This is my first post her and I hope I don't embarass myself. You'll have to excuse the way I phrase my question, I quite frankly don't have the vocabulary to phrase my question in a manner appropriate to the subject matter. I feel a bit like a monkey trying to understand a typewrite in order to type out a last will and testament. I'm afraid you will have to bear with me.

My question is this: Thebe, one of Jupiter's moons, is close enough to Jupiter to have been completely captured by Jupiter's gravity even ceasing it's rotation and bulging towards Jupiter. I'm unsure of thebe is inside the Roche limit. I'm to understand that Jupiter has several rings of fine power not generally visible ala Saturn's rings. What I'm getting at is: Rings are known to be the remains of failed/dead mons that were crushed by gravitational forces when they fell past the Roche limit. What occured to me is couldn't jupiter's rings not be the beginning of the destruction of Thebe, ie a slow destruction of this moon (or another or all of th einner 4 moons) instead of some 5th moon that has been destroyed already.
Before anyone kills me for suggestign thsi. I realize the suppostion has to be wrong but I'm a bit liek a student who has seen the answer at th eback of the book but can't work the math problem to realize that answer. I know I'm wrong but I don't know WHY I'm wrong.
Does the destruction that happens at the Roche limit happen slowly or is it sudden and violent? Could the slow destruction of thebe or metis or adrastea or amalthea account for the rings of fine dust? Any assistance killing off my bad astronomy before I unleash it on my unsuspecting wife would be greatly appreciated.

sorry for the post length

G'day Hoganshero and welcome to the BABB.

I may have this wrong, but I understand that the Roche limit applies only to larger objects. IIRC, there's some tidal aspect to the Roche limit, which means that if you're small, there's not enough tidal difference across your diameter for the main body's gravity to pull you apart.

If I'm correct, I don't know whether that applies to Thebe, but it might be somewhere to start...

You're absolutely right, Peter. The tidal forces that pull a moon apart are quite simple, and depend on the diameter of the moon and the radius of the orbit. What resists these forces is the material strength of the moon, and if it's large enough the gravity of the moon also starts becoming significant.

Hogan's Hero: Your theory is at least possible; I would need to check the numbers to give a more definitive answer, but I would guess that at least part of Jupiter's rings comes from the inner moons. Whether this is due to tidal forces or meteorite impacts I can't answer.

Also: Don't panic. We're harmless - mostly.

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Thanks for the replies guys helping me clear a niggling bit of bad astonomy of my own creation will be most helpful to putting me on the right track my soon I'll have the vocab to hold an intelligent discussion here

Thebe, one of Jupiter's moons, is close enough to Jupiter to have been completely captured by Jupiter's gravity even ceasing it's rotation and bulging towards Jupiter. I'm unsure of thebe is inside the Roche limit.

Tidal lock (one side of a secondary always facing the primary) is not based exclusively on the distance from the primary. Many secondaries are tidally locked.

Thebe orbits Jupiter at about 222,000km. It's Roche limit is about 89,000km.

There's a complicated equation to calculate Roche limits, but the quick and dirty is 2.5 times the primary's radius. (That's where the 89,000 figure derives; the more rigorous calc gives about 52,000.)

There's a pretty good write up on tidal lock, Roche limit, and ring systems here:
http://www.uwgb.edu/dutchs/planets/resonanc.htm

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I don't think we know that for certain.

Do a google search for "Roche limit". You should find at least some answers to your questions. For instance, whether any of Jupiter's moons are close to their Roche limit (I doubt it).

You can go here for some formulae: http://scienceworld.wolfram.com/physics/RocheLimit.html

The Roche limit is the distance at which tidal forces on the satellite exceed the gravitational attraction of the satellite. A satellite can orbit within its Roche limit, but it'll need something other than gravity to hold it together.

The Roche limit is independent of the size of the satellite, but dependent on its density. This gets a bit annoying, in that a given planet doesn't have a unique Roche limit. For small bodies like the Earth, you could pretend that the density of the satellite was the same as the density of the planet and calculate a Roche limit. Or you could pick the density of your average asteroid and calculate a Roche limit.

Is this correct? The quick formula assumes that the primary and the satellite have the same density. But Thebe is more than twice as dense as Jupiter. (Also, Jupiter's equatorial radius is about 72,000 km, so 2.5 times its radius is 180,000 km - not 89,000. Perhaps you mistook the radius for the diameter.)

My rough calculations give Thebe a Roche limit of about 131,000 km, so it's well outside.

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Arrrgh!! I divided Jupiter's radius by two to get the radius! ops:

__________________
Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov

Interesting question, Hoganshero.

The closest Jovian moons are Metis and Adrastea, at a radius of ~ 128,000km. and 129,000 km, respectively. This is typically considered INSIDE the Roche' limit.

Also both are tidally locked and inside the synchronous orbit of Jupiter, meaning they orbit faster than Jupiter's rotation. They orbit in about 7 hours. That's a velocity of >110,000 km/hr; over 30 km/ sec.!! It seems to me any space debri even slightly impacting Metis would begin to erode it away; especially since the surface gravity is so low on a 60 km. chunk of rock, (Adrastea is around 20 km.)

Asteroidal impact with the inner moons, by the way, is the method of inner ring development supposedly discovered by Galileo. See here:
http://www.windows.ucar.edu/tour/lin...leo_rings.html 8)

G^2

Well, I can't help you out to much but it looks like i wouldn't have to anyway. Just thought I'd say welcome to the board.
It's nice to see some more canadians. TSC (the supreme canuck) will be pleased. You live in London? I am from Brantford.

Nice to meet you.

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- Commander Tuvok

The roche limit is easy to calculate:

If I see even one I call Orkin.

I hate roches.

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J Pax