Why doesn't ice in Saturn's ring sublime away?

[KenJackson]

Brian Cox on Wonders of the Solar System (episode: Order Out of Chaos) on the Science Channel says the rings of Saturn are made up of chunks of water ice.

Water ice? In space?

I would expect a chunk of water ice in space would experience a near zero vapor pressure. Wouldn't it? And if so, why wouldn't all those chunks have sublimated long ago?

 

[Vanadium 50]

It's cold there. What's the vapor pressure of water at -170?

 

[qraal]

It's cold there. What's the vapor pressure of water at -170?

So low that water ice is stable over the age of the solar system.

 

[qraal]

The other point I forgot is that water molecules and/or hydroxyl ions have been observed coming off the rings, so they do have a very tiny vapor pressure.

 

[Borek]

Google for "snow line" (AKA "frost line").

 

[KenJackson]

...water molecules and/or hydroxyl ions have been observed coming off the rings, so they do have a very tiny vapor pressure.

If the chunks of ice are loosing molecules and ions, they are loosing mass. So either the mass is being replaced or they are very young in astronomical terms.

How is the mass being replaced, or where did it recently come from?

 

[qraal]

If the chunks of ice are loosing molecules and ions, they are loosing mass. So either the mass is being replaced or they are very young in astronomical terms.

How is the mass being replaced, or where did it recently come from?

Large chunks lose it slower than small due to their higher volume/area ratio. Slow enough loss means the chunks can last billions of years.

 

[Vanadium 50]

All we know for certain is that the rings are more than 400 years old. However, they are probably hundreds of millions of years old, possibly billions.

How long they last is a quantitative question, and it depends on the vapor pressure of ice, as well as the rate of redposition: if a water molecule sublimes and then condenses back onto a different piece of the ring system the equilibrium can be stable. But the vapor pressure of water is really, really low. Water ice forms a significant portion of the outer moons, and you don't see them evaporating.

 

[Chronos]

Saturns rings are probably fairly recent [< billion years old]. They are believed to be remnants of an icy moon - possibly destroyed by tidal forces or an impact event. It is also known that at least one of Saturn's moons is emitting water vapor that may be replenishing the rings. There are also numerous small moons in the vicinity that have the potential to replenish the rings via collisions or tidal effects.