<p>Ring lifetimes vary tremendously depending on the direction of ring rotation and the rotation period of the planet. Retrograde rings will re-enter after a few million years.</p>

<p>If a planet rotates slower than 4.48 times the low orbit period then all parts of the 
rings under 2.7 rp will be tidally slowed and will eventually re-enter. 
If, like Saturn, the planetary period is shorter than 4.48 times the low orbit period
then the outer rings  will be tidally supported and will have extended lives.</p>

CC BY-SA 4.0
2019-10-24T17:29:28.230Z
These are very specific numbers. Can you explain how they arise (or point to references) and why retrograde rings would have short lifetimes (and why a few million years rather than a few thousand or a few billion)?
ProfRob
CC BY-SA 4.0
2019-10-24T18:56:24.447Z
@Ray Moses  Is this your hypothesis or an established fact? Tides indeed discriminate between the moons located below the synchronous orbit (Phobos) and those above it (Deimos). This, however, is inapplicable to rings, because a tidal deformation caused by a ring is (a) extremely small, (b) or order m=0 (which, simply speaking, means that this is a waist, not a bulge).
Michael_1812
CC BY-SA 4.0
2019-10-24T15:53:09.313Z
2019-10-24T15:53:09.313Z
Ray Moses
<p>It appears (and I am no expert) that Saturn's ring evolution is governed mainly by "viscous spreading" - collisions between ring particles; and also by interactions with Saturn's moons (resonances); and bombardment with meteoritic material.</p>

<p>There appears to be no consensus on how old the ring system is. Most theories of their formation have this taking place early in the life of the solar system, but there are observations that suggest little contamination by meteoritic material and hence that the rings are young.</p>

<p>According to <a href="http://adsabs.harvard.edu/abs/2010Icar..209..771S">Salmon et al. (2010)</a>  the viscosity in the ring is critical to its evolution and therefore estimates of ages and lifetimes. As the disc spreads due to viscosity, the viscosity itself will reduce, slowing the evolution and thus the ring system may not empty for many billions of years. On the other hand, comparing viscous spreading models with the current density profile of Saturn's rings suggests they are much less than a billion years old. However, it should be noted that other authors have used similar models to argue that Saturn's rings did indeed form 4.5 billion years ago and were much more massive (by a factor $&gt;100$) then they are today! <a href="http://adsabs.harvard.edu/abs/2011Icar..216..535C">(Charnoz et al. 2011)</a></p>

CC BY-SA 3.0
2018-04-09T06:18:33.353Z
A Cassini update to this answer.    They think Saturn's rings formed within the last few hundred million years.    http://www.sciencemag.org/news/2017/12/saturn-s-rings-are-recent-addition-solar-system-cassini-observations-show
userLTK
CC BY-SA 4.0
2019-10-24T16:06:44.643Z
@userLTK: That interpretation is based on the dirtiness of ring ice and how fast it can be 'cleaned' or 'dirtied'. This is still highly debated, and at this years EPSC/DPS meeting there were arguments brought forth for both sides - the old and the young rings.
AtmosphericPrisonEscape
CC BY-SA 3.0
2014-12-26T12:33:46.983Z
2014-12-26T19:06:50.787Z
2014-12-26T19:06:50.787Z
ProfRob
<p>I'm surprised that this question hasn't been asked before (here or on Physics), to the best of my knowledge. It's one that I might have asked when I was a bit younger, and one that I think other people will ask.</p>

<p>Anyway, it's clear that <a href="https://astronomy.stackexchange.com/questions/1509/will-saturn-rings-become-a-moon">Saturn's rings won't form a moon</a>, and the same is likely to be true for other ring systems. However, I'm guessing that they won't last forever (it's just a guess).</p>

<p>How long do planetary rings in general last? What mechanisms could cause them to dissipate/fall apart/end? I'm guessing the <a href="https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect" rel="noreferrer">Poynting-Robertson effect</a> could come into play, but I'm not sure.</p>

<p>And for anyone curious, yes, I checked just for the fun of it, and Yahoo Answers had a bunch of really, really bad, unsourced and most likely inaccurate answers (given that there was no consensus), ranging from '3 million years' to '13-18 billion years' to 'forever'.</p>

<saturn><planetary-ring>
How long do planetary rings last?
HDE 226868
# How long do planetary rings last?

