<p>It's mainly by the clear definition of the rings, and their mere existence.</p>

<p>Without a moon the rings would be short-lived, hence unlikely to be detected just in time shortly after they've formed. And they would tend to wash out to broader rings. The space between the rings seems to be empty. That's easier to explain by one or more moons keeping the gap clean, and keeping the rings on their track (<a href="http://en.wikipedia.org/wiki/Planetary_ring" rel="nofollow">shepherd moons</a>).</p>

<p>A way of thinking of the shepherd moons:
Our Earth's moon slows down Earth's rotation by tidal forces and friction.
Think of a ring a little inside the orbit of a small moon as an analogon of Earth inside the orbit of Moon: It's slowed down by tidal forces and friction (non-elastic collisions between ring particles). Slowing down ring particles results in an orbit closer to the planet/asteroid. This way a small moon can act repulsive to a nearby ring, confining it.</p>

<p>More details about the dynamics of planetary rings, including confinement by shepherd moons, see <a href="http://adsabs.harvard.edu/full/1982ARA&amp;A..20..249G" rel="nofollow">this paper</a>, p. 491 ff.</p>

CC BY-SA 3.0
2014-03-28T17:41:02.197Z
I still do not understand the physics behind how the moon/s can create those rings. (the gap I get)
KingsInnerSoul
CC BY-SA 3.0
2014-03-29T15:46:31.353Z
Moons can act repulsive on rings. Details are non-trivial. Two papers on ring dynamics: http://www.lpl.arizona.edu/~shane/PTYS_206/lectures/PTYS_206_saturns_rings_moons.pdf, http://www.agnld.uni-potsdam.de/~frank/Schmidt_etal_Chap14.pdf. May be understanding horseshoe orbits can help a bit: http://en.wikipedia.org/wiki/Horseshoe_orbit
Gerald
CC BY-SA 3.0
2014-03-28T17:36:12.050Z
2014-03-30T13:32:25.233Z
2014-03-30T13:32:25.233Z
Gerald
<p>I was watching this <a href="http://www.space.com/25177-asteroid-between-saturn-and-uranus-has-rings-animation.html" rel="nofollow">video</a> and they said that the rock probably has a moon. Is the reason they assume it has a moon is because there is a gap between the rings created by the moon's orbit around the rock? Or the rings created by the moon breaking down in its orbit and leaving debris behind in its orbit?  </p>

<the-moon><asteroids><planetary-ring>
What is the connection between moons, rings, and gaps?
KingsInnerSoul
# What is the connection between moons, rings, and gaps?

(Asked by: username_0 on 2014-03-28)

I was watching this [video](http://www.space.com/25177-asteroid-between-saturn-and-uranus-has-rings-animation.html) and they said that the rock probably has a moon. Is the reason they assume it has a moon is because there is a gap between the rings created by the moon's orbit around the rock? Or the rings created by the moon breaking down in its orbit and leaving debris behind in its orbit?

**Tags:** the-moon, asteroids, planetary-ring


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## 1 answers

### Answer by username_1 on 2014-03-28. Score: 3 | Accepted answer

It's mainly by the clear definition of the rings, and their mere existence.

Without a moon the rings would be short-lived, hence unlikely to be detected just in time shortly after they've formed. And they would tend to wash out to broader rings. The space between the rings seems to be empty. That's easier to explain by one or more moons keeping the gap clean, and keeping the rings on their track ([shepherd moons](http://en.wikipedia.org/wiki/Planetary_ring)).

A way of thinking of the shepherd moons:
Our Earth's moon slows down Earth's rotation by tidal forces and friction.
Think of a ring a little inside the orbit of a small moon as an analogon of Earth inside the orbit of Moon: It's slowed down by tidal forces and friction (non-elastic collisions between ring particles). Slowing down ring particles results in an orbit closer to the planet/asteroid. This way a small moon can act repulsive to a nearby ring, confining it.

More details about the dynamics of planetary rings, including confinement by shepherd moons, see [this paper](http://adsabs.harvard.edu/full/1982ARA&A..20..249G), p. 491 ff.

> **Comments:**
> **username_0**: I still do not understand the physics behind how the moon/s can create those rings. (the gap I get)  
> **username_1**: Moons can act repulsive on rings. Details are non-trivial. Two papers on ring dynamics: http://www.lpl.arizona.edu/~shane/PTYS_206/lectures/PTYS_206_saturns_rings_moons.pdf, http://www.agnld.uni-potsdam.de/~frank/Schmidt_etal_Chap14.pdf. May be understanding horseshoe orbits can help a bit: http://en.wikipedia.org/wiki/Horseshoe_orbit  

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