Impactor Populations in the Saturnian System: Constraints from the Cratering

Introduction: When using a planetary surface’s impact cratering record to investigate the object’s geologic history, it is important to understand the sources of impactors generating the unmodified record. For Saturn’s satellites, this implies that impactor populations pertinent to the Saturn system should be characterized in order to acceptably interpret cratering records modified by geologic activity [1-8]. Here, as a first step, we compile and analyze the crater distributions of apparently unmodified, heavily cratered terrains on Mimas, Tethys, Dione, Rhea, and Iapetus to place constraints on characteristics of impactor population(s) in the Saturn system (see also [9]). This work provides a preliminary survey of these impactor populations. Numerical modeling, along with this analysis of the cratering records, will be required for further insight into properties of these populations. Previous Work: Imaging from Voyagers 1 and 2 provided the first opportunity to characterize the impactor population(s) relevant to the Saturn system from cratering records of the satellites. Generally, the data seemed to imply that cratering of the satellites was dominated by two populations [7, 8, 10-15]. One was characterized by a greater number of large craters (Saturn Population I in Voyager literature) and the other by a relative deficiency of large craters (Saturn Population II). Furthermore, Population I was indicated on older terrains, such as Rhea and Iapetus, while Population II appeared to only be found on young terrains of Mimas, Enceladus, Tethys and Dione. Characteristics of Population I seemed to be most compatible with heliocentric comets. Meanwhile, characteristics of Population II seemed to be most compatible with smaller planetocentric impactors created by escaped secondaries from large basins and disrupted satellites. Imaging by Cassini, which improves the spatial and resolution coverage over that obtained by Voyager, allows us to compile new, expanded crater distributions for the satellites to analyze in an attempt to support or refute this hypothesis. Methods: Impact crater distributions are compiled from controlled global mosaics of Mimas, Tethys, Dione, Rhea and Iapetus, and high-resolution images of Tethys, Dione, Rhea, and Iapetus all derived from Cassini ISS images. From these images the diameter and position of the crater is recorded. Results of our crater counts are presented in the relative (R) sizefrequency distribution plot format [16] (Fig. 1). The Rplot is the ratio of our distributions to a distribution with differential slope equal to 3, plotted against crater diameter in log-log space with ± √N error bars (N is the number of craters counted in that bin). Both averaged (Fig. 1A) and raw (Fig. 1B) R-values are plotted for completeness. R-values are averaged to simplify the plot and make comparisons more straightforward. The raw data points for each terrain shown have been averaged where data overlap from different source images and plotted as a connected line without error bars (the line is dashed when it goes through a diameter bin with no data). Where data is averaged the raw R-values are weighted by their errors (similar to the method of [17, p.45]), so that raw values computed using a large number of measurements, which are more reliable, are given more importance in the calculation of the average.