The Cassini spacecraft took seven years to reach Saturn. But for Carolyn Porco, who leads the Cassini imaging team at the Space Science Institute (SSI) in Boulder, Colorado, the images it sent back were well worth the wait. Most exciting of all was the revelation that one of the planet's moons may have the essential ingredients to support life.

A veteran of the 1980s Voyager space mission, Porco was well aware that the outer Solar System is not the barren wasteland it was once thought to be. Images from Voyager, for example, had shown some of Jupiter's and Neptune's moons to be geologically active. But this knowledge didn't dampen the thrill of Cassini's discovery that Enceladus, one of Saturn's 60 moons, spews jets of vapour containing organic material and tiny, icy particles from its south pole. This spectacular finding demonstrates present-day geological activity on a small, cold moon.

Enceladus is only about 500 kilometres in diameter, but the jets can shoot thousands of kilometres into space. The particles they contain eventually make their way into Saturn's E ring — the diffuse outermost ring, which is composed of microscopic icy particles. Long before Cassini arrived at Saturn, scientists had suspected Enceladus to be the E ring's source of material, but they never expected this dramatic phenomenon. And the drama matches the implications: “It's not out of the question that these jets are the result of liquid water,” says Porco.

Infrared measurements by Cassini showed south pole 'hot spots' almost 100 kelvin warmer than the surface temperature expected from thermal equilibrium with sunlight. The hot spots are associated with four distinctive linear cracks — dubbed 'tiger-stripe' fractures — in the moon's surface. “We were in a tizzy,” says Porco, “because, if we could confirm the presence of water in addition to organic materials and warmer temperatures, we may have stumbled upon a habitable zone in our Solar System. This is an explorer's dream come true.”

The next step was to determine the locales of the jets. Porco asked SSI planetary scientist Joseph Spitale to triangulate the surface locations of each jet. To ensure that the measurements were made without prejudice, she didn't tell him her hypothesis of an association between the hot spots and the jets.

Spitale's measurements “hit the jackpot”, Porco says — all of the prominent jets emerge from one of the four tiger-stripe fractures, and most coincide with one of the hot spots, confirming a causal relationship between the south pole's anomalous heat and jet activity (see page 695).

Geysering activity powered by pressurized liquid water trapped beneath the south pole is a possible mechanism for the jets; another is water vapour evaporating from warm ice. Unfortunately, no single observation from Cassini can answer this question. Although it will take several lines of evidence, Porco says that the prospect of finding an extraterrestrial habitat suitable for life is the greatest thrill any scientist could hope to experience.

Cassini's mission extends until 2010. A handful of additional Enceladus flybys are planned, and Porco and Spitale predict that other hot spots will be found in the fracture area. Planning future observations while analysing incoming data is all-consuming, says Porco, adding that “Cassini hasn't been a mission so much as a way of life”. That lifestyle looks set to continue: Porco already thinks Enceladus merits another mission. “In my mind, it's the go-to place for investigating issues of astrobiological interest,” she says.