Polar wander of an ice shell on Europa
Polar wander of an ice shell on Europa
复制标题
欧罗巴冰壳的极地漂移
DOI：
10.1016/0019-1035(89)90053-5
复制
发表时间：
1987
期刊：
影响因子：
3.2
通讯作者：
D. Stevenson
中科院分区：
文献类型：
--
作者：
G. Ojakangas;D. Stevenson
木卫二上的冰壳通过一层液态水与硅酸盐核心分离，由于冰内表面温度和潮汐耗散的空间变化，其热平衡厚度剖面随其表面位置的变化而变化（G.W. Ojakangas 和 D.J. Stevenson 1989），Icarus 81, 220–241）。这些厚度变化以及壳上存在的任何化石旋转和潮汐凸起的二次球谐度分量对物体的惯性张量有贡献。问题在于行星弹性岩石圈从下方承受地形负载。继 R.J.威尔曼和 D.L. Turcotte (1981, Proc. Lunar Planet. Sci. B 12, 837–851)，我们开发了描述当二次谐波地形被添加到地壳底部时物体惯性张量变化的方程。对于木卫二上的冰壳，我们发现热平衡状态可能涉及惯性主轴的异常方向（当忽略流体静力凸起时），其中中间主力矩和最大主力矩互换。为了达到同步卫星的首选方向，热平衡壳必须围绕卫星-行星方向执行 90° 的净重新定向。我们提出了一个在圆形轨道上同步旋转的刚性卫星的简单模型，其中主力矩差 B - C 随时间增加，当 t > 0 时变为正值。该模型表明，预期的重新定向确实是动态有利的。然后，我们考虑一个更现实的模型，其中假设木卫二的外壳和海洋作为一个单一实体重新定向，独立于核心，仅受到外壳内粘性耗散的阻碍。 G.W. 提出了壳与海洋的这种耦合，而与核的耦合缺乏。 Ojakangas（1988 年，行星体的耦合热与动力演化，博士论文，加州理工学院）。该模型表明，外壳中的摩擦消除了极地漂移的可能性，除非低电导率风化层将近地表温度提高了几十度，因此风化层正下方的冰在极地漂移时间尺度上表现出粘性。然而，如果赤道到极点近地表温度差的减小量超过与模型相关的临界值，则壳的惯性张量在热平衡中不再具有不稳定的形式。或者，如果预先存在的表面裂缝（例如，由于潮汐应力）延伸到冰表现出粘性的深度，则可能会发生极地漂移。这些裂缝必须得到润滑（也许通过下面的液态水）。如果风化层底部或表面裂缝的温度 Tfat > 125°K，则模型表明，在 B - C 变为正值后，极移发生的时间尺度 <∼2 x 106 年（随着 Tfin 的增加而减小）。如果没有耗散，极移将在约 x 103 年内发生。大规模的极地漂移可能会间歇性地发生，间隔时间约为壳层的热扩散时间（~107年），尽管极地缓慢、连续漂移的状态也是可能的。冰底地形粘性流的时间尺度也接近107年。极地漂移是使冰破裂的一种非常有效的手段，并且可能有助于观察到欧罗巴冰的整体破裂系统。
An ice shell on Europa that is decoupled from the silicate core by a layer of liquid water has a thermal-equilibrium thickness profile that varies with position over its surface because of spatial variations in the surface temperature and tidal dissipation within the ice (G.W. Ojakangas and D.J. Stevenson 1989), Icarus 81, 220–241). The second spherical harmonic degree components of these thickness variations and of any fossil rotational and tidal bulges present on the shell contribute to the inertia tensor of the body. The problem is that of a planetary elastic lithosphere that is topographically loaded from below. Following R.J. Willemann and D.L. Turcotte (1981, Proc. Lunar Planet. Sci. B 12, 837–851), we develop equations describing the variations in the inertia tensor of a body when second harmonic degree topography is added to the base of the crust. For an ice shell on Europa, it is found that a state of thermal equilibrium may involve an unusual orientation of the principal axes of inertia (when hydrostatic bulges are ignored) in which the intermediate and maximum principal moments are interchanged. To reach the preferred orientation for synchronous satellites, a thermal-equilibrium shell must execute a net reorientation of 90° about the satellite-planet direction. We present a simple model of rigid, synchronously rotating satellite in a circular orbit for which the principal moment difference B - C increases with time, becoming positive for t > 0. The model demonstrates that the expected reorientation is indeed dynamically favored. We then consider a more realistic model in which Europa's shell and ocean are assumed to reorient as a single entity, independently of the core, hindered only by viscous dissipation within the shell. Such coupling of the shell with the ocean, and lack of coupling with the core, is suggested by G.W. Ojakangas (1988, Coupled Thermal and Dynamical Evolution of Planetary Bodies, Ph.D. thesis, California Institute of Technology). The model suggests that friction in the shell eliminates the possibility of polar wander unless a low-conductivity regolith increases the near-surface temperature by a few tens of degrees, so the ice just below the regolith behaves viscously on the polar wander time scale. However, if the equator-to-pole near-surface temperature difference is decreased by more than a critical, model-dependent amount, the shell's inertia tensor no longer has an unstable form in thermal equilibrium. Alternatively, polar wander may occur if preexisting surface fractures (e.g., due to tidal stresses) extend to a depth where the ice behaves viscously. These fractures must be lubricated (perhaps by liquid water from below). If the temperature Tfat the base of the regolith or the surface fractures is > 125°K, the model suggests that polar wander occurs on a time scale <∼2 x 106years (decreasing as Tfincreases), after B - C becomes postive. In the absence of dissipation, polar wander would occur in ∼few x 103years. Large-scale polar wander may occur episodically, separated by periods on the order of the thermal diffusion time for the shell (∼107years), although a state of slow, continuous drifting of the pole is also possible. The time scales of viscous flow of topography at the base of the ice is also near 107years. Polar wander is a very effective means for fracturing the ice and may have contributed to the observed global fracture systems in Europa's ice.