Unraveling Iapetus’s mysterious past using polygonal craters

Iapetus’s mysterious surface exhibits an equatorial bulge, likely frozen in during a past higher spin rate, an overprinting equatorial ridge, and a substantial leading-trailing hemisphere albedo dichotomy, pointing to a complex geologic history. In this work, we utilized polygonal impact craters (PICs) as tools to investigate small and/or regolith buried fractures, otherwise not visible in available spacecraft images, to address some of the major mysteries of Iapetus’s past. Our work resulted in the following: (1) We identified 55 PICs out of the 595 craters analyzed, indicating that Iapetus exhibits more tectonized terrain than previously recognized. (2) Inferred fracture orientations are inconsistent with the equatorial ridge being structural in origin. Instead, our results better support a formation mechanism involving the deposition of unconsolidated material, possibly derived from a ring or moonlet that once orbited Iapetus. (3) The latitudes and orientations of most inferred fractures are consistent with stresses related to despinning or spin-up events. However, the potential effects of basin-induced fracturing should also be considered. Our results are inconsistent with inferred fracture formation by obliquity, diurnal tides, and nonsynchronous rotation related stresses. (4) Overprinting relationships indicate that Iapetus’s inferred fractures may postdate the oldest basins and predate Turgis Basin, implying that at least one despinning and one spin-up event could have occurred during this time period. Furthermore, Turgis exhibits a polygonal rim, further pointing towards a younger age of this basin relative to these events. (5) We present an updated possible geologic timeline for Iapetus based on our new findings and results from the literature.