Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan

Abstract

A number of synchronous moons are thought to harbor water oceans beneath their outer ice shells. A subsurface ocean frictionally decouples the shell from the interior. This has led to proposals that a weak tidal or atmospheric torque might cause the shell to rotate differentially with respect to the synchronously rotating interior. As a result of centrifugal and tidal forces, the ocean would assume an ellipsoidal shape with its long axis aligned toward the parent planet. Any displacement of the shell away from its equilibrium position would induce strains thereby increasing its elastic energy and giving rise to an elastic restoring torque. We compare the elastic torque with the tidal torque acting on Europa and the atmospheric torque acting on Titan. For Europa, the tidal torque is far too weak to produce stresses that could fracture the ice shell, thus refuting a widely advocated idea. Instead, we suggest that cracks arise from time-dependent stresses due to non-hydrostatic gravity anomalies from tidally driven, episodic convection in the interior. Two years of Cassini RADAR observations of Titan's surface are interpreted as implying an angular displacement of ~0.24 degrees relative to synchroneity. Compatibility of the amplitude and phase of the observed non-synchronous rotation with estimates of the atmospheric torque requires that Titan's shell be decoupled from its interior. We find that the elastic torque balances the atmospheric torque at an angular displacement <0.05 degrees, thus coupling the shell to the interior. Moreover, if Titan's surface were spinning faster than synchronous, the tidal torque tending to restore synchronous rotation would certainly be larger than the atmospheric torque. There must either be a problem with the interpretation of the radar observations, or with our understanding of Titan's atmosphere and/or interior.