雅努斯和厄庇墨透斯的轮换
The Rotation of Janus and Epimetheus
摘要：
土卫十一是土星的一颗小卫星，它的自转天平动（一种同步自转的振荡）为5.9 ± 1.2度，使土卫十一与地球的月球和火星的火卫一一起成为唯一一颗被发现有强迫自转天平动的天然卫星。强迫天平动是由卫星的轻微偏心轨道和非球形形状引起的。探测卫星的强迫天平动使我们能够通过将测量到的振幅与假设密度恒定的形状模型预测的振幅进行比较来探测其内部。两者之间的差异表明内部密度不对称。对于Epimetheus，形状模型中的不确定性足够大，可以解释测量到的天平动振幅。另一方面，对于Janus，虽然我们不能排除同步旋转，但Janus的最小转动惯量（长轴）和平衡的亚土星点之间的几度永久偏移可能表明Janus确实具有适度的内部密度不对称性。雅努斯和厄庇墨透斯的旋转状态每四年经历一次扰动，因为这两颗卫星“交换”轨道。轨道周期的突然变化产生了关于同步旋转的自由天平动，随后被内部摩擦阻尼。我们计算出，这种自由天平动的振幅很小（<0.1度），衰减很快（最多几周），因此低于目前使用卡西尼图像检测的极限。
英文摘要：
Epimetheus, a small moon of Saturn, has a rotational libration (an oscillation about synchronous rotation) of 5.9 +- 1.2 degrees, placing Epimetheus in the company of Earth's Moon and Mars' Phobos as the only natural satellites for which forced rotational libration has been detected. The forced libration is caused by the satellite's slightly eccentric orbit and non-spherical shape. Detection of a moon's forced libration allows us to probe its interior by comparing the measured amplitude to that predicted by a shape model assuming constant density. A discrepancy between the two would indicate internal density asymmetries. For Epimetheus, the uncertainties in the shape model are large enough to account for the measured libration amplitude. For Janus, on the other hand, although we cannot rule out synchronous rotation, a permanent offset of several degrees between Janus' minimum moment of inertia (long axis) and the equilibrium sub-Saturn point may indicate that Janus does have modest internal density asymmetries. The rotation states of Janus and Epimetheus experience a perturbation every four years, as the two moons "swap" orbits. The sudden change in the orbital periods produces a free libration about synchronous rotation that is subsequently damped by internal friction. We calculate that this free libration is small in amplitude (<0.1 degree) and decays quickly (a few weeks, at most), and is thus below the current limits for detection using Cassini images.
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