Ok so lets see, the point on Ganymede is interesting, though there is always Io, Io has an atmosphere, which is more substancial than Ganymede's (but by Earth standards is about the equivalent of an Exosphere). Of course there is the Sodium Taurus that Io enhabits around Jupiter, and the fact that vulcanism contributes to Io's atmosphere, whereas Ganymede has no such benefit. For Europa I would be none to surprised if beneath all that ice there is an active core.
So, if the Jovian planets condensed out of the Nebular cloud like a star, I can imagine in the early history of the solar system they would have contributed massive ammounts of heat to their moons, heat which would dissipate rather quickly over time, is there anyway to figure how much time it would have taken for the Jovian's to "cool off" to present conditions. Some people claim Uranus is now "inactive" with very little internal heat remaining. Neptune doesn't fit this bill obviously as despite its smaller size it is more massive. It would certainly be a changing environ for nearby moons as the outer planets slowly cooled.
And I realize this thread is achieving a life of its own, but hey its still interesting! Cheers.
solar system creation
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Evil Dr Ganymede
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Io's atmosphere is still 'trace', by any means. In practice, all the Galileans are vacuum worlds.
According to a paper by Adam Burrows et al (The Theory of Brown Dwarfs and Extrasolar Giant Planets, Burrows et al., Reviews of Modern Physics, volume 73, 719-765 (2001) downloadable from:
http://zenith.as.arizona.edu/~burrows/papers/rmp/RMP-final.pdf ) a 4.6 billion year old jovian object of 1 Jupiter Mass has a luminosity of about 0.000000001 Sols. A million years after it formed it had a luminosity of about 0.00001 Sols, dropping logarithmically since that time.
If a million years after formation, Io orbited at the distance where it is today (unlikely, given tidal evolution), its blackbody surface temperature would have been about 300K. At a few tens of million years, it'd be about 170K. At about a hundred million years old, it'd be only about 90K - it'd be heated more by the sun than by Jupiter, and the heating from Jupiter becomes even less relevant as time goes on. The other satellites would of course have been much colder than Io. So basically, for Jupiter the moons are basically only heated while they're forming.
Uranus isn't necessarily inactive because it's got little internal heat... it's just that the heat isn't getting out. It's probably more likely that there's something weird about its internal structure is preventing convection from occuring. Neither Uranus nor Neptune would ever have put out enough heat to affect their moons at all.
According to a paper by Adam Burrows et al (The Theory of Brown Dwarfs and Extrasolar Giant Planets, Burrows et al., Reviews of Modern Physics, volume 73, 719-765 (2001) downloadable from:
http://zenith.as.arizona.edu/~burrows/papers/rmp/RMP-final.pdf ) a 4.6 billion year old jovian object of 1 Jupiter Mass has a luminosity of about 0.000000001 Sols. A million years after it formed it had a luminosity of about 0.00001 Sols, dropping logarithmically since that time.
If a million years after formation, Io orbited at the distance where it is today (unlikely, given tidal evolution), its blackbody surface temperature would have been about 300K. At a few tens of million years, it'd be about 170K. At about a hundred million years old, it'd be only about 90K - it'd be heated more by the sun than by Jupiter, and the heating from Jupiter becomes even less relevant as time goes on. The other satellites would of course have been much colder than Io. So basically, for Jupiter the moons are basically only heated while they're forming.
Uranus isn't necessarily inactive because it's got little internal heat... it's just that the heat isn't getting out. It's probably more likely that there's something weird about its internal structure is preventing convection from occuring. Neither Uranus nor Neptune would ever have put out enough heat to affect their moons at all.