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Jupiter's Io and Neptune's Triton could also prove surprising. Though Io appears largely dehydrated, planetologists don't rule out the possibility of subsurface water, particularly since they think that ordinary steam might provide some of the propulsive muscle behind the moon's volcanoes. Triton presents a greater organic hurdle. At -391[degrees]F, the moon is the coldest known object in the solar system. Nevertheless, it appears heavy with subsurface ice, which seems to have got warm enough, in the past at least, to flow over the landscape in a lava-like slurry. More tantalizing, dark streaks near the poles suggest that occasional geysering on the frozen moon may have spouted carbon or some other organic material. "We don't fully understand what's going on inside Triton," Terrile says, "but something is pumping a lot of energy."
Whether any of the moons will ever be understood fully, of course, is open to question. Before long, however, they will certainly be understood better. Galileo could be functioning until late 1999, with more than 20 passes through the Jovian system still to come. Next fall NASA plans to launch the new Cassini-Huygens spacecraft on a seven-year odyssey to Saturn. In addition to making at least 36 orbital slalom runs through five of Saturn's inner moons, the ship will fire off a probe that will puncture Titan's cloud cover, parachute to its surface and send environmental readings back to Earth.
Even before Cassini's work begins and Galileo's ends, other ships could be on the way to join them in the outer solar system. NASA is tentatively planning several new Europa probes, including one that will photograph its surface and take radar soundings beneath its crust. If the radar picks up the telltale echoes of liquid water, another spacecraft would be sent to land on Europa and release a heated probe designed to melt through the ice layer and look for signs of life in the seas below.
None of these proposed missions will come cheap. Even with NASA's new commitment to building smaller, less expensive spacecraft, interplanetary ships still cost at least $200 million each. Planetologists, however, insist that the potential discoveries could be well worth the money.
"These moons make up one of the most eccentric cosmic families imaginable," says Terrile. "As with any other family, some individuals are underachievers, some are overachievers, and a few may be up to something truly fantastic." It's these last that NASA wants to get to know better.