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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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  • $\begingroup$ Do you have references/links to add to this answer? $\endgroup$ – user8 Dec 4 '13 at 0:35
  • $\begingroup$ Io's own gravity is self-explanatory. Reference for resonances can be found at en.wikipedia.org/wiki/Orbital_resonance $\endgroup$ – Envite Dec 4 '13 at 1:12
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    $\begingroup$ Hmm I was hoping for non-Wikipedia references, like a specific paper about the phenomena $\endgroup$ – user8 Dec 4 '13 at 1:13
  • $\begingroup$ I have not them at hand, just memories from my degree and Google. $\endgroup$ – Envite Dec 4 '13 at 1:23
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    $\begingroup$ 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. $\endgroup$ – astromax Dec 5 '13 at 1:46
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I think the other answer is correct on migration, but there's a flaw in the way this question is asked, which should be addressed. It's also worth looking at the formation of Jupiter as well.

One of the rules of planet formation is that angular momentum remains largely constant. Granted, some angular momentum gets transferred into heat, and some gets lost for any material that escapes from the system and a tiny amount is lost in the escape of thermal radiation (that's a bigger factor with stars than planets). But these tiny variations aside, we can generally say that angular momentum of the contained material is conserved and not all of that falls into the planet. Some remains in orbit around the planet, as moons, a ring or a dust cloud.

Stars empty out their closer-in orbital regions fairly quickly. With planets, that happens much more gradually, so Jupiter likely retained an orbiting nebulous sphere of ice, dust and smaller debris for some time, even after it's moons began forming.

The standard model for Jupiter's moons is that it may have gone through a few generations of moon formation, forming within the orbiting cloud of debris and in time, falling into the planet, while new moons formed and over time, the orbiting disk and gas thinned out. Based on this model, Io is believed to be part of the latest generation of Jupiter's moons formation.

From Wikipedia link above:

Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial fraction (several tenths of a percent) of the mass of Jupiter captured from the solar nebula was passed through it. However, only 2% the proto-disk mass of Jupiter is required to explain the existing satellites.3 Thus there may have been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons then forming from the new debris captured from the solar nebula.3 By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits.4

Jupiter's fast rotation and tidal forces would suggest that it's moons should move away from it similar to our Moon moving away from Earth, but a cloud of orbiting debris tends to slow moon's orbits and cause them to fall into the planet. Jupiter's powerful magnetic field and rapidly moving charged particles may also have an effect, the combination is too hard for me to say whether Io is moving inwards our outwards, there's too many moving parts and even an estimate of how those forces combine is above my pay-grade.


But I digress, though I did want to point out that Io is not believed to have formed with Jupiter but formed later. The question asks how orbiting debris can overcome tidal forces between Jupiter and other larger moons like Ganymede and Callisto.

A cloud of orbiting debris in a disk around a planet can coalesce into a moon provided it's outside the fluid Roche limit. A solid moon begins to break apart usually closer to the rigid Roche limit, closer to the planet, due to some structural integrity.

For moon formation, all that's required is that there be sufficient debris density and that the debris is outside the fluid Roche limit. It doesn't matter that the density of the orbiting ring is low, what matters is that once the coalescing starts, that the proto-moon is outside the Roche limit, it's the Moon's density, not it's size that determines that Roche limit relative to the planet it orbits. A moon information might initialy have a lower density, due to being less compact, so it may have a corresponding Roche limit that's more distant from the planet, but the variation is the cube root of the density, so the roche limit boundary wouldn't be all that much further out at the start of formation.

The proto-moon doesn't need to add the ring-debris all at once, it only needs to be able to hold onto what it gets very close to, and that's a product of being outside the fluid Roche limit. In time, the Moon clears out the region where it orbits, and as pointed out in the other answer, migration likely plays a role in moon formation, but migration isn't the reason Moons form, that's a product of sufficient density of the orbiting disk and gravity.

(hope that makes sense, I'm not sure I explained the last part as well as I could have).

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