Cassini's last look at Saturn's rings During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science, this issue p. 1046, p. eaat2965, p. eaat2349, p. eaau1017; see also p. 1028 Measurement of Saturn’s gravitational field determines the mass of its rings and constrains models of the planet’s interior. INTRODUCTION The interior structure of Saturn, the depth of its winds, and the mass and age of its rings have been open questions in planetary science. The mass distribution inside a fluid and rapidly rotating planet like Saturn is largely driven by the ratio between centrifugal and gravitational forces. In static conditions, the planet should rotate uniformly and its gravity field should be axially and hemispherically symmetric and thus described by even zonal harmonics. However, optical tracking of clouds in the atmospheres of gas giants indicates that their rotation is not uniform. If the velocity field seen at cloud level extends deep into the interior, then there is a redistribution of mass that modifies the gravity field. Gravity measurements therefore constrain both Saturn’s deep interior and the depth of its winds. They also provide the mass of its rings, which dynamical and compositional dating methods have shown is related to their age. RATIONALE In its final 22 orbits, the Cassini spacecraft passed between the rings and the atmosphere of Saturn. In six of these orbits, while the spacecraft was in free fall under the combined attraction of Saturn and its rings, radio tracking from an antenna on Earth was established to measure the radial velocity of the spacecraft with accuracies between 0.023 and 0.088 mm·s−1 at a 30 s integration time. Fitting these data with a dynamical model of the forces acting on the spacecraft allowed us to determine the separate signatures from each zonal harmonic and the ring mass. RESULTS The estimated quantities were a zonal gravity model for Saturn and the gravity from a uniformly distributed mass spanning the A, B, and C rings. Additional small accelerations of unknown origin (possibly related to a time-variable gravity field) were required to obtain a good fit of the data. The determination of Saturn’s static zonal coefficients and ring mass does not depend on the assumed source of these small effects. We found that the measured values of J6, J8, and J10 are so large that they cannot be matched with interior models relying on uniform rotation and plausible compositions, but they are in agreement with interior models that assume deep differential rotation, extending from the equator into the interior up to distances of 0.7 to 0.8 Saturn radii from the spin axis. The equatorial outer layers of Saturn must rotate at an angular velocity that is 4% faster than that of the deep interior, whereas regions at higher latitudes must rotate 1 to 2% slower, regardless of the assumed rotation period. A thermal-wind approach shows that flow profiles that are similar in general character to the observed one yield solutions within the uncertainty of the gravity measurements when extended to a depth of ~9000 km, confirming that the flows are very deep and likely extend down to the levels where magnetic dissipation occurs. We also found that the mass of the rings is 0.41 times that of the Saturnian moon Mimas, which is at the lower end of the expected values. CONCLUSION Saturn’s gravity field measured by Cassini implies a strong and deep differential rotation, extending to a depth of ~9000 km. This differs from Jupiter, where winds are shallower (~3000 km). The gravity measurements are consistent with a mass of Saturn’s core of 15 to 18 Earth masses. The low value of the ring mass suggests a scenario where the present rings of Saturn are young, probably just 10 million to 100 million years old, to be consistent with their pristine icy composition. Nevertheless, the rings may have evolved substantially since their formation and were perhaps once more massive than they are today. Models for a young ring system invoke the chance capture and tidal disruption of a comet or an icy outer Solar System body, suggesting that catastrophic events continued to occur in the Solar System long after its formation 4.6 billion years ago. During the Grand Finale phase of its mission, Cassini passed between the inner edge of Saturn’s D ring and the cloud top, enabling the measurement of the ring mass. This orbit allowed gravitational acceleration of the rings to be disentangled from the much larger acceleration resulting from Saturn’s oblateness, as they pull the spacecraft in opposite directions. The curves show the spacecraft velocity variation due to the rings and the differential rotation. The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn’s gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 1019 kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 107 to 108 years ago.

