The age of the rings and some of the moons of Saturn is an open question, and multiple lines of evidence point to a recent (few hundred Myr ago) cataclysm involving disruption of past moons. The main driver of the evolution of the Saturnian system is the relatively rapid tidal expansion of its largest moon, Titan, which is likely driven by resonant tides within Saturn. The obliquity of Saturn and the orbit of the small moon Hyperion both serve as a record of the past orbital evolution of Titan. Saturn's obliquity was likely generated by a secular spin-orbit resonance with the planets, while Hyperion is caught in a mean-motion resonance with Titan, with both phenomena driven by Titan's orbital expansion. We propose that the breaking of Saturn's spin resonance was also the event in which Hyperion formed, when an outer mid-sized satellite (``Proto-Hyperion'') was destabilized and collided with Titan, with some of the debris accreting into Hyperion. During the instability Proto-Hyperion's perturbations produced the observed orbital inclination of Iapetus. The same event also excited the eccentricity of Titan, which then, through Titan's resonant interaction with the inner moons, led to destabilization, collisional disruption and re-accretion of the inner moon system, including the rings. We present numerical integrations that show that this chain of events has a relatively high probability, and discuss how it fits within our knowledge of the Saturnian system.
The brightness of cloud bands on Jupiter and Saturn as a function of latitude is reported. Bright Jovian bands near the equator are located in regions of anti-cyclonic circulation of the atmosphere. By contrast, bright equatorial bands on Saturn are associated with cyclonic motion. Modern definitions of the cloud band terms 'zone' and 'belt' are distinguished from their old meanings.
Regarding tidal dissipation in Saturn, usually parameterized by Saturn's quality factor Q, there remains a discrepancy between conventional estimates and the latest determination that has been derived from astrometric observations of Saturn's inner satellites. If dissipation in Saturn is as large as the astrometric observations suggest, conventional models predict that Mimas' initial orbit should be located inside Saturn's synchronous orbit or even inside its Roche limit. Using simple structure models and assuming Saturn's core to be viscoelastic, we look for dissipation models which are consi…
Saturn has a dynamically rich satellite system, which includes at least three orbital resonances between three pairs of moons: Mimas-Tethys 4:2, Enceladus-Dione 2:1, and Titan-Hyperion 4:3 mean-motion resonances. Studies of the orbital history of Saturn's moons usually assume that their past dynamics was also dominated solely by two-body resonances. Using direct numerical integrations, we find that three-body resonances among Saturnian satellites were quite common in the past, and could result in a relatively long-term, but finite capture time (10 Myr or longer). We find that these three-body …
During the thirteen years in orbit around Saturn before its final plunge, the Cassini spacecraft provided more than ten thousand astrometric measurements. Such large amounts of accurate data enable the search for extremely faint signals in the orbital motion of the saturnian moons. Among these, the detection of the dynamical feedback of the rotation of the inner moons of Saturn on their respective orbits becomes possible. Using all the currently available astrometric data associated with Atlas, Prometheus, Pandora, Janus and Epimetheus, we first provide a detailed analysis of the Cassini Imagi…
In this work we analyze the orbital evolution and the dynamical stability in the vicinity of the small Saturnian moons Aegaeon, Methone, Anthe and Pallene. We numerically resolve the exact equations of motions to investigate the orbital motion of thousands of test particles within and near to the domain of the 7/6, 14/15, 10/11 mean motion resonances of Aegaeon, Methone and Anthe with Mimas, respectively. We show that, for massless small moons, the orbits of particles initially restricted to the resonant domains remain stable for at least $10^4$ yr. We also conduct numerical simulations consid…
