Global Geologic Mapping of Ganymede Light and Dark Material at 1:15m. G

Introduction: The Galilean satellites represent a series of bodies that all formed from the same proto-Jovian nebula but are distinctly different in their physical properties, surface geology, and thermal evolution [1-4]. The explanation for many of these disparate characteristics is likely related to the radially varying conditions of the nebula during the earliest period of their formation [5-6]. However, an explanation for the differences in the geological evolution of Ganymede and Callisto, given their similar size, density, and bulk composition, remain one of the most fundamental questions in comparative planetology [6, 7]. Understanding the global distribution of geologic units on Ganymede in space and time can offer constraints on our ideas of how and why this icy satellite evolved to its present state. The Voyager mission provided important information about the nature of the surface of Ganymede at moderate resolution and these data were used to produce a series of geologic maps covering the imaged surface at the 1:5M scale [e.g., 8-10]. The Galileo mission has provided a host of new data (high-resolution monochromatic, color, and stereo imagery, polarimetry, near-infrared spectral imagery, etc.) and we are compiling a global geologic map of Ganymede (at the 1:15M scale) that will represent the most recent understanding of the satellite on the basis of this data. This contribution builds on important previous accomplishments in the study of Ganymede [11-15] and here we summarize our progress toward the completion of this global mapping project. Unit definitions: The first task we undertook in developing a global geologic map of Ganymede was to reassess the units identified at Voyager resolution using Galileo resolution data [16, 17]. The result of this reas-sessment was a revised Description of Map Units (DOMU) in which the units have been divided into five terrain types: 1) light, 2) dark, 3) reticulate, 4) palimp-sest, and 5) crater material. Light material: This material has been subdivided into four units: grooved, subdued, irregular, and undi-vided. The grooved unit (lg) is arranged in domains characterized by parallel, roughly evenly spaced grooves and ridges oriented in a single dominant direction. The subdued unit (ls) is similar to the grooved unit but appears smooth or finely grooved at Galileo and/or Voyager resolution except where secondary craters and crater chains are superposed. The irregular unit (li) is similar to the subdued unit but contains isolated grooves with no preferred orientation. The undi-vided unit (l) represents all materials of …