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- W46123043 abstract "Planetary rings are swarms of objects orbiting a central planet with vertical motions that are small compared to their motions within a common plane. This characteristic arises because their planets rotate fast enough that they bulge at their equators, thus defining a preferred orbital plane. This is a major contrast between rings and other astrophysical disks, such as spiral galaxies and young solar systems, which do not derive their shapes from an external gravity field but from the average angular momentum of the disk itself (in both cases, once a preferred plane is established, collisions among particles damp out the motions perpendicular to it). Planetary rings are also distinguished by a large planet/ring mass ratio, which greatly enhances the flatness of rings (their aspect ratios are as small as 10-7). However, rings do have a number of similarities with other astrophysical disks, which add to the motivation for studying them. Unlike other known disk systems that are either many light-years away or (like the early stages of our solar system) far back in time, planetary rings can be studied up close and in real time. By necessity, a dense ring resides inside its planet’s p0015 Roche limit, the distance from a planet within which tides can pull a moon apart. If, contrariwise, a dense ring were outside the Roche limit, it would most likely accrete into one or more moons. The Roche limit is not a sharp boundary; materials that are less dense or more porous can remain dispersed as a ring at the same location where denser material will accrete. Also, dense rings near the transition develop a microstructure as they try to accrete and are frustrated by tides (Section 4.1). Furthermore, this limit operates only for rings dense enough that the particles frequently collide with each other; tenuous rings (Section 6) can resist accretion regardless of their location because their particles do not interact much. In this chapter, we will first discuss how we learn about rings in Section 2. Then we will give an overview of the known ring systems (Figure 41.1), as well as systems where rings are unconfirmed but plausible, in Section 3. More detailed descriptions of various ring structures organized by type, with a focus on finding commonalities among rings in different locations that share certain qualities, can be found in Sections 4e6. In Section 7 we will discuss ways by which rings tell us about their surroundings, and in Section 8 we will discuss the age and origins of ring systems." @default.
- W46123043 created "2016-06-24" @default.
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- W46123043 date "2014-01-01" @default.
- W46123043 modified "2023-09-27" @default.
- W46123043 title "Planetary Rings" @default.
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- W46123043 doi "https://doi.org/10.1016/b978-0-12-415845-0.00041-4" @default.
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