Hi Lee

I tend to agree, though I am unsure about the cosmic-ray thing.

What'd be really interesting is floating colonies in the atmospheres of the big boys, though aerostats in a hydrogen/helium atmosphere would be a real challenge.

The three lesser giants have orbital velocities that are reachable via atmospheric vehicles - nuclear ramjets - and they have near-in moons suitable for mining for heavier elements. You could melt all the way into the core of Mimas, Miranda or Proteus, to extract heavier elements. I suspect though that the methane in the atmosphere would be the main structural feedstock - carbon for nanotubes and graphene.

There's some doubt about the potentially liquid layers of Uranus and Neptune. They might be too hot for high pressure oceans, or they might not. Would be interesting to know for sure.

Finally there are two really curious big moons - Titan and Triton - which are potentially in possession of deep oceans under the ice. Triton might have a relatively "thin" ice layer, kept warmer than Titan's by its orbital decay from an elliptical orbit. It's a moon we really need to explore in depth.

Adam

Final thought is what could be done to improve conditions on a moon like Triton. Stephen Baxter has suggested altering Nereid's orbit into a collision. A head on collision would be at about 10.4 km/s, thanks to Triton being retrograde and Nereid being already highly elliptical (e = 0.75), so the smash would pack quite a wallop. Some models I have read suggest a lot of carbon dioxide, which would be released in quantity after a massive impact. A bit too much CO2 actually, but fortunately physics would cause it to be sequestered away. The pressure from nitrogen ice sublimation would probably make quite a heavy atmosphere, initially. Once it cooled below 31 degrees C, the carbon dioxide (probably several hundred bars worth) would start liquefying and sinking beneath any water ocean already present, being quite a bit denser. What didn't sink would dissolve and probably precipitate as huge amounts of carbonate. Eventually a small residual would remain mixed with the nitrogen and methane. If enough ammonia was present the water/ammonia mix would remain unfrozen down to quite low temperatures. Heat loss would take centuries, probably. Perhaps a huge soletta could be built to warm it to Saturn-level insolation, but keeping the oceans warm via a higher surface pressure (20 bar would keep it at 200 K.) Wouldn't be especially earth-like but it might suit genengineered lifeforms that like ammonia/water eutetics.