The solar system, to be brutally honest, has turned out to be something of a bust. There was a time when the planets seemed to have a lot of potential, but only if scientists didn't look too closely. Once they did, things got ugly fast. The planets were either flash-frozen or deep-fried, uninhabitable gas giants or uninhabitable rocky pellets, smothered by a toxic atmosphere or almost totally airless--altogether poor company for a glamour world like Earth.
But planets aren't all there is to the sun's family. The solar system is also packed with moons--more than five dozen of them. Increasingly, astronomers are appreciating that these cosmic offspring may be far more remarkable than the parent worlds they orbit. Unlike most of the planets, the moons have oceans, the moons have continents, the moons even have active volcanoes.
And now, it seems, a moon may be the best place yet to look for the most remarkable thing of all: extraterrestrial life. Last week the sturdy Galileo space probe finished beaming back the sharpest images ever taken of Jupiter's ice-covered satellite Europa. The pictures revealed more clearly than ever before that the moon's frosty rind is nothing more than a planet-wide ice cap floating atop a globe-girdling ocean of ordinary water.
What's more, the spacecraft spotted brown stains on the ice that could conceivably be a mix of hydrogen cyanide and other life-related chemicals. "If this is indeed hydrogen cyanide," says Richard Terrile, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, "we have organic chemicals mixed into a bath of water. That's a recipe for life."
Whether there's anything so dramatic waiting on any of the other moons in the solar system is unclear, but NASA wants to find out. The agency is contemplating five or more missions to the planetary satellites in about the next 10 years. Says Terrile: "We're beginning to appreciate that within our solar system, there are all these minisystems worth exploring."
It was in 1979, during the Voyager 1 spacecraft's first encounter with the Jovian moon Io, that astronomers began to suspect there might be more to the moons than met the telescopic eye. While 43 of the 61 satellites measure less than 300 miles or so in diameter, most of the others are more than 1,200 miles across. Bodies with this kind of bulk are capable of supporting an atmosphere--a big plus when you're trying to incubate life. What just about all the moons lacked was the heat needed to get biological chemistry going.
Or so it seemed. In 1979, however, when the first pictures of Io were beamed to Earth, NASA got a shock. Rising from Io's face were the unmistakable plumes of up to 10 erupting volcanoes. "Suddenly," says Torrence Johnson, project scientist for the Galileo mission, "it was clear that other bodies in the solar system could be geologically active."