The orbiting Cassini spacecraft has spotted what appear to be water geysers on one of Saturn's icy moons, raising the tantalizing possibility that the celestial object harbors life.
The surprising images from the moon Enceladus represent some of the most direct and dramatic evidence yet of liquid water beyond the Earth. Previous claims have been mostly circumstantial, based on scientists' analysis of rocks and other indirect data.
Excited by the discovery, some scientists said Enceladus should be added to the short list of places within the solar system most likely to have extraterrestrial life.
Cassini recently snapped high-resolution images showing geyser-like eruptions of ice particles and water vapor at Enceladus' south pole, scientists said.
The pictures do not actually show any liquid water, but scientists surmise that the ice and vapor are coming from underground reservoirs of liquid water near the surface.
Torrence Johnson, a Cassini scientist from the Jet Propulsion Laboratory in Pasdena, said this marks the first time that scientists have seen evidence of liquid water so close to the surface on another body beyond Earth.
If Enceladus does harbor life, it probably consists of microbes or other primitive organisms capable of living in extreme conditions, scientists say.
The findings were published in Friday's issue of the journal Science.
David Morrison, a senior scientist at NASA's Astrobiology Institute, cautioned against rushing to judgment about whether the tiny moon could support life. Scientists generally agree habitats need several ingredients for life to emerge, including water, a stable heat source and the right chemical recipe.
"It's certainly interesting, but I don't see how much more you can say beyond that," Morrison said.
Scientists believe Mars and Jupiter's icy moons might have — or once had — conditions hospitable to life. But the evidence is indirect.
In the case of Mars, scientists have never seen any flowing water. But based on their study of rocks, they believe water once existed there. They say magnetic readings of Jupiter's moon Europa strongly suggest that it has an ocean of liquid water, covered by ice.
Saturn is around 800 million miles from Earth. Enceladus measures 314 miles across and is the shiniest object in the solar system.
Enceladus was long thought to be cold and still, in part because it receives so little sunlight. But scientists now believe it is a geologically active moon that possesses an unusually warm south pole and a significant atmosphere.
The south pole "hot spot" is still frigid by Earth standards. The temperature there is minus-297 degrees Fahrenheit — about 20 degrees warmer than the neighboring region.
The water is believed to come from underground reservoirs that are under high pressure. Porco said the venting has probably been going on for at least several thousand years, perhaps indicating a lasting heat source underground.
Cassini found the geysers are mostly made up of water and ice particles with significant amounts of carbon dioxide and trace amounts of methane — all of which probably help to replenish the moon's atmosphere.
The Cassini-Huygens mission is a joint NASA-European Space Agency project. The spacecraft was launched in 1997 and went into orbit around Saturn in 2004, exploring its spectacular rings and many moons.
Cassini made three flybys of Enceladus last year and is expected to fly within 220 miles of the moon again in 2008.