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 Ring material has long been thought to enter Saturn’s atmosphere, modifying its atmospheric and ionospheric chemistry. This phenomenon, dubbed “ring rain,” involves the transport of charged dust particles from the main rings along the planetary magnetic field. RATIONALE At the end of the Cassini mission, measurements by onboard instruments tested this hypothesis as well as whether ring material falls directly into the equatorial atmosphere. The final 22 orbits of the Cassini mission sent the spacecraft through the gap between the atmosphere and the innermost of the broad ring system, the D-ring. RESULTS The Magnetospheric Imaging Instrument—designed to measure energetic neutral atoms, ions, and electrons—recorded very small dust grains [8000 to 40,000 unified atomic mass units (u), or roughly 1- to 3-nm radius] in two sensors. At 3000-km altitude, a peak rate of ~300,000 counts s–1 was detected by one sensor as Cassini crossed the equatorial plane. At lower altitude (1700 to 2000 km), a second sensor recorded positively charged dust in the upper atmosphere and ionosphere over a size range of ~8000 to 40,000 u (~1 to 2 nm, assuming the density of ice). Consistent with this observation, larger dust in the 0.1- to 1-µm range was detected by the Cassini Dust Analyzer and the Radio and Plasma Wave Science instrument. CONCLUSION We modeled the interaction of dust with the H and H2 exospheric populations known to populate the gap. Collisions between small dust grains and H atoms provide sufficient drag to de-orbit the dust, causing it to plunge into the atmosphere over ~4 hours. The analysis indicates that at least ~5 kg s−1 of dust is continuously precipitating into the atmosphere. At 3000-km altitude, the dust is distributed symmetrically about the equator, mostly between ±2° latitude with a peak density of ~0.1 cm−3. On the wings of the distribution, consistent with ring rain transport along the magnetic field, almost all of the dust was observed to be charged. At the 2000 to 1700 km altitude, the dust has reached a diffusive terminal velocity and, although showing some bias toward the equator, is ordered mostly by a scale height of ~180 km in altitude. The most probable source for this dust population is the innermost bright ringlet of the D-ring, known as the D68 ringlet. We predict that this kinetic process generates a highly anisotropic neutral hydrogen population, concentrated near the equatorial plane with periapses between ~4000 and 7000 km, and apoapses ranging to as high as 10 Saturn radii, with a small fraction on escape trajectories. Ring dust. (Top left) Data and model fits for the equatorial dust population near 3000-km altitude for three orbits through the D-ring gap. HV, plate detector high voltage. Red line uses left scale (percent). (Bottom left) Dust counts (blue) from ~2000 to 1700 km (modulated by sensor energy/charge steps, green), ordered by altitude and latitude, consistent with diffusive transport. (Right) Model trajectory of a dust particle in three frames of reference, as collisions with exospheric hydrogen degrade its velocity. Saturn has been shrunk to expand the gap for clarity. The sizes of Saturn’s ring particles range from meters (boulders) to nanometers (dust). Determination of the rings’ ages depends on loss processes, including the transport of dust into Saturn’s atmosphere. During the Grand Finale orbits of the Cassini spacecraft, its instruments measured tiny dust grains that compose the innermost D-ring of Saturn. The nanometer-sized dust experiences collisions with exospheric (upper atmosphere) hydrogen and molecular hydrogen, which forces it to fall from the ring into the ionosphere and lower atmosphere. We used the Magnetospheric Imaging Instrument to detect and characterize this dust transport and also found that diffusion dominates above and near the altitude of peak ionospheric density. This mechanism results in a mass deposition into the equatorial atmosphere of ~5 kilograms per second, constraining the age of the D-ring.

