A Catalog of Impact Craters on Ganymede

Introduction: Previous crater catalogs for Ganymede were based on Voyager analysis but Galileo data provides improved resolutions in the areas where it covered the moon and also helped fill in regions not imaged or imaged poorly by Voyager. Voyager and Galileo images combine to reveal the diversity of impact craters on this icy Jovian satellite. Previous studies cataloged the interior morphologies of these craters primarily as central peak, central pit, and central dome [1]. It was shown that craters with central peaks were the most abundant of the interior morphology on all terrains followed by the central dome craters and then the central pit craters. The results also showed no strong correlation between the morphology and geologic unit [1]. Studies of ejecta morphologies associated with Ganymede craters show that some are surrounded by layered ejecta morphologies, displaying both single layer and double layer ejecta structures [2-4]. Similar morphologies surrounding martian impact craters are commonly attributed to the role of subsurface ice [5-7]. We are studying the similarities and differences between the layered ejecta morphologies on Ganymede and Mars in order to constrain the role of target volatiles on layered ejecta formation. However, this report will focus on the current status of our study of interior morphologies found in Ganymede craters. Methodology: Data sources used in this project are the Voyager and Galileo imagery from the PDS system and the geologic and shaded relief maps from the US Geological Survey (USGS). We also use the USGS GIS maps of Ganymede available through PIGWAD. Diameters are measured and coordinates of crater center are obtained through use of the Ganymede PIGWAD GIS maps. When completed, this catalog will include all craters 3 km in diameter and larger across Ganymede. Crater rim diameters are measured along four different directions and averaged over these four measurements. The catalog also includes interior and ejecta morphologies, geologic unit, and a generalized preservational scale (0 through 3, with 0 very degraded and 3 a pristine, very fresh crater). The geologic units are obtained from the USGS geologic maps. Current Status of Catalog: At present the catalog contains 1200 craters of which 465 craters display interior morphologies. Our current study region covers the regions with longitude ranging from -180° to -115° and 0° to 91.8° and latitude ranging from +85.6° to -29.7° and +86.7° to -32.2° (Fig 1). The diameter range of the craters included thus far is from 5 km to 85 km. Craters with diameter less than 5 km have not yet been included in the catalog.