Rings Discovered Around a Moon for the First Time

Riding with Robots writes "It turns out that one of the Ringed Planet's moons has rings of its own. The robotic spacecraft Cassini at Saturn has discovered that the icy moon Rhea is orbited by an extensive debris field and at least one ring, the first such system found. 'Many years ago we thought Saturn was the only planet with rings,' said one mission scientist. 'Now we may have a moon of Saturn that is a miniature version of its even more elaborately decorated parent.'"

pff

Wake me up when they find a moon orbiting a ring.

Re:

It might not even be a ring at all..

Due to a decrease in the number of electrons detected by NASA's Cassini spacecraft on either side of the moon, scientists suggest that rings are the likeliest cause of these electrons being blocked before they reach Cass

Re:pff

I'm sure their data, reasoning, and conclusion are much more complex than that single sentence.

Pre-emptive comment

Pre-emptive semi-funny comment involving the Goatse guy, a ring, and mooning.

Dumb question: Why are they 2 dimensional?

You get used to seeing them and maybe don't question it, but why do so many structures in 'outer space' -- low gravity, three-dimensional space -- take on essentially two-dimensional forms? Consider rings around planets, planetary systems around stars, and galaxies, at least. They are all flat discs.

I asked an astrophysicist I know and she said, 'that's the way the math works out'. Ah, thanks. Maybe someone here can be more enlightening.

Disclaimer: For all you nitpickers, I know there are more than three dimensions, and that the structures are not truly two-dimensional. Unless string theory applies here, I think we can leave those facts out of the discussion.

Re:Dumb question: Why are they 2 dimensional?

http://www.faqs.org/faqs/astronomy/faq/part4/section-15.html [faqs.org]

Will answer your question much better than I could.

Re:Dumb question: Why are they 2 dimensional?

It's not how the math works out, it's collisions. When inelastic bodies collide, their post-collision velocities tend to be nearer the (mass-weighted) average of the original velocities. For bodies orbiting a planet, the average motion is generally in the equatorial plane. Thus, for rings (or gas disks around a variety of astronomical bodies), you get flattened features. Saturn's main rings (C, B, and A) are so optically think (think "dense" if you will) that they're very, very flat. Measurements suggest that the B and A rings may be as little as a few meters thick because of all the collisions.

Re:Dumb question: Why are they 2 dimensional?

IANAA (I am not an astrophysicist) but from the physics and astrophysics classes I've taken I can venture a guess. Of course I may be wrong so feel free to correct me if I am.

When interstellar gas contracts to form a solar system it has a certain angular momentum. Now let's assume it has a counter-clockwise rotation about the z-axis as well as a counter-clockwise rotation about the x-axis. Then really it has a counter-clockwise rotation in a plane which intersects the origin at 45 degrees between the x-axis and z-axis. Okay I think I totally screwed that example up... It's too late at night to think in 3-dimensions I think ;). Anyway, the point is you're going to get rotation in a plane. So when the solar system begins to take shape this would be the plane in which it rotates. Planets form in a similar fashion to a solar system, so the spin of the planet would be in a plane and hence the debris which is caught in the planet's gravity would similarly rotate in this plane.

Of course this is all theory on how solar systems/planets form, but to my understanding this is why. I'm sure the explanation for a galaxy would be very similar. At least this is how I understand it to be.

Re:Dumb question: Why are they 2 dimensional?

and maybe don't question it, but why do so many structures in 'outer space' -- low gravity, three-dimensional space -- take on essentially two-dimensional forms? Consider rings around planets, planetary systems around stars, and galaxies, at least. They are all flat discs.

The Flying Spaghetti Monster [wordpress.com] makes flat plate-like shapes because spaghetti likes to rest on plates. See, it all falls into place logically.
             

Re:Dumb question: Why are they 2 dimensional?

Why do so many structures in 'outer space' -- low gravity, three-dimensional space -- take on essentially two-dimensional forms? Consider rings around planets, planetary systems around stars, and galaxies, at least. They are all flat discs.

Try this some day. Take a bit of rope with a ball at the end of it. A tennis ball will do nicely. Bowling balls are just asking for trouble. Now hold the end of the rope and spin around as fast as you can. You now represent a planet, the tennis ball represents a part of a ring and the rope represents gravity. Try not to get dizzy and fall down. Falling down and throwing up doesn't represent anything in astronomy. That's engineering.

Notice that the ball spins in a more or less flat circle. Inertia carries it forwards and the rope pulls it towards you. There really isn't any force pushing it up or down, so it will naturally orbit in a flat plane.

Okay, whoopdie doo. I just told you that a circle is flat. What you're really asking is why millions of little rocks in a ring will all orbit in the same plane instead of going off and doing their own thing, each orbiting in slightly different directions forming a huge cloud.

Are you still spinning that ball around? Good. Now, pick up another one in your other hand and start spinning it as well. Chances are that both balls are spinning at the same speed at opposite ends of the same circle, so everything is fine. Here's where the demonstration gets a bit tricky. You need to unhinge your arms so that you can spin both balls at different angles and slightly different speeds. Since I don't want you to need to undergo major surgery in the name of physics I'll just skip to the ending and tell you what would happen if you could do that.

The balls are going to hit each other. It may not happen right away, but if you have objects moving in intersecting orbits it _will_ happen. If you had a few million balls all spinning around at different angles you would have a better representation of the rings we're talking about with a lot more collisions, but that requires a whole lot of rope and we don't have that much.

