Tectonic and Regional Topography of Io: a New High

Introduction: Regional topography, volcanic and tectonic patterns, and gravity signatures (the latter not yet available for Io) are the primary manifestations of a planets internal dynamics [1,2]. This report is motivated by several new developments, the most important of which are significant upgrades to our stereo topographic mapping capabilities and the successful application of shape-from-shading (photoclinometry, PC) techniques to Io's photometrically complex surface. We summarize these developments and the to-pographic observations and resulting elevation maps completed to date, focusing here on the topographic characteristics of mountains at high resolution and of Io on regional (~1000 km) scales. Topographic Mapping Innovations: As described in a companion abstract [3], our primary topographic mapping tools are stereo image mapping and photocli-nometry. We employ several new innovations with each technique. The digital stereo mapping technique used here is described in detail elsewhere [4], but in brief is based on scene recognition software [5]. Stereo mapping of Io is complicated by several unique factors, principally radiation noise damage, complex and variable photometric properties with phase angle, and temporal changes of volcanic features. Other difficulties include the blandness of the extensive Ionian plains in many images. These complications, which manifest themselves as spurious elevation data, are greatly reduced by the use of a Stacked Progressive Cube method of DEM analysis. Briefly this method involves mapping topography at progressively increasing spatial resolutions. The algorithm selects the statistically best DEM elevation value with the best possible resolution for each pixel location. Finally, an anomaly in the original ISIS code that degraded the resolution of the output DEM has been corrected, thus increasing the spatial resolution of our DEMs. Photoclinometry (PC), or shape-from-shading, potentially allows the rapid mapping of topography over large areas with a single images but has seen extremely limited action on Io [6]. We have developed a 2-dimensional PC mapping technique that differs from earlier efforts in two major aspects. The first involves the approach to 2-D PC mapping. Here we allow each parallel profile line to integrate free of the influence of its neighbors, adjusting the lines after integration to produce a DEM. Secondly we use low-phase-angle images to model local albedo in our map regions. Despite these new techniques, PC cannot be considered reliable over distances of hundreds of pixels and we must rely on stereo for regional scale mapping. Fi