Asteroid Capture by Outer Planets

[dougettinger]

I would like to have verified the following assumptions about the capture of asteroids by the outer gas giants. This topic was brought up during the discussion of the moon, Dione, but got lost in the flurry of thoughts. The assumptions are delinated below.

1. A majority of asterioids of the solar system are still orbiting close to the eccliptic plane.

2. These asteroids, if captured by a gas giant, will fall or spiral inward near the region of this ecliptic plane.

3. As they fall inward these asteroid will gain velocity and may easily go faster than the planet's orbiting moons.

4. Some of these asteroids over long periods of time will run into and impact the trailing side of orbiting moons; otherwise, these falling bodies break-up at the Roche Limit or eventually fall into the planet.

5. Collisional debris created by these impacts will eventually be swept by the leading side of the same moon.

Do not these assumptions explain some of the reasons for Dione's appearance with heavier cratering on the trailing side of its surface and a smooth surface on the leading side?

Whatever the answers are, I still have a problem explaining why the rings of Saturn align with the equatorial tilt which is 29 degrees from the ecliptic plane.

Trying to understand Saturn's moon, Dione, and Saturn's rings
Doug

[Chris Peterson]

I would like to have verified the following assumptions about the capture of asteroids by the outer gas giants. This topic was brought up during the discussion of the moon, Dione, but got lost in the flurry of thoughts. The assumptions are delinated below.

1. A majority of asterioids of the solar system are still orbiting close to the eccliptic plane.

Yes.

2. These asteroids, if captured by a gas giant, will fall or spiral inward near the region of this ecliptic plane.

Asteroids are very unlikely to be captured by a gas giant, given that they lie inside the orbits of all the gas giants, and in order to be moved outward they need to undergo a complex and unlikely interaction with a third body. Gas giants are much more likely to capture some sort of cometary body (that is, a body from the scattered disc or Oort cloud), and there's little reason to expect that capture to lie near the ecliptic.

3. As they fall inward these asteroid will gain velocity and may easily go faster than the planet's orbiting moons.

4. Some of these asteroids over long periods of time will run into and impact the trailing side of orbiting moons; otherwise, these falling bodies break-up at the Roche Limit or eventually fall into the planet.

5. Collisional debris created by these impacts will eventually be swept by the leading side of the same moon.

All of these probably greatly oversimplify things- to the point where they may not describe physical reality very closely.

Whatever the answers are, I still have a problem explaining why the rings of Saturn align with the equatorial tilt which is 29 degrees from the ecliptic plane.

Ring systems can only exist at non-zero inclinations with respect to the equator if the planet is perfectly spherical. All of the gas giants are oblate, so the only stable ring orbits are over their equators. Orbits with non-zero inclinations will experience tidal forces that will disrupt them or drive them towards zero inclination.

[dougettinger]

Thank you for this prompt reply, Chris.

I totally agree with you answer to assumption number 2. Perhaps I incorrectly referred to comets (known to be much like asteroids in composition) as asteroids. Although some asteroids in highly elliptical orbits can be both in the inner and outer solar system, unless by definition these asteroids are always called comets.

Indeed, assumptions 3,4,and 5 are over-simplified, but, I am trying arrive at an understandable answer to Dione's appearance. What is your simplified version or your more detailed assumptions for Dione's appearance. I do not like the reason that impacts are rotating Dione bit by bit; why is the moon, as discovered, perfectly aligned with distinct trailing and lead surfaces?

Yes, you now make me remember about oblateness aligning rings about the equator. I suppose Uranus has enough oblateness to create a similar ring alignment.
What do you suppose is the time period required to re-align pieces of a smaller body broken up by the Roche Limt at zero equatorial inclination? I am leading toward a follow-up question: Why do we not see any mis-aligned rings at this moment for any of the outer planets ?

Seeing rings in my belfry,
Doug

[Chris Peterson]

What do you suppose is the time period required to re-align pieces of a smaller body broken up by the Roche Limt at zero equatorial inclination? I am leading toward a follow-up question: Why do we not see any mis-aligned rings at this moment for any of the outer planets?

I don't know the time required. In fact, we do see misaligned rings that are parts of existing ring systems, however.

[dougettinger]

Excellent. Our Great Protectors keep doing their duty. And the solar system keeps doing what we suspect but at a hopefully much diminished rate. It is very strange that I missed or neglected this observational data.

I am still very curious what you personally think happened to Dione in order to have different aurfaces for its leading and trailing sides.

Doug

[Chris Peterson]

I am still very curious what you personally think happened to Dione in order to have different aurfaces for its leading and trailing sides.

I don't have a well formed opinion. The radical variations we see in the surfaces of planetary moons suggests that a wide range of processes are important, and we simply have too little information right now- both in terms of those processes, and in terms of knowledge of the history of the bodies involved.

The most I'd venture is that the difference is probably related to the inherent asymmetry of a tidally locked body and some sort of resurfacing process.

[dougettinger]

Before we leave this subject, what is "asymmetry of a tidally locked body" and how does it affect hemispherical-wide surface differences ?

Doug

[Chris Peterson]

Before we leave this subject, what is "asymmetry of a tidally locked body" and how does it affect hemispherical-wide surface differences ?

It is because it is tidally locked that it even has a leading and trailing face- that's the asymmetry.

[dougettinger]

Sorry, I never heard of asymmetry used in these terms. But, yes, tidally locked bodies can lead to asymmetry.

So not too much was ventured in saying Dione was created by asymmetry of tidal locking and resurfacing processes.

Doug

[Chris Peterson]

So not too much was ventured in saying Dione was created by asymmetry of tidal locking and resurfacing processes.

Only as much as I'm comfortable that the facts support. Always a good position when discussing scientific matters.

[dougettinger]

I am not asking to place your life or career on the line, but certainly ones reputation is important. I was only asking for one or two possibilities that should be in the range of scientific thought. You were still very helpful.

Doug

[Chris Peterson]

I am not asking to place your life or career on the line, but certainly ones reputation is important. I was only asking for one or two possibilities that should be in the range of scientific thought. You were still very helpful.

Please understand, how I state my views on scientific matters has nothing to do with any concern for career or reputation. I see no value in idle speculation. If I don't have a reason to believe something, I don't believe it. If I don't have enough knowledge on a matter to offer a reasonable (even if unlikely) explanation, I don't offer an explanation.