# How long do planetary rings last?

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, 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 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'.

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) 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)

• A Cassini update to this answer. They think Saturn's rings formed within the last few hundred million years. sciencemag.org/news/2017/12/… – userLTK Apr 9 '18 at 6:18
• @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 Oct 24 at 16:06

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.

• 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)? – Rob Jeffries Oct 24 at 17:29
• @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 Oct 24 at 18:56