And what might be swimming in that warm, dark ocean? "Europa's ocean is probably in contact with the mantle below, so there could be hydrothermal vents," says Robert Pappalardo, a planetary geologist who works with Head. Around similar vents on the Earth's sea floor, an other-worldly web of life thrives in total darkness. It is fueled not by the sun's energy, but by chemicals melted out of newly erupted sea-floor rocks by hot seawater circulating through the cracks.

Europan life-forms would have to work hard to look more alien than Earth's own tulip-headed, sulfur-fueled tube worms. "And we can't even rule out photosynthesis in Europa's ocean very near hot hydrothermal vents," says Pappalardo.

The biggest moon in our entire solar system, Ganymede, like Europa and our Earth, generates a magnetic field. The field might be generated by a molten iron core or, more likely, by the motion of a thin, salty ocean. So Ganymede may have a buried ocean, too.

Since Ganymede is unaffected by tidal heating, says Pappalardo, its rocky core may be coated with ice. "So, this ocean might be more like an ice sandwich, with water in the middle," he says. Without a hot core adding useful heat and minerals to any ocean, life is less likely. Nonetheless, the cracks in Ganymede's ice crust suggest that even this moon occasionally swings close enough to Jupiter to get a gravity warm-up.

Oddly enough, Ganymede may be suffering a bad case of Io dandruff: Bright spots at its poles, says Pappalardo, may be fresh ice churned up by charged particles raining down on its poles. The source of the particles could be dust ejected from Io's volcanoes and widely distributed by Jupiter's vigorous magnetic fields.

And distant Callisto? "Callisto is a big mystery," says Head. Both Callisto and Ganymede are poorer in rock and richer in ice than are the inner two big moons. Callisto does have a magnetic field, which suggests a hidden ocean. But its icy surface is very old and very stable, compared to Ganymede and Europa.

Callisto almost twinkles, where spots of freshly revealed ice show through its dark skin. Those bright spots, blasted open by meteorite impacts, are an indication of Callisto's stolid character. Here on Earth, and on Io, Europa and even Ganymede, geological events like erosion, eruption, and melting slowly erase the craters left by incoming asteroids and comets. The huge number of craters on Callisto argue that this moon, like Earth's own, no longer has the energy to recycle its surface.

With Galileo slowly running out of steam, NASA is deciding whether it's worth the effort to wring more pictures from the spacecraft. Which moons should it return to while on its last legs?

"We're like kids in a candy store," Head says. "We want everything! But that's the beauty of exploration. Anything we learn is incredible."