Mimas (pronounced either "My-mass", or "Mee-mass") is only 240 miles in diameter, but is of great interest for two reasons. First, it has a relatively large crater, Herschel, which is nearly 1/3 the size of Mimas. This is a very unusual situation, because the force required to make such a large crater could easily have broken the moon into pieces. In fact, as indicated by the large fractures on the opposite side of the moon, the collision which made Herschel probably did come close to destroying Mimas. It should be mentioned, however, that the low density of Mimas (only 17% denser than water) suggests that it is made mostly of water and other ices, and an alternative explanation of the fractures has also been proposed, involving a partial melting and then freezing of the moon, resulting in cracks caused by the expansion of water ice as it freezes.
There are a number of moons in the outer Solar System which bear scars of this sort, although none quite so dramatic as those on Mimas, and we suspect that early in the history of the Solar System, soon after the planets and their moons were first formed, rubble which was left over from their formation included so many large pieces that many of the smaller moons, such as Mimas, were smashed to pieces one or even more times. After being broken up, however, since the pieces of the moons would still have been in orbit around their planets, they would have tended to collide with each other and recombine to form new versions of the previous moons. If by the time they did so the larger objects had pretty nearly been swept up and only smaller pieces of rubble were left over, the "second generation" craters produced on the new moons' surfaces would be much smaller. In this view of things Herschel would have been produced when, after Mimas had been broken up and then reformed, it happened to run into a survivor from the larger pieces of rubble which had mostly been already swept up.
The other reason that Mimas is of interest is that it creates a large gap, the Cassini Division, in the rings of Saturn. The rings are made of countless snowballs of various sizes. One interesting feature of the rings is that despite being 150,000 miles wide, they are less than a mile thick. The only way to explain this extreme thinness is that all of the snowballs in any given region must be orbiting Saturn with almost exactly the same velocity. Unlike the asteroid fields shown in movies in which rocks move around with rapid chaotic motions, in a region with lots of objects such as Saturn's rings, everything actually has to move in almost exactly the same way. To see how this works, imagine that there were objects orbiting Saturn at an angle to the rings. Each time they went through the plane of the rings there would be a good chance that they would run into something, and as a result of the resulting collision, end up sharing their vertical motion with other ring particles. After a while the verticle motions that gave the particles an inclined orbit would have been somewhat damped out. The fact that the rings are only 1/200000 as thick as they are wide means that any vertical motions must be less than 1/200000 of the average orbital velocity, or only a fraction of a mile per hour, so the damping effect of such collisions must be extremely effective in preventing motions from being anything other than very nearly identical. The same argument can be applied to motions that are inwards or outwards or forwards or backwards relative to the overall motion, thus every object in a given region must be orbiting Saturn with a velocity that varies by less than 1/200000 of the average velocity.
However, although the ring particles have almost exactly the same velocity in any given part of the rings, different parts of the rings move with different velocities. Kepler's Third Law requires orbital motions to be faster closer to a planet and slower further away from the planet, just as the inner planets orbit the Sun more quickly and the outer planets orbit it more slowly. We can easily verify that the ring particles orbit in this way by spreading the light reflected by the rings into a spectrum and examining the Doppler effect caused by the motion of the rings. Particles on the side of the planet which are approaching us have a "blueshift" (a shift of their absorption lines to shorter wavelength) as a result on their motion, while those on the other side, which are going away from us, have a "redshift" (a shift of their absorption lines to longer wavelengths). By measuring the size of these wavelength changes in different parts of the rings, we find that the inner parts orbit more quickly and the outer parts more slowly, exactly as predicted by Kepler's Third Law.
As the ring particles orbit the planet they occasionally lap Mimas, because they are closer to the planet and have shorter orbital periods than it does. Because Mimas is the innermost moon of reasonably large size (although there are closer moons, they are all considerably smaller), it can produce a moderate gravitational tug on the ring particles which are lapping it, making them "wobble" a little bit relative to the uniform orbital motion that they would otherwise have. In most parts of the rings, the ring particles lap Mimas (and any other moons which can produce similar effects) in different places at different times, but if the ring particles happen to be in an orbit which has a period exactly half that of Mimas' orbit, they will always lap it in exactly the same place (a particle lapping Mimas in a particular place in one orbit would go twice around the planet while Mimas goes once around, and lap it exactly where it did the previous time). Because the tug that Mimas gives a particle in such an orbit is always exactly the same, over time the successive tugs can add up and gradually change the orbit to one of a different size. As a result, there are very few ring particles in the region which is close to the orbital size which has half the orbital period of Mimas, and there is a gap, the Cassini Division (named after the astronomer who first noticed the gap several hundred years ago), in that part of the rings. There are also other, less pronounced gaps at other places where the orbital period of the ring particles would be commensurate
with, or in resonance
with Mimas' orbit, and in fact such relationships are quite common in the Solar System, and will be discussed in some detail when we cover the origin and evolution of the Solar System.
Rather interestingly, Mimas also affects the outer edge of the "A" ring of Saturn. The position of that ring is controlled by one of the shepherd moons of Saturn, Atlas
, which orbits just outside the "A" ring. However, the fact that Atlas has the particular orbit that it does is probably caused by a 3 to 2 resonance between the orbital periods of Mimas and Atlas.