(Asked by: username_0 on 2014-12-02)

I'm surprised that this question hasn't been asked before (here or on Physics), to the best of my knowledge. It's one that I might have asked when I was a bit younger, and one that I think other people will ask.

Anyway, it's clear that [Saturn's rings won't form a moon](https://astronomy.stackexchange.com/questions/1509/will-saturn-rings-become-a-moon), and the same is likely to be true for other ring systems. However, I'm guessing that they won't last forever (it's just a guess).

How long do planetary rings in general last? What mechanisms could cause them to dissipate/fall apart/end? I'm guessing the [Poynting-Robertson effect](https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect) could come into play, but I'm not sure.

And for anyone curious, yes, I checked just for the fun of it, and Yahoo Answers had a bunch of really, really bad, unsourced and most likely inaccurate answers (given that there was no consensus), ranging from '3 million years' to '13-18 billion years' to 'forever'.

**Tags:** saturn, planetary-ring

---

## 2 answers

### Answer by username_2 on 2014-12-26. Score: 11 | Accepted answer

It appears (and I am no expert) that Saturn's ring evolution is governed mainly by "viscous spreading" - collisions between ring particles; and also by interactions with Saturn's moons (resonances); and bombardment with meteoritic material.

There appears to be no consensus on how old the ring system is. Most theories of their formation have this taking place early in the life of the solar system, but there are observations that suggest little contamination by meteoritic material and hence that the rings are young.

According to [Salmon et al. (2010)](http://adsabs.harvard.edu/abs/2010Icar..209..771S) the viscosity in the ring is critical to its evolution and therefore estimates of ages and lifetimes. As the disc spreads due to viscosity, the viscosity itself will reduce, slowing the evolution and thus the ring system may not empty for many billions of years. On the other hand, comparing viscous spreading models with the current density profile of Saturn's rings suggests they are much less than a billion years old. However, it should be noted that other authors have used similar models to argue that Saturn's rings did indeed form 4.5 billion years ago and were much more massive (by a factor $>100$) then they are today! [(Charnoz et al. 2011)](http://adsabs.harvard.edu/abs/2011Icar..216..535C)

> **Comments:**
> **username_4**: A Cassini update to this answer.    They think Saturn's rings formed within the last few hundred million years.    http://www.sciencemag.org/news/2017/12/saturn-s-rings-are-recent-addition-solar-system-cassini-observations-show  
> **username_5**: @userLTK: That interpretation is based on the dirtiness of ring ice and how fast it can be 'cleaned' or 'dirtied'. This is still highly debated, and at this years EPSC/DPS meeting there were arguments brought forth for both sides - the old and the young rings.  

---

### Answer by username_1 on 2019-10-24. Score: 3

Ring lifetimes vary tremendously depending on the direction of ring rotation and the rotation period of the planet. Retrograde rings will re-enter after a few million years.

If a planet rotates slower than 4.48 times the low orbit period then all parts of the
rings under 2.7 rp will be tidally slowed and will eventually re-enter.
If, like Saturn, the planetary period is shorter than 4.48 times the low orbit period
then the outer rings will be tidally supported and will have extended lives.

> **Comments:**
> **username_2**: These are very specific numbers. Can you explain how they arise (or point to references) and why retrograde rings would have short lifetimes (and why a few million years rather than a few thousand or a few billion)?  
> **username_3**: @Ray Moses  Is this your hypothesis or an established fact? Tides indeed discriminate between the moons located below the synchronous orbit (Phobos) and those above it (Deimos). This, however, is inapplicable to rings, because a tidal deformation caused by a ring is (a) extremely small, (b) or order m=0 (which, simply speaking, means that this is a waist, not a bulge).  

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