Submitted by barbadizzy t3_10hu1zr in askscience
I was always under the assumption that they were made of many little bits of rock/ice and depending on the density can appear brighter or thicker. Is it possible for them to have had so much debris that it melded together into a single solid disk? Just saw a close-up of Saturn's rings and now I'm questioning everything I thought I knew about rings.
Alternatively, if they were to get dense enough to even approach being solid, they’d collapse into one or more moons.
Isn't that a function of distance from the planet (the Roche limit) rather than the total mass of the rings?
The ring will have its own urge to turn into moons, and if has lots of mass, that moony limit will apply.
Even if you had such a fantasy strong material to build it from, a rigid ring doesn't have a stable orbit. Once the tiniest thing knocks it slightly off center, like a single meteor impact, or even the gravity of a passing meteor that doesn't even hit it, the ring would quickly shift further and further off center until one side hits the planet. The orbit isn't self-correcting once you make the object rigid. Quite the opposite - whichever side is closer to the planet gets a stronger pull that pulls it even closer to the planet.
If you wonder why that doesn't happen to individual orbiting satellites, that's because individual orbiting objects that are knocked closer the the planet take on an eccentric elliptical orbit shape which is still stable. But a rigid ring is still stuck in a circle shape even when the "proper" orbit that would remain stable should be an eccentric ellipse.
This raises an interesting question of whether a ringworld that was flexible such that it could be deformed into any ellipse could keep a stable orbit. It still probably couldn't because the same energy orbit that was a circle, when it deforms into an ellipse, ends up being an ellipse with a larger circumference than it had as a circle - so it would have to be both infinitely flexible and infinitely stretchy. That's why a field of rocks and dust can form a stable orbit ring shape (they can form a shape that is infinitely flexible and stretchy since they're not really one joined object.) While a single-object ring really can't.
Is this also why the Halo rings from, well, halo, aren't really feasable?
Not really because Halo's don't orbit around things the same way a ringworld would. Not that that makes it any more feesable, its just not THAT particular reason it would be a problem.
I never played Halo and don't know the game lore, so I can't really answer the question - I don't know what halo rings are supposed to be.
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No. The ring particles are in orbit, and orbits that are closer to the planet always go faster. So the inner particles are always moving faster than the outer particles, and can’t stick together.
If you took Saturn’s rings and added more ice particles to them, they would not weld together. Instead, collisions between particles would knock more of them into higher orbits and more into lower orbits. The ring would get wider, and a lot of the mass you add would fall into Saturn.
Even if you tried to build a solid structure shaped like Saturn’s rings from scratch out of steel, the difference in gravity between inside and outside would create tremendous forces that would rip it apart.
And even if you could somehow build a solid ring (perhaps by making it out of scrith :P) it wouldn't remain in a stable "orbit" of Saturn.
What would be the type of circumstance where they could collapse into moon(s)?
The rings would have to be lifted outside of Saturn's roche limit in order to form mons.
If enough of it found its way into the same place to agglomerate gravitationally.
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No. What you are seeing is many small pieces of dust and debris which from a distance seem dense and or solid. Take clouds for example. Before we knew about the precipitation process, we thought clouds could possibly hold structures.
Orbital mechanics, has something to say about stable orbits. Material closer to planet must travel faster, material further away must travel slower. A ring of any sizeable width will swirl over time, spinning faster or slower depending on distance from the body it's orbiting.
Not an astrophysicist, but I believe that's the same driving force that can make rings. When a moon's orbit degrades to the point where the difference in gravity between the near-side and far-side is greater than the moons own binding gravity, the near side will naturally accelerate away from the moon while the far-side luffs off behind.
So no, a ring cannot be a solid plate. But a series of hoops with bearings between them? Well probably not that either since rings aren't flat but undulate up and down in the plane of their orbit due to... Science.
Rings form because something big and heavy got too close to whatever it was orbiting and the near side and far side wanted to orbit at different speeds. These forces rip the moon apart and a ring forms.
If you tried to place a solid disk in orbit around a planet, the same thing would happen.
Well first of all, matter doesnt compress in that way. A ring would be far too thin and mostly empty to form. Not only that but tidal forces and impacts would damage a ring very quickly. Technically a solid ring around a planet is possible, but hardly if ar all through natural formations, and would probably have to be made
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If you work out the math, it turns out a solid circular ring around a gravity well, while it can be in equilibrium, it's not in stable equilibrium. It's in unstable equilibrium. This means once it forms it's not going to be staying that way. The slightest tiniest offcenter effect, including the teeny peterbations from other planets in the solar system, will knock it off center and once that happens the orbit will degrade quickly, until one side of the ring gets closer and closer and hits the parent planet.
In order for the orbit of a ring to be a stable orbit, the material that makes up the ring MUST NOT behave like a single solid rigid object. It has to behave like separate particles each in their own individual orbit. Thus a ring of dust or a ring of rocks works, but something like Larry Niven's Ringworld does not.
(This became a major plot point addressed in the second book in the series, where after being told by fans that the Ringworld as he envisioned it wouldn't stay in orbit, The author invented the notion that the ring was artificially stabilized by having been built with ramjet thrusters along the rim that would constantly turn the solar wind into propulsion thrusting back at the sun. So the closer the ring got to the sun the stronger those thrusters would work, pushing back away from the sun, automatically stabalizing what would otherwise be an unstable system.)
I guess if it's thin enough it's mathematically possible. However, it would be really sensitive to any pertubations. These stresses would break it to pieces sooner than later.
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Ringworld is a popular science fiction based on (sort of) the same idea. The ring in it is essentially a huge ring around a star instead of a planet, some 100s of million miles in diameter, and is artificial. Also, it's facing the sun. The science isn't actually accurate I think but it's a good story if you want to read.
Essentially an offshoot of a Dyson sphere. I‘ve always wondered about the mechanics of those. ;-)
Either one requires a rigid ring or hollow sphere that can be evenly 'balanced' around the central star or planet in (usually) a rotation equal to a circular orbit at the sphere's equator. Closer to that ideal spin rate, the less the ring/sphere tries to implode or throw itself apart. Unfortunately, it still needs fantastical materials to be rigid enough even if it's perfectly spun for the ring, and the sphere's poles present a massive problem...
What if it was a planet-like, a rigid ring but with inner fluidish magma layers? Would the external forces and gravity allow the ring crust to remain while moving around the magma mass?
Dorocche t1_j5ave5l wrote
Rings are under a lot of pressure from all kinds of conflicting and chaotic forces. So they need to be able to move fairly fluidly.
If they melded into a single solid structure, they would immediately be ripped apart again unless they were made of a fantasy super-strong material.