Why are planetary rings flat

The individual particles in the uranian rings are nearly as black as lumps of coal. While astronomers do not understand the composition of this material in detail, it seems to consist in large part of carbon and hydrocarbon compounds. Organic material of this sort is rather common in the outer solar system. Many of the asteroids and comets are also composed of dark, tarlike materials. In the case of Uranus, its ten small inner moons have a similar composition, suggesting that one or more moons might have broken up to make the rings.

There are only four of them, and the particles are not uniformly distributed along their lengths. Because these rings are so difficult to investigate from Earth, it will probably be a long time before we understand them very well. It is full of information about the rings and their interactions with moons; check out their press-release images of the Saturn ring system, for example. Using the occultations of stars by the rings of Saturn, astronomers have been able to measure details in the ring structure to a resolution of 10 km.

This is a much higher resolution than can be obtained in a conventional photo of the rings. Substituting in the above formula and solving for the angular resolution, we get. This is not possible for our telescopes to achieve. For comparison, the best resolution from either the Hubble Space Telescope or ground-based telescopes is about 0.

This is why such occultation measurements are so useful for astronomers. How close to Saturn would a spacecraft have to be to make out detail in its rings as small as 20 km, if its camera has an angular resolution of 5 arcsec?

Much of our fascination with planetary rings is a result of their intricate structures, most of which owe their existence to the gravitational effect of moons, without which the rings would be flat and featureless. Indeed, it is becoming clear that without moons there would probably be no rings at all because, left to themselves, thin disks of small particles gradually spread and dissipate. A resonance takes place when two objects have orbital periods that are exact ratios of each other, such as or Such a particle would be nearest Mimas in the same part of its orbit every second revolution.

The repeated gravitational tugs of Mimas, acting always in the same direction, would perturb it, forcing it into a new orbit outside the gap. In this way, the Cassini Division became depleted of ring material over long periods of time.

Unlike the earlier Voyager flybys, Cassini was able to observe the rings for more than a decade, revealing a remarkable range of changes, on time scales from a few minutes to several years. Many of the features newly seen in Cassini data indicated the presence of condensations or small moons only a few tens of meters across imbedded in the rings.

As each small moon moves, it produces waves in the surrounding ring material like the wake left by a moving ship. Even when the moon is too small to be resolved, its characteristic waves could be photographed by Cassini. One of the most interesting rings of Saturn is the narrow F Ring , which contains several apparent ringlets within its kilometer width.

In places, the F Ring breaks up into two or three parallel strands that sometimes show bends or kinks. Clearly, the gravity of some objects must be keeping the particles in these thin rings from spreading out. As we have seen, the largest features in the rings of Saturn are produced by gravitational resonances with the inner moons, while much of the fine structure is caused by smaller embedded moons. A similar situation applies to the Epsilon Ring of Uranus , which is shepherded by the moons Cordelia and Ophelia.

These two shepherds, each about 50 kilometers in diameter, orbit about kilometers inside and outside the ring.

You can see the dark side of Pandora on this image because it is being illuminated by the light reflected from Saturn. Theoretical calculations suggest that the other narrow rings in the uranian and neptunian systems should also be controlled by shepherd moons, but none has been located.

The calculated diameter for such shepherds about 10 kilometers was just at the limit of detectability for the Voyager cameras, so it is impossible to say whether they are present or not.

Given all the narrow rings we see, some scientists still hope to find another more satisfactory mechanism for keeping them confined. One of the outstanding problems with understanding the rings is determining their ages.

Have the giant planets always had the ring systems we see today, or might these be a recent or transient addition to the solar system?

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