Why do Jovian moons line up?

[crovax]

All the times I've been observing Jupiter I saw its largest moons lined up. Is there any specific reason for that? As far as I understand Jupiter is a huge planet that simply catches everything in its gravity-well. So its moons are simply large objects that were unlucky enough to fly by too close to it. But shouldn't they have random orbits then? How comes that three (and may be more, it's just I can clearly see only 3) create almost perfect line together with Jupiter.

This first confused me last year and that is when I first got involved into astronomy. I knew quite some about physics and astronomy but didn't have any practice so didn't know that planets are visible by naked eye and are so bright. Actually half of the summer I thought that Jupiter is a lamp on the top of the telecom tower in the village where I have country house. Then one night when I noticed that it has moved too far away from the tower, I understood that it some sky object. Then I found an ooold telescope (or spyglass, I am still not sure what that thing is) and was amazed to discover that it is visible as ball and looks nothing like stars that are visible as tiny dots. Then we with friends started doing many guesses and based on that line we for quite a long time were wondering around "human made object" theories. Then afterall we moved out from it because we couldn't think out any object that will hang on every night in the same position (we often see balloon and similar stuff there). Then after observing it carefully we noticed colouring and it became obvious that it is planet. However first we thought that the colouring is Saturn rings but after checking with google I recognized Jupiter. But in the end geometry of Jovian moons still leaves me wondering and I don't hjave any idea how that happened

Can it be that it is the result of more complex gravitational interactions? So they had random orbits before but then were affected by objects like sun and other planets and all those objects altered the orbits till they reached some sort of equilibrium?

[brianb]

As far as I understand Jupiter is a huge planet that simply catches everything in its gravity-well. So its moons are simply large objects that were unlucky enough to fly by too close to it.

Not necessarily, Jupiter and its four main moons probably formed from the same protoplanet - too much angular momentum to concentrate into a single object. The moons are in orbit over the planet's equator, all in the same plane - and as Jupiter's axis is almost perpendicular to the line of sight from Earth, we see the satellites shuttling back and forth rather than going round and round, as we would if Jupiter's pole were pointed at us.

But in fact even if the moons were in random orbits, mutual gravitational attractions would have reduced the orbits to something close to a common plane, just as the trillions of moonlets that make up Saturn's rings are in a flat plane.

[crovax]

Wow, thanks for the reply! I guess I am starting to understand now! However after thinking about all this I've ended up with another question

Why does only Saturn have rings? I mean massive ones. I know that other gas giants have them too but as far as I understand they can't be compared to Saturnian rings. Those rings definitely consist of stuff caught by a planet because I can't imagine how a big proto-planet can end up in such a weird shape (heap of tiny picies). But shouldn't that stuff be distributed equally between the huge planets? I really don't see any reason why Saturn should catch moonlets at a much higher rate. After thinking I was able to imagine only a big Saturnian moon that was crashed by some huge asteroid and that the picies were attracted back to Saturn orbit. However I am not really sure if a big moon can be crashed into tonns of tiny picies. Was there any research on the nature of Saturn rings?

[brianb]

There's been a fair amount of research into ring system dynamics and basically the question is still open. Some people think that Sautrn's rings are fairly short lived (by the standards of the solar system); the material must be in more or less constant collision, which means the bits will get ground ever smaller, and the smaller pieces will spiral in. So what we have is a large comet, or small icy planetoid, being captured, getting within the Roche limit & consequntly being shattered into small pieces. Other people think the ring system is more stable and can last some billions of years, in which case the ring material is simply bits which are too close to Saturn to coalesce into small moons (the Roche limit again). The problem with the last theory is that, if true, it's hard to explain why other planets don't have bright ring systems.

[661-pete]

All of the Solar System planets are to some degree oblate (flattened at the poles) but in Jupiter, with its large mass and fast rotation, the effect is more pronounced than most - and obvious when seen in even a small telescope.

It can be proved by the laws of mechanics (but don't ask me how - my maths is 40 years in the past and even then the stuff with tensors was never my strong point!) that a body orbiting such a planet at an orbit inclined to its equator, is not stable; the gravitational effect of the primary's equatorial bulge will tend, given time, to perturb the orbit into one aligned with the planet's equator. Certainly this is the case with most of the moons of the gas giants: the few which are not (e.g. Saturn's Phoebe) are probably recent (astronomically speaking) captures for which the process has not played out yet.

In the case of Jupiter the appearance of the moons is also affected by the fact that the planet has a very slight axial tilt, and virtually no seasons. We almost always see Jupiter from a point of view directly over its equator, so its rotational plane is presented edge-on.

In the case of Saturn which is more strongly inclined to the ecliptic, of course the moons appear somewhat more often 'spread out' from the centre line (except this year when Saturn happens to be edge-on).