Moons of Mystery

"Oh, it's so hard!" he protests. "Each of these bodies has a personality. It's like a miniature solar system. And the differences and the details are so incredible!"

He rattles off a list of intriguing possibilities: molten lava within cracks on Io, the question of water on Europa, how boulders surface on Callisto. "I know I've said this 10 times, but it's just unbelievable!"

Head, a geology professor at Brown University in Providence, Rhode Island, is on the team that tells the Galileo spacecraft what to photograph as it rambles among Jupiter's brood of 16 satellites. Since Galileo began beaming home images in late 1995 of the four largest moons of the largest planet in our solar system, he's been up to his eyeballs in extraordinary revelations.

Each of the four almost-planets is wondrously different, so it is understandably difficult to choose the most fascinating. The candidates:

Io, which orbits closest to massive Jupiter, is a brass-colored ball riddled with belching volcanoes. When Io's great rivers of super-hot lava encounter a patch of sulfur-dioxide ice, the ice vaporizes in luminous, blue plumes that can rise 60 miles into Io's sky. The 3,600-degree F lava itself can shoot a mile into the air. It's faintly disturbing to consider that our own blue and green Earth may have looked quite similar in its molten youth.

And how does Io stay so hot in this frosty neighborhood, five times farther from the warming sun than the Earth is?

Well, if you knead a ball of clay between your hands, the friction will soon warm and soften it. Io is likewise kneaded by the gravitational tug of Jupiter on one side, and the two nearby outer moons on the other. Galileo's latest information suggests this "tidal heating" keeps Io so hot that its thin, sulfur-yellowed crust may float on a deep, global ocean of melted rock.

Beautiful as Io is, this moon is too waterless to be very interesting to Earthlings in search of extraterrestrial companionship.

Europa, on the other hand, is another story. Though the moon's surface is 90 percent ice, and it is stained with reddish stuff as uninviting as battery acid, there is probably a watery ocean deep below, warmed by the same tidal kneading that melts Io's rocks.

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."