Diapir-induced Reorientation of Enceladus

Introduction: The south polar region of Enceladus, a small icy satellite of Saturn, consists of young, tectonically deformed terrain and has an anomalously high heat flux [1,2]. We find that the pole-centered location of this region can be explained by reorientation of Enceladus induced by a large, low-density ice diapir within a relatively thick ice mantle. Poleward reorientation requires that Enceladus have a near-surface elastic ice layer in excess of ~1 km thickness. Reorientation of Enceladus: We consider the circumstances under which a large, low-density diapir could have caused reorientation of Enceladus to move the diapiric region towards the satellite’s maximum intertia (spin) axis. We model a large-scale low-density region embedded within an ice mantle and beneath an ice lithosphere (Fig. 1). If the overlying lithosphere has negligible rigidity, the mass deficit at depth will be compensated by a surface mass excess, generated by upwarped topography. Because that mass excess is closer to the surface than the interior mass deficit, the net effect is to generate a positive geoid anomaly, which would tend to reorient the region toward the equator. In contrast, for the case of an infinitely rigid lithosphere, there would be no surface topography, thus producing a net negative geoid anomaly, tending to reorient the region toward the pole. A subsurface diapir therefore can result in either poleward or equatorward reorientation, depending on the rigidity (or elastic thickness) of the lithosphere [3]. Reorientation is opposed by the frozen-in component of the triaxial satellite’s tidal and rotational bulges [4,5]; thus, a satellite with lower rigidity will have smaller permanent bulges and is more likely to undergo reorientation. The viscous relaxation timescale for a conductive ice layer is typically less than 1 Myr [6], so density anomalies which persist for periods long compared to this timescale are likely to lead to reorientation [5,7]. Following the approach of Matsuyama et al. [5], we find for a synchronously rotating satellite the angular reorientation δ due to an imposed geoid anomaly is