Scientists have found evidence that cold, Yellowstone-like geysers of water are issuing from a moon of Saturn called Enceladus, apparently fueled by liquid reservoirs that may lie just tens of yards beneath the moon's icy surface.
The surprising discovery, detailed in Friday's issue of the journal Science, could shoot Enceladus to the top of the list in the search for life elsewhere in our solar system. Scientists described it as the most important discovery in planetary science in a quarter-century.
"I think this is important enough that we will see a redirection in the planetary exploration program," Carolyn Porco, head of the imaging team for the Cassini mission to Saturn, told MSNBC.com. "We've just brought Enceladus up to the forefront as a major target of astrobiological interest."
The readings from Enceladus' geyser plumes indicate that all the prerequisites for life as we know it could exist beneath Enceladus' surface, Porco said.
"Living organisms require liquid water and organic materials, and we know we have both on Enceladus now," she said. "The plumes through which Cassini flew last July contain methane, contain CO2, propane — they contain several organic materials."
The third necessary ingredient — energy for fueling life's processes — could exist around hydrothermal vents around the bottom of Enceladus' water reservoirs, just as it does around Earth's deep-ocean hydrothermal vents.
The results impressed University of Colorado planetary scientist Robert Pappalardo, who has studied Enceladus and other icy moons but was not involved in the newly published research.
"I think the discovery of activity on Enceladus is about the most exciting discovery in planetary science since the volcanoes of Io," he said, referring to the detection of volcanic activity on Jupiter's moon by the Voyager probe in 1979.
The findings unveiled Thursday are based on imagery as well as temperature readings from Cassini, a U.S-European spacecraft that has been studying the ringed planet and its moons since 2004.
The precise sources of the geysers could not be spotted directly, because Cassini's camera isn't quite good enough to spot the bright spray of water and ice crystals against the bright ice on Enceladus' surface, said imaging team member Andrew Ingersoll, an atmospheric scientist at the California Institute of Technology.
However, Cassini's camera repeatedly recorded the spray of ice crystals and water vapor from Enceladus' south polar region, backlit by sunlight. That imagery allowed researchers to trace the source back to the mysterious dark "tiger stripes" previously seen on Enceladus.
Researchers investigated several possibilities for the origin of the geysers, including the idea that the contents were driven by warm ice turning directly into vapor, or consisted of a cometlike slurry of ice and dirt.
The Cassini team found that the spray from the geysers was composed of equal proportions of ice and water vapor. That ruled out the "warm ice" model as well as the "comet" model, Porco said. The best remaining model was that the geyser was driven by liquid water beneath the surface.
"We arrived at our last model, and in some ways somewhat reluctantly, because this is a pretty extraordinary result," she said. "Like [the late astronomer] Carl Sagan was fond of saying, 'Extraordinary claims require extraordinary evidence.'"
The imaging team's conclusion was supported by the temperature readings from Cassini's infrared spectrometer: Although the surface temperatures were far below freezing, the readings showed relatively warm spots in the south polar region, centering on the tiger stripes. Scientists traced the internal heating patterns that could create such warm spots, and concluded that temperatures could be above freezing mere yards beneath the surface.
"It can be warm enough 10 meters or so beneath the surface," Porco explained, "and there's enough pressure to keep liquid water stable at that depth."
Still more supporting evidence came from an analysis of the ice surrounding the "tiger stripe" cracks. That ice was amorphous and virtually crater-free, indicating that it welled up relatively recently.
Cassini's images showed that the geysers rose hundreds of miles above the surface. Based on the imagery, researchers concluded that most of the ice crystals fell back to the surface as snow. Some of the ice escaped Enceladus' gravity field to become part of a wide, thin ring of Saturn known as the E ring.