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Jun21-11, 09:10 PM
P: 210
Quote Quote by qraal View Post
To tie this discussion to engineering let's look at the physical requirements of the task of providing Ganymede with 10 bars of hydrogen atmosphere. The radius is 2634.1 km and surface gravity is 1.428 m/s2, letting us compute the total mass of atmosphere as 6.1E+19 kg. Sourcing that much hydrogen from the gas giants will be challenging, but not ludicrous in the context of a solar system wide economy. Of course we're a long way from that point as yet.

The question for would be terraformers of the gas giant Moons is just what to do about the ice? The outer Galileans and Titan are half ice, implying very deep oceans if they fully melted. The obvious answer is to keep their average temperature sub-glacial, but then the Moons aren't much better than the Antarctic ice-caps as habitats and a lot further away.
You're nowhere near to the point where the ice can become a problem.

Your assumption that the entire mass of the atmosphere would be subject to the surface gravity of the moon is wildly wrong. The more you pile on, the further out of the gravity well it extends and the less the atmosphere you're piling on contributes to the surface pressure. Titan, with very similar size and surface gravity, has an atmosphere of nitrogen (about 14 times the density of hydrogen at a given temperature and pressure), with "only" 1.47 bar of surface pressure. That atmosphere extends out a thousand km from the surface, at its current frigid temperatures. It's not just a matter of piling more atmosphere on...what do you suppose happens when the molecules of your upper atmosphere are moving at greater than escape velocity at that altitude? We're not talking the tail of the velocity distribution, either, but the main body...there's just no way you're getting a deep, dense, warm atmosphere to stick to such a low-mass object. The quoted paper referred to a super-earth three times the mass of the largest terrestrial planet in our solar system, not a tiny moon!