What's the most complex life which could conceivably exist in such a cold environment? Presumably there's very little sunlight penetrating through the ice to the liquid ocean.
The point is that generic small outer solar system bodies have liquid water, whereas the generic inner solar system body doesn’t. If you think liquid water is the most important requirement for life then most of the habitat in the universe is outside the frost line.
It’s totally plausible that simple life could form around geothermal vents on such a moon but harder to believe you could support a complex ecosystem enough to evolve intelligent life without sunlight.
At the high pressures in such an underground ocean I wouldn’t expect our proteins to work, you can kill bacteria with very high pressure. I suspect that proteins that evolved in that environment could work just fine. I can picture a creature from that kind of world drilling up to the surface somehow, but the low pressure would be deadly and in a system like Jupiter where Io ‘breeds’ radiation (vapor from volcanos gets ionized, gets accelerated, ionizes more vapor) the surface could be a dangerous place.
Such a creature though might have a leg up on starfaring as getting into orbit might be easier (never mind building a beanstalk) and rather than being tempted by boring destinations like Mars (smaller planet, smaller civilization) they might go straight to cutting up wet/carbonaceous asteroids to build large (100-1000x more habitable area than Earth) habitats and if they developed D-D fusion they might be quicker to see the advantages of a comet-hopping lifestyle.
But just to be sure, I got in touch with a friend of mine who works at a research reactor, and asked him what he thought would happen to you if you tried to swim in their radiation containment pool.
“In our reactor?” He thought about it for a moment. “You’d die pretty quickly, before reaching the water, from gunshot wounds.”After all, remember some things in a reactor don't react well to bulletsch.
https://solarsystem.nasa.gov/planets/jupiter/in-depth/
> Near the planet, the magnetic field traps swarms of charged particles and accelerates them to very high energies, creating intense radiation that bombards the innermost moons and can damage spacecraft.
So basically like our van Allen belts but supercharged.
Also, though another reply here mentioned Jupiter not emitting its own radiation, this isn't quite correct. It does, almost like a small pulsar.
The hydrothermal vents, where the main sources of energy are hypothesized to be, would be deep enough.