Geissler P E, Greenberg R, Hoppa G, Helfenstein P, McEwen A, Pappalardo R, Tufts R, Ockert-Bell M, Sullivan R, Greeley R, Belton M J, Denk T, Clark B, Burns J, Veverka J
Lunar and Planetary Laboratory, University of Arizona, Tucson 85712, USA.
Nature. 1998 Jan 22;391(6665):368-70. doi: 10.1038/34869.
Non-synchronous rotation of Europa was predicted on theoretical grounds, by considering the orbitally averaged torque exerted by Jupiter on the satellite's tidal bulges. If Europa's orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e approximately 0.01 , generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data suggest that there may be a permanent asymmetry in Europa's interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid or ductile ice. Non-synchronous rotation was invoked to explain Europa's global system of lineaments and an equatorial region of rifting seen in Voyager images. Here we report an analysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did so in the past), consistent with the possibility of a global subsurface ocean.
基于理论依据，通过考虑木星对木卫二潮汐隆起施加的轨道平均扭矩，预测了木卫二的非同步自转。如果木卫二的轨道是圆形的，或者该卫星由无潮汐耗散的无摩擦流体组成，那么这个扭矩的平均值将为零。然而，木卫二由于与木卫一和木卫三的动力学相互作用而产生了约0.01的小强迫偏心率e，这应该会导致该卫星的平衡自旋速率略快于同步速率。最近的引力数据表明，木卫二内部质量分布可能存在永久性不对称，其程度足以抵消潮汐扭矩；因此，如果观测到非同步自转，其表面可能通过一层地下液态或韧性冰层与内部解耦。非同步自转被用来解释木卫二的全球线性构造系统以及航海者号图像中看到的赤道裂谷区域。在此，我们基于伽利略号航天器的初步观测结果，报告对这些表面特征的方向和分布的分析。我们发现证据表明木卫二的自转速度比同步速率快（或者过去是这样）——这与存在全球地下海洋的可能性相符。
