's Middle-sized Icy Satellites: Global Image Mosaics and Topography

Introduction: With the approach of Cassini to the Saturn system, attention naturally focuses on the planet, its rings and Titan, but the Saturn system is also populated by a number of smaller satellites. The seven middle-sized icy satellites, along with those of Uranus, (between 400 and 1500 km wide) are distinctly different geophysically and geologically from their much larger Galilean-class brethren [e.g., 1]. Topographic mapping of these bodies is a critical part of understanding their geologic evolution. Here we describe our recent efforts to map the topography of these satellites using Voyager data. Topographic Mapping Techniques: Topographic mapping of the Saturnian satellites has been limited in the past to limb profiles describing the general shape of the satellites [e.g., 2] and very limited photoclinometry (PC) line profiles of selected features [3]. Our new topographic mapping of these satellites is based on two methods, stereo image analysis and PC of low-sun regions , and allows us to produce digital elevation models (DEMs) of large surface areas. Stereo analysis is based on an automated scene recognition algorithm successfully applied to the Galilean satellites [4] and recently updated to improve surface resolution of the DEM [5]. Areal coverage of stereo DEMs is limited by the rapid nature of the Voyager flybys. Rhea (Fig. 1) and Dione have the best stereo coverage, with ~25% of the surface mappable at better than 500 m vertical resolution and better than 5 km horizontal resolution on both satellites. Some stereo coverage was obtained for Tethys, Enceladus, and Mimas, but these satellites all revolve more rapidly than Rhea and Dione and changes in solar illumination during the imaging sequence restrict useful stereo coverage. Iapetus was viewed at no better than 15 km/pixel resolution by Voyager. Photoclinometry (PC) greatly extends topog-raphic coverage, with the proviso that long-wavelength topographic variations are suspect using this technique, unless controlled by coincident stereo coverage. We employed a new PC algorithm developed by the first author for rapid 2-dimensional PC mapping that includes modeling of local albedo changes [6]. PC mapping is generally limited to areas within 30° of the local terminator. A necessary precursor to topographic mapping using imaging is the existence of an accurate coordinate control network for the Voyager images. Extensive effort was made to adjust the control on the relevant