PINWHEEL RIFTS RADIAL TO THE SOUTH POLAR TERRAIN OF ENCELADUS

The highly tectonized south polar terrain (SPT) of Enceladus displays prominent structural features, including the eruptive tiger stripe fissures, numerous tectonic fracture sets in different orientations, and an encircling band of complex deformation called the dichotomy boundary. A system of narrow rifts (or chasmata) extends ~260 km northward from this boundary up to and beyond the equator, implying that the tectonic reach of SPT deformation processes extends far beyond the SPT itself. Structural features surrounding the SPT, exterior to the dichotomy boundary, have been described in the literature; however, there has been no rigorous scientific analysis of the geometry, formation mechanisms, structural affinity, or relationship to quantifiable stress fields of these structures. In this study we present detailed mapping of the dichotomy boundary and adjacent deformation, including the rifts emanating from the SPT. We use shape-from-shadow methods on spacecraft images to characterize these features and model tidal stress fields using SatStress. North of the dichotomy boundary and surrounding the SPT, there are additional terranes of less-defined troughs and ridges with contrasting patterns of structural reworking, encircled by a second dichotomy boundary-like feature that could be an indication of a past northern extent of the SPT. Hence, the boundary of the SPT may have receded poleward over time, possibly in response to a change in the size of the south polar thermal anomaly. The rifts radiate out from northward cuspate protrusions along the inner dichotomy boundary and curve in a clockwise fashion as they extend across the outer terrane boundary, producing geometries that inspired the descriptive term pinwheel rifts. These relationships indicate that pinwheel rift development is strongly linked to the evolution of the inner dichotomy and hence recent SPT activity. The curved clockwise geometry of the rifts may signify a shift in the regional stress fields beyond the influence of the SPT, possibly in response to nonsynchronous rotation related stresses. By characterizing the geometries of these large structural features that interact with the SPT, we can elucidate the rift evolution process on Enceladus and the possibility of ongoing rift-formation away from the SPT.