Why do planets have rings?
by Sam Atkins
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The side of Earth nearest to the moon feels a stronger gravitational pull from the moon, than the center of the Earth, and even more than the furthest side of Earth. Different parts of Earth are pulled with different amounts of strength.
Why do planets have rings?
It has to do with the tidal force. As you may know, the gravity a celestial object exerts weakens with distance. This means that the gravity the Moon exerts is stronger on the near side of Earth than on the far side of Earth. When one side of an object is pulled harder than the other, it causes the object to stretch. As the Earth rotates, different sides are being stretched by the Moon’s gravity. We experience this as the rise and fall of the water levels twice a day, which we call the tides. NOTE: Mind you that this is a very simplistic explanation.
This graph shows the scaling of the strength of gravity with distance. The vertical numbers represent the strength of gravity as a fraction of the original gravity (1). The horizontal numbers represent the distance from the object as a factor of the original distance (For example, 4 equals 4 times the original distance).
Now, you have to remember that the strength of gravity weakens with a square of the distance. If the distance is multiplied by 2, the gravity is divided by 4. If the distance is multiplied by 10, the gravity is divided by 100. A consequence of this is that there is a region in the immediate space around a massive celestial object in which the drop off in gravity, and thus the tidal force, is really intense. So intense that sufficiently large objects cannot maintain their own structural integrity under that amount of stress.
This region is known as the Roche limit.
As a moon gets close enough to the planet, the gravitational pull shoots up dramatically. Keep in mind that the moons are large and thus their bodies encompass a greater amount of difference in tidal force than an object that is only a kilometer across. Both the size and density of the moon are important factors to where the Roche limit lies.
Imagine a moon slowly approaching a gas giant. From a far enough distance, the weakening of gravity is largely flattened, never reaching zero but the tidal force exerted on the moon is negligible. As the moon gets closer and closer, the gravity gets higher at a faster rate. The moon gradually starts to stretch as the tidal forces become stronger. Eventually, the moon gets to a point where the gravity exerted by the gas giant shoots up dramatically. The pull on the near side is so much stronger than the pull on the far side that the moon fractures, breaking apart into millions of pieces.
This creates a massive debris field that swirls around the gas giant in a chaotic swarm with each dust particle, rock or boulder moving at different speeds and directions, bouncing and hurling each other around. Over time, a dominant orbit will develop amongst the countless gravitational interactions and collisions. The gravitational pull of the planet will cause the particles in the ring to spread out and flatten into a disk shape. This is because objects closer to the planet will move faster than those further away. The gap between the innermost ring and the planet surface is where the planet’s gravity is too strong to maintain its orbit and the material falls into the planet. Ultimately, we end up with a flat ring structure made up of hundreds of thousands to millions of “lanes” of rock and/or ice.
Saturn has 13 very reflective rings made up of trillions of chunks of ice. There are also numerous small moons that glide around and in between rings that act as gravitational "shepherds" that maintain the ring's shape.
The most prominent example in our solar system are the thirteen majestic rings of Saturn. They are believed to have developed from this break up of one or more moons about 100-200 million years ago. However, these rings will not go on forever. Their orbits are expected to gradually decay, and the rings will dissipate from the inside out, falling into the planet over time.
Meanwhile, other planets may develop rings of their own in the future. It is expected in the next 25 to 50 million years that the Martian moon, Phobos, will decay in its orbit to the Roche limit and be broken up by tidal forces and create a ring around the crimson planet that may last between 1 and 100 million years.
Image credit: NASA