中文翻译：

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尘埃颗粒从土星的 D 环落入赤道高层大气

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 长期以来，人们一直认为环材料会进入土星的大气层，从而改变其大气层和电离层的化学性质。这种现象被称为“环雨，”涉及带电尘埃粒子从主环沿着行星磁场传输。基本原理在卡西尼号任务结束时，机载仪器的测量验证了这一假设以及环物质是否直接落入赤道大气中。卡西尼号任务的最后 22 个轨道使航天器穿过大气层和宽环系统最里面的 D 环之间的间隙。结果 磁层成像仪器——设计用于测量高能中性原子、离子和电子——在两个传感器中记录了非常小的尘埃颗粒 [8000 到 40,000 统一原子质量单位 (u)，或大约 1 到 3 纳米半径]。在 3000 公里的高度，当卡西尼号穿过赤道平面时，一个传感器检测到约 300,000 次 s-1 的峰值速率。在低海拔（1700 至 2000 公里），第二个传感器记录了高层大气和电离层中带正电荷的尘埃，其大小范围为~8000 到 40,000 u（~1 到 2 nm，假设冰的密度）。与这一观察结果一致，卡西尼尘埃分析仪和无线电和等离子波科学仪器检测到了 0.1 到 1 微米范围内的较大尘埃。结论 我们模拟了尘埃与已知填充间隙的 H 和 H2 外层种群的相互作用。小尘埃颗粒和 H 原子之间的碰撞提供了足够的阻力使尘埃脱离轨道，使其在大约 4 小时内坠入大气层。分析表明，至少约 5 kg s-1 的灰尘不断沉淀到大气中。在 3000 公里的高度，尘埃围绕赤道对称分布，主要在±2° 纬度之间，峰值密度为~0.1 cm-3。在分布的翅膀上，与沿磁场的环雨传输一致，几乎所有的尘埃都被观察到带电。在 2000 至 1700 公里的高度，尘埃已达到扩散终端速度，虽然显示出一些向赤道的偏向，但主要按海拔约 180 公里的标度高度排序。这个尘埃群最可能的来源是 D 环最里面的明亮小环，被称为 D68 小环。我们预测，这个动力学过程会产生高度各向异性的中性氢群，集中在赤道平面附近，近点在~4000 到 7000 公里之间，远点的范围高达 10 土星半径，只有一小部分在逃逸轨迹上。环尘。（左上）数据和模型适用于通过 D 形环间隙的三个轨道在 3000 公里高度附近的赤道尘埃群。HV，板探测器高压。红线使用左刻度（百分比）。（左下）从约 2000 公里到 1700 公里（由传感器能量/电荷步骤调制，绿色）的灰尘计数（蓝色），按高度和纬度排序，与扩散传输一致。（右）在三个参考系中模拟尘埃粒子的轨迹，因为与外层氢的碰撞会降低其速度。为清楚起见，土星已缩小以扩大差距。土星环粒子的大小从米（巨石）到纳米（尘埃）不等。环的年龄的确定取决于损失过程，包括灰尘进入土星大气的传输。在卡西尼号飞船的大结局轨道上，它的仪器测量了构成土星最内部 D 环的微小尘埃颗粒。纳米级尘埃与外层（高层大气）氢和分子氢发生碰撞，迫使它从环中落入电离层和低层大气。我们使用磁层成像仪器来检测和表征这种尘埃传输，还发现扩散在电离层密度峰值上方和附近占主导地位。这种机制导致赤道大气中每秒约 5 公斤的大量沉积，从而限制了 D 环的年龄。这迫使它从环坠落到电离层和低层大气中。我们使用磁层成像仪器来检测和表征这种尘埃传输，还发现扩散在电离层密度峰值上方和附近占主导地位。这种机制导致赤道大气中每秒约 5 公斤的大量沉积，从而限制了 D 环的年龄。这迫使它从环坠落到电离层和低层大气中。我们使用磁层成像仪器来检测和表征这种尘埃传输，还发现扩散在电离层密度峰值上方和附近占主导地位。这种机制导致赤道大气中每秒约 5 公斤的大量沉积，从而限制了 D 环的年龄。