Apex-antapex Asymmetry of Impact Crater Density on Ganymede’s Dark Terrain

Introduction: Synchronously rotating satellites bombarded by a heliocentric impactor population should have a difference in crater density between their apex and antapex of motion. Predictions by Zahnle et al. [1] indicate the crater density at the apex could be up to ~70x greater than at the antapex. An absence of such a difference could be caused by saturation crater-ing, periods of nonsynchronous rotation, or impacts by planetocentric debris/secondaries [1]. An apex-antapex crater asymmetry was found by Zahnle et al. [1] on the bright terrain of Ganymede. The difference, however, is much smaller than predicted , with the density at the apex being only 4X greater than at the antapex. From this they concluded that either the distribution was saturated or, perhaps more interestingly, Ganymede rotated nonsynchro-nously at some point in its history. The goal of this project is to similarly analyze Ga-nymede's dark terrain, which is approximately twice the age of the resurfaced, ~2 Gyr old bright terrain [2]. As with bright terrain, a significantly diminished or absent difference in cratering density on Ganymede's dark terrain may indicate some interesting events taking place in the satellite's history. Methods: Crater diameters and coordinates were measured and recorded from a 1 km/pixel global mosaic. In the case for palimpsests (large, circular bright patches, likely the remains of viscously relaxed craters [e.g., 3]), the formula D p = 2.442 D c 0.906 [4] was used to calculate the unrelaxed original crater diameter, D c. D p is our measurement of the bright patch from edge to edge. From the collected data, relative plots, termed R-plots, were generated. An R-plot ratios the distributions determined here to a standard distribution with a differential slope of-3, with √N error bars [5]. In order to examine the variation in crater density with increasing distance from the apex (0º, 90º), the data was grouped into 10º bins with centers every 5º from the apex. The areas of each slice were calculated in order to obtain the crater density. The crater density per 10 6 km 2 vs. the angular distance from the apex was then plotted on a linear graph to show the variation with √N error bars. Results: In Figure 1, the crater density vs. apex angle is shown for Perrine, Marius, Nicholson, and Galileo Regio for craters with D > 10 km, as well as for bright terrain. The data for Galileo Regio was collected by …