Polygonal impact craters on Ganymede

Polygonal impact craters (PIC) are impact craters that have at least one straight rim segment in planform [1-8]. Among all impact craters, PICs represent a small percentage. They exist on both rocky and icy planetary bodies [9]. To our knowledge no studies on PICs have been carried out for Ganymede. Here we are examining the straight segments of PICs and their relationship with adjacent lineaments or fractures. We use the global mosaic prepared by [10], which combines the best high-resolution images from Voyager 1, Voyager 2, Galileo and Juno spacecrafts. Despite the resolution limits and different illumination angles, we identified and mapped 459 PICs across Ganymede whose diameter range from 5 km to 153 km.  PICs, which were superimposed by other craters or terrains are not considered for this study. The number of straight segments possessed by PICs ranges from 1 to 9 with quadrangular, hexagonal and octagonal shapes being most common. Most of these PICs exhibit a central peak or a pit, with a minor fraction of them showing a dome. Straight rim segments of PICs align with the linear features adjacent to them and indicate that such lineaments are not exclusively surface features but lead to a localization of deformation and influence the cratering process. Straight rim segments of PICs in the dark cratered terrain (dc) are oriented along fractures and furrows. For instance, Galileo Regio have many PICs because of the NW-SE trending furrows and a high density of faults and fractures.  Here, most of the PICs have hexagonal shape with two of the straight segments parallel to the orientation of furrows and rest of the segments are at approximately perpendicular angle. Also, the presence of PICs suggests that they formed after formation of the linear features. The majority of linear features on anti-Jovian hemisphere trends in NW-SE direction while the preferred orientation of linear features on sub-Jovian hemisphere is in NE-SW direction [11]. However, the preliminary orientation analysis of straight segments of PICs using rose diagrams does not show a preferred orientation for the anti-Jovian and sub-Jovian hemispheres.

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