<p>The currently leading answer is correct to say that moon formation inside the Roche limit is unlikely.</p>

<p>However, the disk is evolving due to viscosity between the particles, and as a consequence it "spreads", so that material is able to move to outside the Roche limit.</p>

<p>In fact this is a leading possible explanation for the formation of the inner moons of Saturn - that an initially much more massive ring system underwent viscous evolution and spreading, and that material spread outside the Roche limit was able to condense into the inner moons. See for example <a href="http://arxiv.org/abs/1109.3360">http://arxiv.org/abs/1109.3360</a></p>

<p>Whether such a process can continue is doubtful. Models for viscous disk evolution show that the initial evolution is very rapid and that subsequent evolution is very much slower, so that the rate of mass transferral to outside the Roche limit is now quite small. It maybe that it is too slow to form anything new and that any mass would be just accreted onto existing Saturnian satellites.</p>

CC BY-SA 3.0
2014-12-26T19:21:15.730Z
2014-12-26T19:21:15.730Z
ProfRob
<p>It will not happen.</p>

<p>Existing moons have orbits that avoid these matter fragments to become bigger, by posing tidal forces into them.</p>

CC BY-SA 3.0
2014-12-30T02:28:53.507Z
It's not primarily the tides of the moons that prevent the ring particles from coalescing. It's the tide exerted by Saturn itself. See [Gerald's answer](http://astronomy.stackexchange.com/a/1511/53).
Keith Thompson
CC BY-SA 3.0
2014-01-23T00:13:44.360Z
2014-01-23T00:13:44.360Z
Envite
<p>The answer to the headline question is: No. Most of Saturn's rings are below the <a href="http://en.wikipedia.org/wiki/Roche_limit">Roche limit</a> of about 2.5 Saturn radii. Hence tidal forces will prevent that part of the rings to form a (large) moon.</p>

<p>Actually, part of the rings may be caused by loss of material from some of Saturn's moons, as <a href="http://saturn.jpl.nasa.gov/science/moons/enceladus/enceladusfeedring/">suspected from observations of Enceladus</a>.</p>

<p><a href="http://www.lpi.usra.edu/meetings/metsoc99/pdf/5060.pdf">Accretion of Earth</a> is still going on. So any number between millions of years and billions of years of accretion time for a planet can be justified.
Half of Earth's mass should have accreted within 10 million years, see <a href="http://www.pnas.org/content/early/2011/10/10/1108544108.full.pdf">this paper</a>.</p>

CC BY-SA 3.0
2014-01-23T00:53:12.293Z
Didn't know about the Roche limit, thank you
Eduardo Serra
CC BY-SA 3.0
2014-01-23T00:24:08.153Z
2014-01-23T00:24:08.153Z
Gerald
<p>Planets form from disks of matter orbiting around a star; some moons form from disks of matter orbiting planets. If this were going to happen around Saturn, approximately how much time would it take?</p>

<gravity><formation><natural-satellites><saturn><planetary-ring>
Will Saturn's rings become a moon?
Eduardo Serra
# Will Saturn's rings become a moon?

(Asked by: username_0 on 2014-01-22)

Planets form from disks of matter orbiting around a star; some moons form from disks of matter orbiting planets. If this were going to happen around Saturn, approximately how much time would it take?

**Tags:** gravity, formation, natural-satellites, saturn, planetary-ring

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## 3 answers

### Answer by username_4 on 2014-01-23. Score: 24 | Accepted answer

The answer to the headline question is: No. Most of Saturn's rings are below the [Roche limit](http://en.wikipedia.org/wiki/Roche_limit) of about 2.5 Saturn radii. Hence tidal forces will prevent that part of the rings to form a (large) moon.

Actually, part of the rings may be caused by loss of material from some of Saturn's moons, as [suspected from observations of Enceladus](http://saturn.jpl.nasa.gov/science/moons/enceladus/enceladusfeedring/).

[Accretion of Earth](http://www.lpi.usra.edu/meetings/metsoc99/pdf/5060.pdf) is still going on. So any number between millions of years and billions of years of accretion time for a planet can be justified.
Half of Earth's mass should have accreted within 10 million years, see [this paper](http://www.pnas.org/content/early/2011/10/10/1108544108.full.pdf).

> **Comments:**
> **username_0**: Didn't know about the Roche limit, thank you  

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### Answer by username_1 on 2014-12-26. Score: 11

The currently leading answer is correct to say that moon formation inside the Roche limit is unlikely.

However, the disk is evolving due to viscosity between the particles, and as a consequence it "spreads", so that material is able to move to outside the Roche limit.

In fact this is a leading possible explanation for the formation of the inner moons of Saturn - that an initially much more massive ring system underwent viscous evolution and spreading, and that material spread outside the Roche limit was able to condense into the inner moons. See for example <http://arxiv.org/abs/1109.3360>

Whether such a process can continue is doubtful. Models for viscous disk evolution show that the initial evolution is very rapid and that subsequent evolution is very much slower, so that the rate of mass transferral to outside the Roche limit is now quite small. It maybe that it is too slow to form anything new and that any mass would be just accreted onto existing Saturnian satellites.


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### Answer by username_2 on 2014-01-23. Score: 2

It will not happen.

Existing moons have orbits that avoid these matter fragments to become bigger, by posing tidal forces into them.

> **Comments:**
> **username_3**: It's not primarily the tides of the moons that prevent the ring particles from coalescing. It's the tide exerted by Saturn itself. See [Gerald's answer](http://astronomy.stackexchange.com/a/1511/53).  

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