The geologically active south pole of Enceladus generates a plume of micron-sized particles, which likely form Saturn's tenuous E-ring extending from the orbit of Mimas to Titan. Interactions between these particles and satellites have been suggested, though only as very thin surficial phenomena. We scrutinize high-resolution images with a newly developed numerical shape model of Helene and find that the leading hemisphere of Helene is covered by thick deposits of E-ring particles, which occasionally collapse to form gully-like depressions. The depths of the resultant gullies and near-absence …
We revisit the dynamics of Atlas. Using Cassini ISS astrometric observations spanning February 2004 to August 2013, Cooper et al. (2015) found evidence that Atlas is currently perturbed by both a 54:53 corotation eccentricity resonance (CER) and a 54:53 Lindblad eccentricity resonance (LER) with Prometheus. They demonstrated that the orbit of Atlas is chaotic, with a Lyapunov time of order 10 years, as a direct consequence of the coupled resonant interaction (CER/LER) with Prometheus. Here we investigate the interactions between the two resonances using the CoraLin analytical model (El Moutami…
The reconstruction of the orbital evolution of the propeller structure Blériot orbiting in Saturn's A ring from recurrent observations in Cassini ISS images yielded a considerable offset motion from the expected Keplerian orbit (Tiscareno et al., 2010). This offset motion can be composed by three sinusoidal harmonics with amplitudes and periods of 1845, 152, 58 km and 11.1, 3.7 and 2.2 years, respectively (Sremčević et al., 2014). In this paper we present results from N-Body simulations, where we integrated the orbital evolution of a moonlet, which is placed at the radial position of Blériot u…
Aegaeon, Methone, Anthe, and Pallene are four Saturnian small moons, discovered by the Cassini spacecraft. Although their orbital characterization has been carried on by a number of authors, their long-term evolution has not been studied in detail so far. In this work, we numerically explore the long-term evolution, up to $10^5$ yr, of the small moons in a system formed by an oblate Saturn and the five largest moons close to the region: Janus, Epimetheus, Mimas, Enceladus, and Tethys. By using frequency analysis we determined the stability of the small moons and characterize, through diffusion…
We present thermal model fits for 11 Jovian and 3 Saturnian irregular satellites based on measurements from the WISE/NEOWISE dataset. Our fits confirm spacecraft-measured diameters for the objects with in situ observations (Himalia and Phoebe) and provide diameters and albedo for 12 previously unmeasured objects, 10 Jovian and 2 Saturnian irregular satellites. The best-fit thermal model beaming parameters are comparable to what is observed for other small bodies in the outer Solar System, while the visible, W1, and W2 albedos trace the taxonomic classifications previously established in the li…
We present astrometric observations of the Saturnian satellites Mimas, Enceladus, Tethys, Dione and Rhea from Cassini Imaging Science Subsystem (ISS) narrow-angle camera (NAC) images. Image sequences were designed to observe mutual occultations between these satellites. The positions of satellite centres were estimated by fitting ellipsoidal shape models to the measured limbs of the imaged satellites. Spacecraft pointing corrections were computed using the UCAC2 star catalogue. We provide a total of 2303 astrometric observations, resulting in 976 pairs, the remainder consisting of observations…
We fit an isothermal oscillatory density model of Saturn's protoplanetary disk to the present-day major satellites and innermost rings D/C and we determine the radial scale length of the disk, the equation of state and the central density of the primordial gas, and the rotational state of the Saturnian nebula. This disk does not look like the Jovian and Uranian disks that we modeled previously. Its power-law index is extremely steep ($k=-4.5$) and its radial extent is very narrow ($ΔR\lesssim 0.9$ Gm), its rotation parameter that measures centrifugal support against self-gravity is somewhat la…
