Dimensions of Central Pits in Ganymede Craters

Introduction: Central pit craters are an unusual class of impact crater seen most commonly on the icy Galilean satellites and Mars. They are characterized by terraced rims and flattened floors with a pit at or near the center (see Fig 1). Their association with ice-rich surfaces has led most hypotheses for the origins of central pits to rely on the volatility or weakness of water ice [e.g. 1, 2, 3, 4]. Knowledge of pit dimensions is necessary to reveal the depth of pit influence, and can also be used to infer the quantities of material involved in the formation process. Such information will have important implications for the development and testing of pit formation theories; for example, variation of pit volume in craters formed on different terrain types may indicate a material property dependence of the pit formation mechanism. We will present topographic profiles of central pit craters on Ganymede. Measurements from these data are used to construct scaling trends to be used in future testing of numerical models. Method: Digital elevation models (DEMs) were created from Galileo Solid State Imager (SSI) images, with the use of the stereo scene-recognition algorithm developed by [6]. Once each DEM of a crater was obtained, spurious patterns or shape distortions created by radiation noise or data compression artifacts were removed through the use of standard image noise filters, and manually by visual inspection of the DEM and original image(s). Four cross-sectional profiles were taken across each crater (Fig. 1); terrain type was noted so that any differences in crater trends on bright and dark terrains could be documented. Crater diameters were determined from an average of the 4 rim-to-rim distances. Measurement of other crater dimensions including pit depth and diameter were made from all profiles across each crater. Obvious outliers in these values were discarded and the maximum reasonable value adopted for each feature measurement; height and width values in this work should therefore be viewed as an upper