Department of Geosciences, Raymond &Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, 69978, Israel.
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 76100, Israel.
Nature. 2015 Apr 9;520(7546):202-4. doi: 10.1038/nature14278. Epub 2015 Mar 25.
The alignment of Saturn's magnetic pole with its rotation axis precludes the use of magnetic field measurements to determine its rotation period. The period was previously determined from radio measurements by the Voyager spacecraft to be 10 h 39 min 22.4 s (ref. 2). When the Cassini spacecraft measured a period of 10 h 47 min 6 s, which was additionally found to change between sequential measurements, it became clear that the radio period could not be used to determine the bulk planetary rotation period. Estimates based upon Saturn's measured wind fields have increased the uncertainty even more, giving numbers smaller than the Voyager rotation period, and at present Saturn's rotation period is thought to be between 10 h 32 min and 10 h 47 min, which is unsatisfactory for such a fundamental property. Here we report a period of 10 h 32 min 45 s ± 46 s, based upon an optimization approach using Saturn's measured gravitational field and limits on the observed shape and possible internal density profiles. Moreover, even when solely using the constraints from its gravitational field, the rotation period can be inferred with a precision of several minutes. To validate our method, we applied the same procedure to Jupiter and correctly recovered its well-known rotation period.
土星的磁极与自转轴对齐，这使得无法通过测量磁场来确定其自转周期。此前，航海者号飞船通过无线电测量确定其自转周期为 10 h 39 min 22.4 s（参考文献 2）。当卡西尼号飞船测量到的周期为 10 h 47 min 6 s 时，并且还发现该周期在连续测量之间发生变化，这就清楚表明不能使用无线电周期来确定行星的整体自转周期。基于对土星测量风场的估计，进一步增加了不确定性，给出的数值比航海者号的自转周期更小，目前认为土星的自转周期在 10 h 32 min 到 10 h 47 min 之间，这对于这样一个基本性质来说是不理想的。在这里，我们报告了一个周期为 10 h 32 min 45 s ± 46 s 的结果，该结果是基于使用土星测量的引力场以及对观测到的形状和可能的内部密度分布的限制进行优化得到的。此外，即使仅使用其引力场的约束条件，也可以以数分钟的精度推断出自转周期。为了验证我们的方法，我们将相同的程序应用于木星，并正确恢复了其众所周知的自转周期。
