Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science, this issue p. eaat5434, p. eaat1962, p. eaat2027, p. eaat3185, p. eaat2236, p. eaat2382 INTRODUCTION During the Cassini spacecraft’s Grand Finale mission in 2017, it performed 22 traversals of the 2000-km-wide region between Saturn and its innermost D ring. During these traversals, the onboard cosmic dust analyzer (CDA) sought to collect material released from the main rings. The science goals were to measure the composition of ring material and determine whether it is falling into the planet’s atmosphere. RATIONALE Clues about the origin of Saturn’s massive main rings may lie in their composition. Remote observations have shown that they are formed primarily of water ice, with small amounts of other materials such as silicates, complex organics, and nanophase hematite. Fine-grain ejecta generated by hypervelocity collisions of interplanetary dust particles (IDPs) on the main rings serve as microscopic samples. These grains could be examined in situ by the Cassini spacecraft during its final orbits. Deposition of ring ejecta into Saturn’s atmosphere has been suggested as an explanation for the pattern of ionospheric H3+ infrared emission, a phenomenon known as ring rain. Dynamical studies have suggested a preferential transport of charged ring particles toward the planet’s southern hemisphere because of the northward offset of Saturn’s internal magnetic field. However, the deposition flux and its form (ions or charged grains) remained unclear. In situ characterization of the ring ejecta by the Cassini CDA was planned to provide observational constraints on the composition of Saturn’s ring system and test the ring rain hypothesis. RESULTS The region within Saturn’s D ring is populated predominantly by grains tens of nanometers in radius. Larger grains (hundreds of nanometers) dominate the mass density but are narrowly confined within a few hundred kilometers around the ring plane. The measured flux profiles vary with the CDA pointing configurations. The highest dust flux was registered during the ring plane crossings when the CDA was sensitive to the prograde dust populations (Kepler ram pointing) (see the figure). When the CDA was pointed toward the retrograde direction (plasma ram pointing), two additional flux enhancements appeared on both sides of the rings at roughly the same magnetic latitude. The south dust peak is stronger and wider, indicating the dominance of Saturn’s magnetic field in the dynamics of charged nanograins. These grains are likely fast ejecta released from the main rings and falling into Saturn, producing the observed ionospheric signature of ring rain. We estimate that a few tons of nanometer-sized ejecta is produced each second across the main rings. Although this constitutes only a small fraction (<0.1%) of the total ring ejecta production, it is sufficient to supply the observed ring rain effect. Two distinct grain compositional types were identified: water ice and silicate. The silicate-to-ice ratio varies with latitude; the global average ranges from 1:11 to 1:2, higher than that inferred from remote observations of the rings. CONCLUSION Our observations illustrate the interactions between Saturn and its main rings through charged, nanometer-sized ejecta particles. The dominance of nanograins between Saturn and its rings is a dynamical selection effect, stemming from the grains’ high ejection speeds (hundreds of meters per second and higher) and Saturn’s offset magnetic field. The presence of the main rings modifies the effects of the IDP infall to Saturn’s atmosphere. The rings do this asymmetrically, leading to the distribution of the ring rain phenomenon. Confirmed ring constituents include water ice and silicates, whose ratio is likely shaped by processes associated with ring erosion processes and ring-planet interactions. Schematic view of the nanometer-sized ring ejecta environment in the vicinity of Saturn. CDA measurements were taken during Cassini’s Grand Finale mission. The measured dust flux profiles, presented by the histograms along the spacecraft trajectory, show different patterns depending on the instrument pointing configuration. The highest dust flux occurred at the ring plane under Kepler ram pointing (yellow). The profiles registered with plasma ram pointing (green) show two additional, mid-latitude peaks at both sides of the rings with substantial north-south asymmetry. This signature in the vertical profiles indicates that the measured nanograins in fact originate from the rings and are whirling into Saturn under the dynamical influence of the planet’s offset magnetic field. Blue and orange dots represent the two grain composition types identified in the mass spectra, water ice and silicate, respectively. Saturn’s main rings are composed of >95% water ice, and the nature of the remaining few percent has remained unclear. The Cassini spacecraft’s traversals between Saturn and its innermost D ring allowed its cosmic dust analyzer (CDA) to collect material released from the main rings and to characterize the ring material infall into Saturn. We report the direct in situ detection of material from Saturn’s dense rings by the CDA impact mass spectrometer. Most detected grains are a few tens of nanometers in size and dynamically associated with the previously inferred “ring rain.” Silicate and water-ice grains were identified, in proportions that vary with latitude. Silicate grains constitute up to 30% of infalling grains, a higher percentage than the bulk silicate content of the rings.

