同步卫星的弹性冰壳：木卫二裂缝和土卫六非同步旋转的意义
Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan
摘要：
许多同步卫星被认为在其外层冰壳下拥有海洋。一个地下海洋从内部包裹着贝壳。这导致了一个建议，一个弱的潮汐或大气扭矩可能会导致壳旋转差异相对于同步旋转的内部。由于离心力和潮汐力的作用，海洋将呈现椭圆形，其长轴朝向母行星。壳体离开其平衡位置的任何位移都会引起应变，从而增加其弹性能并产生弹性恢复扭矩。我们比较的弹性扭矩与潮汐扭矩作用在木卫二和大气扭矩作用在土卫六。对木卫二来说，潮汐扭矩太弱，不足以产生可能使冰壳破裂的应力，从而驳斥了一个广泛提倡的观点。相反，我们认为，裂缝产生的时间依赖性的应力，由于非流体静力学重力异常潮汐驱动，幕式对流在内部。卡西尼雷达对土卫六表面两年的观测被解释为相对于同步性的角位移约为0.24度。观测到的非同步旋转的幅度和相位与大气扭矩的估计值的兼容性要求土卫六的外壳与其内部解耦。我们发现，在角位移<0.05度时，弹性扭矩平衡大气扭矩，从而将壳体与内部耦合。此外，如果土卫六的表面旋转速度比同步旋转速度快，那么趋向于恢复同步旋转的潮汐扭矩肯定会大于大气扭矩。要么是雷达观测的解释有问题，要么是我们对土卫六大气层和/或内部的理解有问题。
英文摘要：
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.
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