Planetary rings

[Slatiebartfarce]

how do planetary rings form? how do they move? what are some theories?

[Ambitwistor]

Various theories have been proposed, including tidal disruption of satellites, and collisions with satellites. This link:

http://www-astro.physics.uiowa.edu/~lam/research/rings/

says that the latter theory is preferred today.

As for how they move, it's mainly just orbital motion due to gravity, but there are collisions too, electromagnetic effects sometimes when ring particles are charged, etc.

[Phobos]

Originally posted by Ambitwistor
As for how they move, it's mainly just orbital motion due to gravity,

Correct. It's important to remember that planetary rings are not a single solid disk, but rather a collection of countless small pieces (each like a tiny moon).

And of course, each of the 4 gas planets in our solar system has a ring system (Saturn's being the most spectacular).

And it's possible that Earth once had a ring system (that later formed the moon).

[Ambitwistor]

Originally posted by Phobos
And it's possible that Earth once had a ring system (that later formed the moon).

Really? I thought that the favored theory of the Moon's formation nowadays was the "giant impact" model, in which a Mars-sized object struck the Earth (thus explaining the Moon's similarity to the Earth's mantle, the Moon's iron depletion, etc.)

[enigma]

Yeah, but in the time period immediately following the impact, there was a ring of debris which was still coalescing into the moon.

[Phobos]

Exactly. Of course, it could have been a VERY short-lived ring. Probably so short, I shouldn't have mentioned it.

from http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html
A giant impact would lead to a ring of very hot debris in orbit around the young Earth. Calculations indicate that the Moon could have formed from that debris in ten years or less!


another good link...
http://www.swri.org/3pubs/ttoday/spring99/moon.htm

[FZ+]

This raises a question:

What made earth's ring coallesce into a moon so quickly, and makes the rings of saturn etc remain seemingly so stable?

[Ambitwistor]

Originally posted by FZ+
What made earth's ring coallesce into a moon so quickly, and makes the rings of saturn etc remain seemingly so stable?

I don't know much about the Moon's formation. Saturn's ring's aren't stable; that's how we know they weren't formed when Saturn was. But they do last for hundreds of millions of years. I think this is partly due to the shepherd moons.

[Janus]

Originally posted by FZ+
This raises a question:

What made earth's ring coallesce into a moon so quickly, and makes the rings of saturn etc remain seemingly so stable?

Basically, the Earth's ring formed outside the Roche limit, and Saturn's rings are inside of the Roche limit.

[FZ+]

Ok...

But the questions continue... how did the debris field from the proposed object/earth collision get outside the roche limit? Surely some would have ended up within it, leaving Earth still with some remnants of this dust ring?

Or is Earth's roche limit simply too small?

[Phobos]

Originally posted by FZ+
But the questions continue... how did the debris field from the proposed object/earth collision get outside the roche limit? Surely some would have ended up within it, leaving Earth still with some remnants of this dust ring?


More stable, but not permanently stable. Even Saturn's rings are expected to be gone in about 100 million years, IIRC. Earth's Moon-forming collosion was like 4.5 billion years ago, so any ring remnant is long gone.

[Nereid]

Originally posted by FZ+
But the questions continue... how did the debris field from the proposed object/earth collision get outside the roche limit? Surely some would have ended up within it, leaving Earth still with some remnants of this dust ring?

Or is Earth's roche limit simply too small? Now here are some really good questions!

Anyone care to make some a priori guestimates of the physical parameters and processes which a good model of the formation of the Moon (Mars-sized Earth impactor theory) would require to answer FZ+'s questions (suitably extended and quantified), to ~10% accuracy? And which are the three (five) most imporant?

Yes, it would have been an inelastic collision; now for the details ...