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Jupiter's moon Io is arguably one of the most volcanically active bodies in the Solar System. The reason, according to NASA's page Scientists to Io: Your Volcanoes Are in the Wrong Place is believed to be caused by Io being

caught in a tug-of-war between Jupiter's massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit further from Jupiter – Europa and Ganymede. Io orbits faster than these other moons, completing two orbits every time Europa finishes one, and four orbits for each one Ganymede makes. This regular timing means that Io feels the strongest gravitational pull from its neighboring moons in the same orbital location, which distorts Io's orbit into an oval shape. This in turn causes Io to flex as it moves around Jupiter.

So, how did Io form in the first place, given the tidal stresses acting upon it? Does this suggest (and what evidence is there) that Io 'migrated' into its present orbit?

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No, it is not just a matter of migration. You need to take into account two facts.

One is that (as experience shows) Io's own gravity is enough to avoid it breaking by tidal forces. It has been like that through all its history: Io could not have been formed if it started aggregating today, but it was formed at the same time Europa and Ganymede did: they three were growing in parallel.

Other is that of the orbital resonances, which makes precisely that orbit with so simple integer number relations with those of Europa and Ganymede a stable one. Io could not have been formed in another place.

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Do you have references/links to add to this answer? – user8 Dec 4 '13 at 0:35
Io's own gravity is self-explanatory. Reference for resonances can be found at – Envite Dec 4 '13 at 1:12
Hmm I was hoping for non-Wikipedia references, like a specific paper about the phenomena – user8 Dec 4 '13 at 1:13
I have not them at hand, just memories from my degree and Google. – Envite Dec 4 '13 at 1:23
I think it would add a lot of extra value to your answer if you found a paper discussing how orbital resonances may constitude evidence that Io could not have formed elsewhere. Wikipedia sources are a good starting point, but sometimes it lacks sufficient detail to answer questions like these. – astromax Dec 5 '13 at 1:46

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