Finding the trigger to Iapetus' odd global albedo pattern: Dynamics of dust from Saturn's irregular satellites
土星的卫星土卫八的正面卡西尼区域，其半径只有其后缘的十分之一。这种神秘的二分法的起源已经争论了40多年，但随着新的数据，一个更清晰的画面正在出现。出于卡西尼号雷达和成像观测，我们调查索特的模型的黑暗外源性尘埃打击原本更明亮的土卫八的动力学建模的黑暗尘埃从外部逆行卫星菲比环下的相关扰动。特别是，我们研究了粒子撞击Iapetus的概率，以及它们在Iapetian表面的预期空间分布。我们发现，在长寿命的粒子（大于约5微米）中，大多数粒子尺寸（大于约10微米）几乎肯定会撞击土卫八，并且它们在表面上的计算分布与卡西尼区域的纵向跨度很好地匹配。卫星的极地区域被观察到是明亮的，可能是因为那里沉积了冰。因此，在纬度方向上，我们估计极地尘埃沉积速率，以帮助约束用于解释亮极的热迁移模型（Spencer & Denk 2010）。我们还分析了来自其他不规则外卫星的尘埃，确定其中很大一部分物质最终将覆盖在土卫八上-也许可以解释为什么土卫八的黑暗物质的光谱与菲比的光谱有所不同。最后，我们追踪了没有撞击土卫八的尘埃颗粒，发现大部分落在土卫六上，只有一小部分击中了土卫八。正如先前所证实的，这种外来尘埃，加上木星的混沌旋转，可能会产生大致各向同性的、中等旋转的木星表面。
Influence of the coorbital resonance on the rotation of the Trojan satellites of Saturn
卡西尼号太空船收集土星卫星的高分辨率图像，并揭示了这些新世界的表面。卫星的形状和旋转可以从卡西尼号成像科学子系统数据中确定，采用边缘坐标和立体测量控制点。Epimetheus（Tiscareno et al. 2009）就是这种情况，它开启了新旋转模型的阐述（Tiscareno et al. 2009; Noyelles 2010; Robutel et al. 2011）。特别是，Epimetheus的特点是它的马蹄形轨道和交换的存在是必不可少的，以明确引入旋转模型。在土星系统的旅程中，卡西尼号航天器积累了其他卫星的观测数据，并有可能确定其中几个卫星的旋转参数。为了准备这些未来的观测，我们建立了共轨（也称为特洛伊）卫星Telesto、Calypso、Helene和Polydeuces以及Janus和Epimetheus的旋转模型。事实上，Telesto和Calypso围绕土星-Tethys的L_4和L_5拉格朗日点运行，而Helene和Polydeuces是Dione的共轨道。本研究的目的是了解如何偏离开普勒运动引起的摄动的共轨体，影响这些卫星的旋转。为此，我们利用Erdi（1977）提出的表示共轨运动的形式，在转动方程中明确地引入了微扰，从而以紧凑的形式描述了包括Janus和Epimetheus在内的共轨的转动运动。
Tidal dissipation inside giant planets is important for the orbital evolution of their natural satellites. It is conventionally treated by parameterized equilibrium tidal theory, in which the tidal torque declines rapidly with distance, and orbital expansion was faster in the past. However, Lainey et al. (2017) find that some Saturnian satellites are currently migrating outward faster than predicted by equilibrium tidal theory. Resonance locking between satellites and internal oscillations of Saturn, proposed by Fuller et al. (2016), naturally matches the observed migration rates. Here, we sho…
We observe similar structures in the orbital radii of satellite and ring systems of Uranus, Jupiter and Neptune. This stimulates an investigation that the evolution of these systems and Saturn's system is initiated by a common mechanism that involves the interaction of radiation with their subnebulae (protosatellite disks). A model is presented that is characterized by resonance created through stimulated radiative molecular association (SRMA) reactions. In this model thermal energy is extracted from a protoplanetary disk at specific distances from the protoplanet wherever there is a match bet…
Using numerical integrations, we find that the orbital eccentricity of Saturn's moon Iapetus undergoes prominent multi-Myr oscillations. We identify the responsible resonant argument to be $\varpi-\varpi_{g5}+Ω-Ω_{eq}$, with the terms being the longitudes of pericenter of Iapetus and planetary secular mode $g_5$, Iapetus's longitude of the node and Saturn's equinox. We find that this argument currently (on a $10^7$~yr timescale) appears to librate with a very large amplitude. On longer timescales, the behavior of this resonant angle is strongly dependent on the resonant interaction between Sat…
Saturn's rings act as a system of innumerable test particles that are remarkably sensitive to periodic disturbances in the planet's gravitational field. We identify 15 additional density and bending waves in Saturn's C ring driven by the planet's internal normal mode oscillations. Taking advantage of a highly accurate absolute radius scale for the rings, we are able to detect weak, high-wavenumber (up to m=14) waves with km-scale radial wavelengths. From a systematic scan of the entire C ring, we report the discovery and identification of 11 new Outer Lindblad Resonances (OLRs), two counterpar…
Saturn's icy satellites and rings investigated by Cassini - VIMS. III. Radial compositional variability
