POLYGONAL IMPACT CRATERS ON IAPETUS

Iapetus’ inclined orbit and tidal bulge point toward unknown orbital event(s) in the past that would have generated global-scale stresses across its surface. For example, true polar wander may have occurred after large impact events formed Engelier, Falsaron, and Turgis basins, possibly aligning these large basins near Iapetus’ axis of spin. Furthermore, Iapetus is thought to have experienced despinning. These and other orbital events have distinct ‘tectonic signatures’ that can be analyzed using global stress patterns, reflected by surface fractures and other structural features. However, many fractures have likely been buried by Iapetus’ regolith, dust from Phoebe, and/or are below the spatial resolution of the available images, thereby limiting investigation of these stress patterns. In this work, we are analyzing polygonal impact craters (PICs), to investigate inferred faults and fractures on Iapetus that may be buried by regolith or below the available image resolutions.

We are applying a technique to identify and analyze PICs on Iapetus using a series of statistical tests. We first test for uniform azimuth distributions for each crater. A set of multi-line segment azimuths and lengths is utilized to test for a uniform distribution of crater rim azimuths. A subsequent test is applied to exclude degraded craters that may be falsely classified as PICs due to non-uniform rim azimuth distributions. The prominent rim azimuth(s) is then determined for identified PICs.

Our preliminary results indicate that PICs are present on Iapetus. In some locations, PICs are proximal to visible linear features and exhibit straight rim segments that parallel these features, supporting the interpretation that they are extensional tectonic fractures with a shared background stress pattern. PICs are also present in regions with no visible tectonic features, indicating that a stress pattern exists although it was not preserved by any visible tectonic features in that area. We are using the 2D numerical program SatStressGUI to model global stresses across the surface of Iapetus to compare the patterns with mapped PIC straight rim segments and visible fractures. Stress and associated modeled fracture maps will be analyzed to investigate the possible global stress mechanisms.