Saturn is known for its beautiful set of rings.
But why are there rings there? Why don't the Rings come together to form a moon?
We know that gravity tends to form objects into spherical shapes like the Sun, the planets, and the moon.
So why don't the particles in the ring
gravitationally come together to form a moon?
It's because of tides.
The moon's gravity pulls on the earth, but one side of the earth is closer to the moon.
So the moon pulls on that side harder and pulls less on the opposite side.
Also, if we compute a vector that points to the center of the moon this vector changes direction over the length of the earth.
The result is that there's a tidal force. This force
compresses one side of the earth and expands the other.
The earth spins one time per day.
The moon is also moving but not as quickly.
As the Earth spins, we experience a high tide, then a low tide, then a high tide, then a low tide.
Every day we get two of each.
But these tidal forces don't just happen on earth.
They happen between any planet and its moons and these forces aren't just acting on the planet,
they're also affecting the moons and they can be extremely powerful.
The tidal force is trying to stretch and pull apart the moon
But each moon has its own gravity and its gravity is holding it together.
So tidal forces are pulling it apart and their gravity is holding it together.
If you bring a moon closer to its planet the tidal force gets stronger and stronger.
At some point the tidal force is so strong
that it overcomes the gravity holding the moon together and the moon rips apart.
The place where this happens is called the Roche limit.
Named after Edouard Roche who discovered it.
Inside the Roche limit, you get rings.
Outside the Roche limit, you get moons.
Saturn's rings are inside the Roche limit which is why they're no moons there. Moons cannot form there
because of tidal forces. And we've seen this happen. In 1994 the comet Shoemaker-Levy 9
got too close to Jupiter and was ripped apart by tidal forces.
The comet fragments then smashed into Jupiter creating several brown spots. And
tides have other effect. One of Jupiter's moons Io is
constantly being stretched and compressed by tidal forces so much so that its surface is covered in volcanoes.
Let's do a little experiment. Our moon is well outside the Roche limit.
But what if we move the moon much, much closer. It would look something like this.
The moon is pulled apart by tidal forces.
It breaks into tiny pieces which go spinning round and around the earth
until they form a ring.
Sadly, this would destroy the moon, but on the bright side, we get a beautiful set of rings.
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