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土星重力场和环质量的测量和意义

卡西尼号对土星环的最后一次观察 在卡西尼号任务的最后阶段，航天器在土星和它的环之间飞行，为这个壮观的系统提供了一个新的视角（见 Ida 的透视图）。设定场景，斯皮尔克回顾了卡西尼号在围绕土星运行的 13 年间取得的众多发现。伊斯等人。测量了卡西尼号的引力，将来自行星和环的贡献分开。这使他们能够确定土星的内部结构及其环的质量。布拉蒂等人。目前对位于环内和环周围的五个小卫星的观测。由于环材料的吸积，每个卫星都有独特的形状和组成。蒂斯卡雷诺等。直接近距离观察这些环，寻找由卫星和环粒子之间的引力相互作用雕刻而成的复杂特征。总之，这些结果表明土星环比行星本身年轻得多，并限制了它们起源的模型。科学，这个问题 p。1046 页。eaat2965, 页。eaat2349, 页。eaau1017; 另见第。1028 土星引力场的测量决定了土星环的质量并限制了行星内部的模型。引言土星的内部结构、风的深度以及它的光环的质量和年龄一直是行星科学中的悬而未决的问题。像土星这样的流体和快速旋转的行星内部的质量分布主要是由离心力和重力之间的比率驱动的。在静态条件下，行星应该均匀自转，其重力场应该是轴向和半球对称的，因此可以用偶数带谐波来描述。然而，对气态巨行星大气中云的光学跟踪表明它们的旋转是不均匀的。如果在云层看到的速度场延伸到内部深处，那么质量的重新分布会改变重力场。因此，重力测量限制了土星的深层内部和风的深度。它们还提供了其环的质量，动力学和成分测年方法表明，这与它们的年龄有关。基本原理 在其最后 22 次轨道运行中，卡西尼号宇宙飞船在土星环和大气层之间经过。在其中六个轨道上，当航天器在土星及其环的共同吸引力下自由落体时，建立了来自地球上天线的无线电跟踪以测量航天器的径向速度，精度在 0.023 和 0.088 mm·s-1 之间，积分时间为 30 秒时间。将这些数据与作用在航天器上的力的动力学模型进行拟合，使我们能够确定来自每个纬向谐波和环质量的单独特征。结果 估计的量是土星的纬向重力模型和来自横跨 A、B 和 C 环的均匀分布质量的重力。需要额外的未知来源的小加速度（可能与时变重力场有关）以获得数据的良好拟合。土星静态纬向系数和环质量的确定并不取决于这些小影响的假设来源。我们发现 J6、J8 和 J10 的测量值太大，无法与依赖均匀旋转和似是而非的成分的内部模型匹配，但它们与假设从赤道延伸的深度差动旋转的内部模型一致进入内部，距离自旋轴 0.7 到 0.8 土星半径。土星赤道外层的旋转角速度必须比深部内部快 4%，而高纬度地区的旋转速度必须慢 1% 到 2%，无论假设的旋转周期如何。热风方法表明，当延伸到约 9000 公里的深度时，与观察到的单一特征相似的流动剖面在重力测量的不确定性范围内产生解决方案，证实流动非常深并且可能向下延伸到发生磁耗散的水平。我们还发现，这些环的质量是土星卫星 Mimas 的 0.41 倍，处于预期值的低端。结论卡西尼号测量的土星重力场意味着强烈而深的差动自转，延伸到约 9000 公里的深度。这与木星不同，木星的风较浅（约 3000 公里）。重力测量结果与土星核心质量为 15 至 18 个地球质量一致。环质量的低值表明目前土星环是年轻的，可能只有 1000 万到 1 亿年的历史，与它们原始的冰雪成分一致。尽管如此，这些环自形成以来可能已经发生了很大的变化，并且可能曾经比现在更大。年轻环系统的模型引发了彗星或太阳系外冰体的偶然捕获和潮汐破坏，这表明在太阳系 46 亿年前形成很久之后，灾难性事件仍在继续发生。在其任务的大结局阶段，卡西尼号经过土星 D 环的内边缘和云顶之间，从而能够测量环质量。该轨道允许环的重力加速度与土星扁圆导致的更大加速度分离，因为它们将航天器拉向相反的方向。曲线显示了由于环和差速旋转引起的航天器速度变化。土星的内部结构、其风的深度以及其光环的质量和年龄限制了它的形成和演化。在卡西尼号任务的最后阶段，航天器在行星和它的最内环之间潜水，在云顶上方 2600 到 3900 公里的高度。在其中六次穿越中，与地球的无线电链路受到监测，以确定行星的引力场及其环的质量。我们发现土星的引力偏离了理论预期，需要大气层差旋转延伸至至少 9000 公里的深度。环的总质量为（1.54±0.49）×1019公斤（月球土卫一的0.41±0.13倍），