Enceladus’s Long-Term and Diverse Geologic History

Abstract:

Enceladus’s surface has a diverse array of geologic terrains that includes ancient cratered regions, young ridges, and numerous fractures. Here we focus on three stages of large-scale deformation: an ancient period (~2 Gyr) that resulted in ridge and fractured terrains, a recent period (< 0.2 Gyr) of possible compression on the leading and trailing hemispheres, and a potentially active period of near global-wide extensional driven deformation. Evidence for the ancient deformation is preserved in the cratered Saturnian and anti-Saturnian hemispheres in the form of small ridges (10s m high, 1-15 km long), and short (>10 km) narrow (~10s m) fractures. The next phase of preserved deformation is seen in the much younger ridges of the leading and trailing hemispheres. In the trailing hemisphere are two main sets of ridges: a smaller set (~50 m high), and the larger dorsa (~50 km long, ~800 m high) which can bifurcate in a branching manner with branches that intersect other dorsa at near right angles. We suggest the dorsa are thrust faults that accommodate ~10% shortening. On the leading hemisphere are two different ridge types: a smaller set (1-20 km long, 10s m high), and a larger but less numerous set (~600 m high, 15–35 km long) that are lens-like shape in map view. The larger ridges we interpret to be large-scale thrust faults. The last phase of deformation we explore here is preserved in the numerous fractures extending from the south polar region towards the north. These fractures suggest the leading and trailing hemispheres that were previously dominated by compressional tectonics are now experiencing tension-driven deformation. We suggest the ridges observed in the older cratered Saturnian and anti-Saturnian hemispheres preserve an ancient period of deformation. This was followed much later by a second period of activity that deformed the south polar, leading and trailing hemispheres. We suggest the ice shell is currently thickening from north to south potentially inducing fracturing through the ridged terrains. These distinct phases of deformation suggest Enceladus experiences only episodic overturn on a >1 Gyr year time scale.