The growing understanding that water exists in great abundance beyond the Earth was further strengthened by the push to explore the giant gaseous planets— Jupiter, Saturn, Uranus, and Neptune. With the Pioneer probes in the early 1970s and the Voyagers later that decade and throughout the 1980s, the Galileo probe from 1995 through 2003, and the Cassini-Huygens arrival at Saturn in 2004, planetary scientists have gotten close-up looks not only at the planets but also at their moons. Features like Jupiter's Great Red Spot, the rings of Saturn, and the bands of clouds on both planets were laid bare in enough detail to begin teasing apart their complex structures and dynam�ics. These missions sent back all sorts of surprises as well—the discovery that all four gas planets have rings; that three of them have enormous lightning storms; that Jupiter's moon Io sports sulfur-spewing volcanoes; that the moons of the giant planets come in a bewildering variety of shapes, sizes, and surface features.

But the most intriguing discovery was that the rules about where scientists might look for life beyond Earth had been far too restrictive. Even from terrestrial telescopes, Jupiter's moon Europa has always looked impres�sively bright, suggesting a surface covered in ice. When the Voyagers snapped the first detailed close-up pictures, it became clear that this was the case, but also that Europa was covered with a network of linear features. Galileo's much closer and more prolonged look provided enough high-resolution evidence to prove an audacious hypothesis: The linear features are caused by cracks in a sheet of ice, perhaps many kilometers thick, floating on a subsurface, moon-spanning ocean of liquid water. In a part of the solar system so frigid that every bit of water should be frozen solid, Jupiter's tidal forces squeeze Europa like a rubber ball, creating heat through friction. Basic organic chemicals may swirl through that ocean, given that comets are rich in organics (the European Space Agency's Giotto mission proved this during its 1986 encounter with comet Halley) and that comets crash into moons and planets. And if life is reasonably easy to get started given the right raw materials—something biologists don't know yet—then Europa may be a hotbed of life. A possible future mission, called the Europa Astrobiology Lander, still in the early planning stages, could eventually find out.

Astronomers had also long been intrigued by Titan, the largest of Saturn's moons (and the second largest moon in the solar system—bigger, in fact, than the planet Mercury). Where Europa is white, Titan is orange, suggesting to experts, including the late Carl Sagan, that the moon's atmosphere is rich in methane and other organic compounds. Although Titan is far colder, the chemical mix may resemble Earth's atmosphere when life first arose here. The prospect of seeing Titan up close was one of the things that led to the Cassini-Huygens mission to orbit Saturn (Cassini) and send a lander down to Titan's surface (Huygens).