It is a safe to say that the surfaces of all Galilean moons are lifeless. Extreme coldness combined with hostile radiation and vacuum is not the best place for microbes. The trace amounts of oxygen around these moons (I'm not even sure if the "atmospheres" are thick enough to be called such) are generated by micrometeor impacts, which release water molecules from surface ice. Then ultraviolet radiation breaks these molecules apart. Lighter hydrogen atoms escape easily, but heavier oxygen atoms stay behind. Or so I have understood it.

What comes to subsurface conditions, is completely different matter. It is very possible that Europe has a vast subsurface ocean (but remember, it has not been proved!) Callisto has probably colder, salty ocean, and quite possibly Ganymede too. Europa has plenty of internal energy generated by tidal forces, which could provide the necessary energy for lifeforms to evolve. However, it is very unlikely that multicellular life has formed there.

----

Saturn probably has a solid core, which is larger than many times more massive than Earth. Most of the planet is not gas, but liquid due to high pressure. Some of the hydrogen is in the form strange liquid called "metallic hydrogen", which acts like a metal.

----

Yeah, Triton, which is Neptune's largest satellite, has a visible atmosphere and even geological activity in the form of geysirs.

No atmospheres are found around Uranus' moons which is not surprising given the smaller size of the moons. Innermost and smallest of the large satellites, Miranda, has evidence of past geological activity. Its "coronae" formations resemble the active "tiger strips" on Enceladus. Miranda is only slightly smaller than Enceladus, so it is possible that the moon had similar transient atmosphere as Enceladus has before its activity ceased.

__________________
Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman