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A number of synchronous moons are thought to harbor water oceans beneaththeir outer ice shells. A subsurface ocean frictionally decouples the shellfrom the interior. This has led to proposals that a weak tidal or atmospherictorque might cause the shell to rotate differentially with respect to thesynchronously rotating interior. As a result of centrifugal and tidal forces,the ocean would assume an ellipsoidal shape with its long axis aligned towardthe parent planet. Any displacement of the shell away from its equilibriumposition would induce strains thereby increasing its elastic energy and givingrise to an elastic restoring torque. We compare the elastic torque with thetidal torque acting on Europa and the atmospheric torque acting on Titan. ForEuropa, the tidal torque is far too weak to produce stresses that couldfracture the ice shell, thus refuting a widely advocated idea. Instead, wesuggest that cracks arise from time-dependent stresses due to non-hydrostaticgravity anomalies from tidally driven, episodic convection in the interior. Twoyears of Cassini RADAR observations of Titan's surface are interpreted asimplying an angular displacement of ~0.24 degrees relative to synchroneity.Compatibility of the amplitude and phase of the observed non-synchronousrotation with estimates of the atmospheric torque requires that Titan's shellbe decoupled from its interior. We find that the elastic torque balances theatmospheric torque at an angular displacement <0.05 degrees, thus coupling theshell to the interior. Moreover, if Titan's surface were spinning faster thansynchronous, the tidal torque tending to restore synchronous rotation wouldcertainly be larger than the atmospheric torque. There must either be a problemwith the interpretation of the radar observations, or with our understanding ofTitan's atmosphere and/or interior.

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