"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.