Ring-satellite interactions and orbital evolution in the Saturn system

土星拥有最壮观的光环，多亏了卡西尼号航天器，它也是太阳系中研究最多的光环。到目前为止，卡西尼号的结果显示，一个环状系统在各种过程下不断演变，从与扰动卫星的微妙引力相互作用，到环状部分与可能在环状结构中形成的小卫星之间以每秒几十米的速度撞击。作为距离地球最近的天体物理圆盘，土星环代表着一个独特的实验室，可以在其中研究产生行星的相同过程(但起始条件不同)。卡西尼号数据的现成，加上对环和卫星如何形成和演化的基本了解，使行星环的研究发生了革命性的变化。关于土星光环系统寿命和演化的基本问题，可以通过了解光环物质积累成小卫星的过程以及小卫星如何通过引力与光环相互作用来回答。这项拟议中的研究将以狭窄的F环及其守护卫星普罗米修斯为范例，研究月球通过时可能导致环中引力不稳定的机制。我们相信，在卡西尼号的图像中有证据表明，小卫星可以在几个月到几年的时间尺度上形成和演化。此外，我们可以在数值模拟中模拟这一过程，并通过与卡西尼号数据中看到的实际光环和伴随的小卫星进行比较来检验我们的结果。已经有证据表明，嵌入在土星A环中的小卫星在相邻环材料的引力作用下正在经历缓慢的轨道变化，这只是与土星环寿命有关的更普遍问题的一个方面：目前对土星环寿命的最佳估计表明，卫星轨道的扩张和环轨道的收缩将意味着环只能存活约400 My，这是太阳系年龄的10%。利用从长期监测卫星中提取的轨道数据和卡西尼号图像，我们打算量化这种影响。我们还将研究一种可能的方法来‘锁定’环系，以便角动量被馈送到更大质量的外部卫星，如Mimas。

Saturn has the most spectacular and, thanks to the Cassini spacecraft, the most studied rings in the solar system. The results to date from Cassini show a ring system that is continually evolving under a variety of processes ranging from subtle gravitational interactions with perturbing moons, to impacts at several tens of metres per second between parts of the rings and moonlets which may be formed in the rings themselves. As the closest astrophysical disk to Earth, Saturn's rings represent a unique laboratory in which to study the same processes (but with different starting conditions) that give rise to planets. The ready availability of Cassini data combined with a basic understanding of how rings and moons form and evolve has revolutionised the study of planetary rings. Fundamental questions about the lifetime and evolution of the ring system of Saturn can be answered by understanding the processes by which ring material accumulates into moonlets and how small moons gravitationally interact with the rings. The proposed research will study the mechanisms by which gravitational instability in a ring can be induced by a passing moon using the narrow F ring and its shepherding moon, Prometheus, as a paradigm. We believe that there is evidence in the Cassini images of a process by which moonlets can form and evolve on timescales of months to years. Furthermore, we can model this process in numerical simulations and test our results by comparison with the actual rings and accompanying moonlets seen in the Cassini data. There is already evidence that embedded moonlets in Saturn's A ring are undergoing slow changes in their orbits under the gravitational effect of adjacent ring material and this is just one aspect of a more general problem relating to the lifetime of Saturn's rings: The current best estimate of the lifetime of the rings suggests that expanding satellite orbits and contracting ring orbits will mean that the rings can only survive for ~400My, which is <10% of the age of the solar system. Using orbital data extracted from long-term monitoring of satellites with Cassini images we intend to quantify this effect. We will also be investigating a possible means of 'locking' the ring system so that angular momentum is fed to a more massive, exterior satellite such as Mimas.