Ring-satellite interactions and orbital evolution in the Saturn system

由于卡西尼号宇宙飞船，土星拥有太阳系中最壮观的光环，也是研究最多的光环。卡西尼号迄今为止的结果显示，土星环系统在各种过程中不断演变，从与干扰卫星的微妙引力相互作用，到土星环部分与可能在土星环中形成的小卫星之间每秒几十米的撞击。作为离地球最近的天体物理盘，土星环代表了一个独特的实验室，在这里可以研究行星形成的相同过程（但起始条件不同）。卡西尼号数据的现成可用性，加上对环和卫星形成和演化的基本了解，彻底改变了行星环的研究。通过了解土星环物质积聚成小卫星的过程，以及小卫星如何在引力作用下与土星环相互作用，可以回答有关土星环系统寿命和演化的基本问题。拟议的研究将以狭窄的F环和它的守护卫星普罗米修斯为范例，研究由卫星经过引起环中的引力不稳定性的机制。我们相信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环和伴随的小卫星进行比较来测试我们的结果。已经有证据表明,嵌入式小卫星在土星的环发生缓慢变化的轨道的引力作用下相邻环材料,这是一个更普遍问题的一个方面有关土星环的一生:当前最佳估计数一生的戒指表明卫星轨道扩张和收缩环轨道将意味着~ 400我的戒指只能生存,这是< 10%的太阳系的年龄。利用从卡西尼号卫星的长期监测图像中提取的轨道数据，我们打算量化这种影响。我们还将研究一种可能的“锁定”环系统的方法，这样角动量就会被传递给更大的外部卫星，比如土卫一。

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.