中文翻译：

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原位收集从土星环落入大气的尘埃颗粒

卡西尼号探索的最后阶段 卡西尼号飞船绕土星运行了 13 年；由于燃料不足，轨迹被更改为尚未访问的样本区域。一系列靠近环的轨道之后是大结局轨道，该轨道使航天器穿过土星与其环之间的间隙，然后航天器在进入土星高层大气时被摧毁。本期报告中的六篇论文来自卡西尼号任务的这些最后阶段。多尔蒂等人。测量了靠近土星的磁场，这意味着行星内部存在复杂的多层发电机过程。鲁索斯等人。检测到一个额外的辐射带被困在环内，由自由中子的放射性衰变维持。拉米等人。当卡西尼号飞过发射千米辐射的区域时进行的当前等离子体测量，这些区域与行星的极光相连。许等人。确定了从环落入行星的大型固体尘埃颗粒的组成，而 Mitchell 等人。研究了较小的尘埃纳米颗粒，并展示了它们如何与行星的高层大气相互作用。最后，韦特等人。确定了下落物质中的分子，并直接测量了土星大气的成分。科学，这个问题 p。eaat5434, 页。eaat1962, p. eaat2027，第 eaat3185, 页。eaat2236, 页。eaat2382 介绍 在 2017 年卡西尼号航天器的大结局任务期间，它在土星及其最内部 D 环之间 2000 公里宽的区域执行了 22 次穿越。在这些穿越过程中，机载宇宙尘埃分析仪 (CDA) 试图收集从主环释放的物质。科学目标是测量环材料的成分并确定它是否落入行星的大气层。基本原理 关于土星巨大主环起源的线索可能存在于它们的组成中。远程观察表明，它们主要由水冰形成，还有少量其他材料，如硅酸盐、复杂有机物和纳米相赤铁矿。主环上行星际尘埃粒子 (IDP) 超高速碰撞产生的细粒喷射物可作为微观样本。卡西尼号航天器可以在其最终轨道上对这些颗粒进行原位检查。环形喷射物沉积到土星大气层中被认为是电离层 H3+ 红外发射模式的一种解释，这种现象被称为环形雨。动力学研究表明，由于土星内部磁场向北偏移，带电环粒子优先向行星的南半球传输。然而，沉积通量及其形式（离子或带电颗粒）仍不清楚。卡西尼 CDA 对环喷射物的原位表征计划为土星环系统的组成提供观测约束，并测试环雨假说。结果 土星 D 环内的区域主要由半径为数十纳米的颗粒组成。较大的颗粒（数百纳米）在质量密度中占主导地位，但被狭窄地限制在环平面周围数百公里内。测得的通量分布随 CDA 指向配置而变化。当 CDA 对顺行尘埃种群（开普勒公羊指向）敏感时，在环平面交叉期间记录了最高的尘埃通量（见图）。当 CDA 指向逆行方向（等离子冲头指向）时，在大致相同磁纬度的环的两侧出现了两个额外的通量增强。南尘埃峰更强更宽，表明土星磁场在带电纳米颗粒的动力学中占主导地位。这些颗粒可能是从主环释放并落入土星的快速喷射物，产生了观测到的环雨的电离层特征。我们估计，主环上每秒会产生几吨纳米级的喷射物。虽然这仅占总环形喷射物产量的一小部分 (<0.1%)，但足以提供观察到的环形雨效应。确定了两种不同的颗粒成分类型：水冰和硅酸盐。硅酸盐与冰的比率随纬度而变化；全球平均值范围从 1:11 到 1:2，高于从环的远程观测推断的值。结论我们的观察说明了土星与其主环之间通过带电的纳米级喷射粒子相互作用。土星及其环之间的纳米颗粒的主导地位是一种动态选择效应，源于颗粒的高喷射速度（每秒数百米或更高）和土星的偏移磁场。主环的存在改变了 IDP 对土星大气层的影响。环不对称地这样做，导致环雨现象的分布。已确认的环成分包括水冰和硅酸盐，其比例可能由与环侵蚀过程和环-行星相互作用相关的过程形成。土星附近纳米级环形喷射物环境的示意图。CDA 测量是在卡西尼号的大结局任务期间进行的。由沿航天器轨迹的直方图呈现的测得的尘埃通量分布图显示出不同的模式，具体取决于仪器的指向配置。最高的尘埃通量出现在开普勒冲头指向（黄色）下的环形平面处。使用等离子冲头指向（绿色）注册的配置文件显示另外两个，环两侧的中纬度峰值具有明显的南北不对称性。垂直剖面中的这一特征表明，测得的纳米颗粒实际上来自环，并在行星偏移磁场的动态影响下旋转进入土星。蓝色和橙色点分别代表质谱中确定的两种颗粒组成类型，即水冰和硅酸盐。土星的主环由超过 95% 的水冰组成，其余百分之几的性质尚不清楚。卡西尼号航天器在土星及其最内层 D 环之间的穿越使其宇宙尘埃分析仪 (CDA) 能够收集从主环释放的物质，并表征落入土星的环物质。我们报告了 CDA 撞击质谱仪对土星致密环物质的直接原位检测。大多数检测到的颗粒大小为几十纳米，并且与之前推断的“环形雨”动态相关。硅酸盐和水冰颗粒被确定，其比例随纬度而变化。硅酸盐颗粒占下落颗粒的 30%，高于环的整体硅酸盐含量。