Now we can get back to the original question. Why do all these rocks form flat rings? I could tell you that that's the only way that they won't hit each other, but that doesn't answer the question of how they got there. Suppose that you took about a million little rocks and put them all in random orbits around a planet. At the start they would form a spherical cloud around it -- A ha! A three dimensional structure, just like you were asking for. But the question is "How long can it last?"

All of those rocks are going to start hitting each other, and every time they do they're going to transfer momentum. With enough objects traveling in enough different orbits that's going to happen a _lot_. Do you want to know how much? Look up at the moon some time and count the craters. Back when the solar system was young and not quite so flat, things were smashing into one another all the time. Every time they collided they scrupulously obeyed the law of conservation of momentum and shifted into different directions. Eventually the total momentum of that spherical cloud started to average out and more and more rocks found themselves orbiting in the same flat plane. Why did that happen? Simply because those were the ones that got hit less. Like your friend the astrophysicist said, "That's the way the math works out". It's all about averages, and when you're dealing with millions of rocks smacking into one another over billions of years, that's what matters.

But if we're dealing with _averages_ and _statistics_, why is everything so perfectly flat? Why are all of the planets, moons and rings all in the same plane, and why do all of the billions of stars in the Galaxy move in the same flat orbits?

The simple answers to those questions are "It's not", "They don't" and "That doesn't happen". While the planets all move in

Re:Dumb question: Why are they 2 dimensional?

Whoever told you this was wrong.

Inclination of the orbit has nothing to do with the total angular moment. h = sqrt(G M a (1-e^2)), where h is the specific angular moment, G is Newton's constant, a is the semi-major axis of the orbit, M is the central body's mass (I'm assuming a point source), and e is the eccentricity. Note the lack of the inclination in there. If you think about it, it *has* to be ascent: unlike e and a, the reference plane (and therefore I) is really arbitrary. There are often better choices than others, but they're in no way absolute.

The existence (especially the high frequency of) elliptical and irregular galaxies supports this idea that disks aren't inherently required, even if they are very common.

Our solar system's flatness and the rings or Saturn is also entirely unrelated to the galaxy's shape. If it where related, you'd expect the solar system's plane to be the same as the galaxy (it isn't: prove it to yourself and look at the line of the planets in the night sky and compare it to the line that the galaxy makes). Likewise, Saturn's rings are tilted relative to the ecliptic plane by 26 degrees so that they line in Saturn's equatorial plane.

Why are things flat? Collisions. Collisions average out velocities so they tend to a single plane. (How flat you get depends on collision frequency and any pressure support.)

Photo.

Here's a photo of Rhea [nasa.gov] from nasa.gov. Gives some nice background information on the moon as well.

Funny timing

Just last week my son said something that made me wonder, "could we put a satellite in orbit around our moon"?

Re:Funny timing

Yes of course we could. If you download Celestia [celestia.org] you can see all sorts of interesting things in space.
Now, my version is heavily modded (and it's the alpha version), but I can see Apollo still orbiting good ol' Moon in Celestia. And witness a nice dawn together with Apollo. *sigh* it's a pity that you go through that military drill to become an astronaut. I surely would like to be one.

Essentially, that's the same as putting a satellite around Earth, as Earth orbits Sun like Moon orbits Earth.

What's even more interesting: you could put a spacecraft in the Lagrange-point between Earth and Moon, so it wouldn't move - well with respect to Earth and Moon, of course.

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Its too bad L1-3 are unstable.. you'll still need small maneuvering thrusters/attitude gyros etc. to keep your craft from straying too far from the equilibrium point. L4 and L5 are dynamically stable, but there's also a lot of other cruft just lying around

From the Earth To the Moon

All this talk about orbiting the moon and the Lagrange point reminded me of Jules Verne's "From the Earth To The Moon", a surprisingly accurate description of lunar travel written 140 years ago. I only wish space travel were as simple as he described.

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Yes. In fact, there's already a mission planned: the Lunar Reconnaissance Orbiter [nasa.gov] (LRO). Set for launch later this calendar year, the LRO will be put into a low polar lunar orbit for about 1 year. Among its objectives are the creation of high-resolution lu

Wow.

That's really cool. I was so into the planets when I was young. Loved the Voyager missions (even made a model of the probe out of Contrux... and it was accurate too), and watched as many Nova specials about the Voyager missions as possible. That kid is not dead, he's just taken a place inside of me. I keep an occasional glance at the Cassini mission, just like the Galileo mission to Jupiter.

This is, indeed, a surprise discovery and hopefully there might be more material to study concerning this ring-type.

On a somewhat related-note: It is ironic that this moon has a ring whereas two moons hang out in Saturn's outer rings (they are called the Shepherd Moons).

Can we make some rings around our own moon?

Perhaps there's something relatively simple we can do, to add rings around our moon. Like shooting a missle at an asteroid in the asteroid belt, *just so,* or perhaps the next time a comet comes by.

It'd be a really nice decoration.

Maybe Rings/Dusty Halo

Despite JPL's press-release filled with certainty, this is not a definite detection. The imaging instrument has not seen any ring or halo around Rhea in spite having looked. This does not prove that the putative ring is not there (more observations are planned), but it is contrary evidence and suggests we start asking ourselves what else might cause these data.

Does this mean

they got engaged ?

Yeah, in mythology they were husband and wife

You joke, but Saturn's (Cronus's) wife in mythology was named Rhea. A bit of a coincidence that.

And in next year's news...

The next discovery will be that one of the rocks orbiting Rhea itself has a ring around it.

Wasn't there a movie about this?

'Many years ago we thought Saturn was the only planet with rings,' said one mission scientist...

But they were all of them deceived, for another ring was made...

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Uranus already has rings: http://apod.nasa.gov/apod/ap960430.html [nasa.gov]