Why does Earth have no ring even after exterior body colliding with Earth and launching all the debris in space?
In: Physics
The collision that created the moon would have created rings too, but these tend to last thousands or millions of years, but not billions of years. The rings would have mostly formed the moon or fallen to earth over time, with a small amount escaping.
rings around a planet are really unstable, stuff too close could probably burn up and stuff too far would clump together and make something bigger. The earth probably had a ring when something crashed into it. The 4 gas giants have rings but those are mostly made of ice and ice would melt if it was in orbit around earth.
You can look up the Roche limit or Roche radius. This fine line around a body is where things break up because of the tidal forces acting on something orbiting that body.
To explain this, gravity around a body has an exponential inverse graph. So it starts off very high and dramatically reduces the further away you go from a body.
The Roche limit is where that curve slopes down the fastest (kinda). So it’s the part of the curve where you see the most *curve*. [Something like the graph here](https://imagine.gsfc.nasa.gov/features/yba/M31_velocity/lightcurve/gravity_example.html)
You can see that at that point, a tiny increase in distance results in a dramatic drop in gravity. So take for example the moon. The moon is currently far enough away so that it stays in orbit and doeant fly away from earth. But what if it was at that point on the graph?
One end of the moon would have a very strong gravity acting on it and the other end would have very little gravity and more tendency to fly away from earth. So what happens? The moon would stretch apart and break into lots of pieces.
Two things will happen after that, the pieces at the Roche limit and below will stay there and form that ring and continue to orbit earth and the pieces past the Roche limit will coalesce again and make another moon.
When pieces are too small, the tidal forces at the Roche limit won’t really have an effect on them because they are too small.
So what most likely happened when something hit earth, was that it flew far away from the earth and the little gravitational pull from each of these small pieces come and pull each other together to form the moon which is how things in space coalesce. The moon is said to be much closer to earth millions of years ago than it was today. So it most likely just picked up whatever rubble it came across on its journey away from earth. And things close to earth were a negligible amount and most like burned up in the atmosphere at some point or just the effects of the sun being a deadly lazer
The moon formed a long time ago, back when the sun was a lot hotter and putting out a lot more energy. Most of the dust and particles around Earth got broken up and shoved away by this energy. The outer planets were much farther away and so the sun’s energy wasn’t strong enough to have the same effect.
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