在过去的几年里，卡西尼VIMS，可见光和红外测绘光谱仪，向我们返回了土星冰冷的卫星和环的全面视图。在分析了卫星的光谱特性（Filacchione et al.（2007 a））及其在卫星半球上的分布（Filacchione et al.（2010））之后，我们在本文中继续研究结冰卫星（主卫星和次卫星）的径向变化和主环的平均光谱特性。这一分析是利用卫星的2，264个磁盘综合观测数据和以约125公里/像素的空间分辨率获得的12x700像素宽的环径向镶嵌图进行的。这些数据的比较分析，使我们能够检索水冰和红色污染物分布在整个土星的系统和典型的表面风化层粒度的量。这些测量突出了这里分析的人口中非常显着的差异，从几乎未受污染和水冰丰富的土卫二和卡里普索表面到土卫八的领先半球和菲比的金属/有机物丰富和红色表面。在可见光范围内，环的光谱比冰卫星更红，但显示出更强烈的1.5 - 2.0微米波段深度。光谱斜率、波段深度、可见光波长和相位之间的相关性使我们能够将土星的星族聚集在不同的光谱类别中，这些光谱类别不仅在主卫星和环之间被检测到，而且在共轨的小卫星之间也被检测到。最后，我们应用Hapke的理论来检索土星内部规则卫星的最佳光谱拟合，使用与Ciarniello等人（2011）中讨论的Rhea数据相同的方法。
The distinctive set of Saturnian small satellites, Aegaeon, Methone, Anthe, and Pallene, constitutes an excellent laboratory to understand the evolution of systems immersed in co-orbital dusty rings/arcs, subjected to perturbations from larger satellites and non-gravitational forces. In this work, we carried out a comprehensive numerical exploration of the long-term evolution of Pallene and its ring. Through frequency map analysis, we characterised the current dynamical state around Pallene. A simple tidal evolution model serves to set a time frame for the current orbital configuration of the …
Natural satellite systems present a large variety of orbital configurations in the solar system. While some are clearly the result of known processes, others still have largely unexplained eccentricity and inclination values. Iapetus has a still unexplained 3% orbital eccentricity and its orbital plane is tilted with respect to its local Laplace plane. On the other hand, astrometric measurements of saturnian moons have revealed high tidal migration rates, corresponding to a quality factor Q of Saturn of around 1600 for the mid-sized icy moons. We show how a past crossing of the 5:1 mean motion…
The building blocks of Titan and Enceladus are believed to have formed in a late-stage circumplanetary disk around Saturn. Evaluating the evolution of the abundances of volatile species in this disk as a function of the migration, growth, and evaporation of icy grains is then of primary importance to assess the origin of the material that eventually formed these two moons. Here we use a simple prescription of Saturn's circumplanetary disk in which the location of the centrifugal radius is varied, to investigate the time evolution of the icelines of water ice, ammonia hydrate, methane clathrate…
Most close-in planetary satellites are in synchronous rotation, which is usually the stable end-point of tidal despinning. Saturn's moon Hyperion is a notable exception by having a chaotic rotation. Hyperion's dynamical state is a consequence of its high eccentricity and its highly prolate shape (Wisdom et al. 1984). As many binary asteroids also have elongated secondaries, chaotic rotation is expected for moons in eccentric binaries (Ćuk and Nesvorný, 2010), and a minority of asteroidal secondaries may be in that state (Pravec et al. 2016). The question of secondary rotation is also important…
Key questions surrounding the origin and evolution of Titan and the Saturnian system in which it resides remain following the Cassini-Huygens mission. In-situ measurements performed at key locations on the body are a highly effective way to address these questions, and the aerial-aquatic platform proposed in this report serves to deliver unprecedented access to Titan's northern surface lakes, allowing an understanding of the hydrocarbon cycle, the potential for habitability in the environment and the chemical processes that occur at the surface. The proposed heavier-than-air flight and plunge-…
Over the course of the last decade, observations of highly-inclined (orbital inclination i > 60°) Trans-Neptunian Objects (TNOs) have posed an important challenge to current models of solar system formation (Levison et al. 2008; Nesvorný 2015). These remarkable minor planets necessitate the presence of a distant reservoir of strongly-out-of-plane TNOs, which itself requires some dynamical production mechanism (Gladman et al. 2009; Gomes et al. 2015; Batygin and Brown 2016). A notable recent addition to the census of high-i minor bodies in the solar system is the retrograde asteroid 514107 Ka'e…
