The most recent Scientific American contains a paper on the possible ocean under Saturn's moon Enceladus.  This is really cool news and well worth investigating further.  Europa, however, is still where the action is.  I've resurrected a year-and-a-half old blog entry from my old site that tells of some particularly spectacular possibilities:

A recent paper suggests if that Jupiter's moon Europa does have a subsurface ocean, then that ocean is probably highly oxygenated--i.e., breathable by terrestrial fish.

To quote the article's tantalizing abstract:

...Europa's ocean could reach O2 concentrations comparable to those found in terrestrial surface waters, even if 109 moles yr1 of hydrothermally delivered reductants consume most of the oxidant flux. Such an ocean would be energetically hospitable for terrestrial marine macrofauna. The availability of reductants could be the limiting factor for biologically useful chemical energy on Europa.

To translate: macrofauna=fish, and reductants=food. Current theory suggests that Europa's internal heat comes from the flexing of the planet due to its gravitational interactions with Jupiter and the other moons. This flexing may create enough heat in the moon's core to drive volcanic processes, creating the equivalent to the "black smokers" that pepper the mid-Atlantic ridge on Earth. Scientists have speculated that these vents might support some sort of life, but while they constitute a potential nutrient source, an energy source has been lacking. This paper suggests that energy, in the form of oxygen, might be common.

The mechanism for creating that oxygen appears to be radiation from Jupiter's radiation belts. When substances such as sulfur dioxide and carbon dioxide land on Europa, they sit there and get fried for very long periods. The broken molecules produce Europa's atmosphere, which is pure oxygen. Eventually, the surface ice subducts like a terrestrial continent, pulling the now-split oxygen and carbon et. down and into the ocean. According to the article, at currently projected infall rates, even if a huge amount of that oxygen is immediately bound up with non-biological molecules coming up from below, the total amount available should be comparable to the surface of Earth's oceans.

If this is right, and there really is an ocean, and there really are venting processes at work in the deep, then Europa is habitable now--but not necessarily for us. Consider these mitigating factors:

• It's a salty ocean--but it's Epsom salts (magnesium sulfate) rather than our kind of salt. It may also be really salty (think thick soup), in which case only the most extreme halophile bacteria could survive there.
• There could well be other substances, common to the outer planets, saturating the ocean--such as ammonia. Imagine a sea of Windex.
• Pressure. I've seen no calculations of how much pressure should be crushing down on this sea. But even on a small moon, an ocean under 30 kilometers of ice should have mighty powerful forces compressing it. The ice could be as thin as 800 meters, but we just don't know.

Still--none of these factors makes large, native Europan life forms impossible. And even if the ocean is sterile, in the best case, we might be able to engineer terrestrial fish to withstand a lifelong Epsom salt bath, and populate it ourselves. If the pressure allows, we could dig domes into the ice ceiling and pump them full of nitrogen, and let the oxygen percolate up from below. I used all these ideas in my 2002 novel Permanence, to show what habitable worlds around brown dwarf stars might look like.

All of this makes a Europan mission increasingly important. After all, it may be small, but it's a whole world, and potentially a shirt-sleeve environment for humans. Stocked with fish and other organisms to provide a full food chain, Europa may, shockingly, prove to be a world we can terraform and live on indefinitely (unlike Earth, Europa might even survive the death of the sun). It's definitely worth finding out whether all of that is possible.