Gassy Geysers: Cassini surveys Saturn’s moon

Data from NASA’s Cassini spacecraft raise new questions about the origin of Saturn’s tiny moon Enceladus and the heat driving its plume of ice and water vapor.

Cassini lived to tell the tale of its March 12 brush with the enormous plume of ice, water vapor, and gas spewing from several fissures near the south pole of the moon. Swooping as close as 50 kilometers, Cassini went in like a dog with its tongue flapping out the window, sniffing and tasting the gassy brew for clues to the plume’s origins and composition.

Astronomers had been astonished in 2005 to learn of the giant plumes of ice and water vapor blasting from the “tiger stripe” cracks in Enceladus’ southern hemisphere, and the detection of organic compounds in the gassy mix suggested the moon could support life.

The March 12 fly-through, described at a NASA media briefing this week, got a closer look at the plumes and the moon’s surface. Heat maps generated from the new data suggest that at least three of the tiger-stripe fissures are warm along their entire lengths and are ejecting water molecules at speeds faster than 600 meters per second.

The brightest fracture, dubbed Damascus Sulcus, registered temperatures as high as 180 kelvins. That’s more than 100 kelvins hotter than elsewhere in the polar region. Calculations published in Nature last year suggested that the little moon’s heat comes from friction generated when the fissures’ walls rub against each other as Saturn exerts its gravitational pull. But the new data don’t support that idea, says Larry Esposito of the University of Colorado at Boulder and head of Cassini’s ultraviolet imaging spectrograph team. The moon may be generating heat from somewhere closer to its core, perhaps by the flexing of the moon as it orbits Saturn, comments Princeton University’s Christopher Chyba.

Another intriguing find was the details of the plume’s chemistry, which is reminiscent of both natural gas and comets, says Hunter Waite of the Southwest Research Institute in San Antonio and leader of Cassini’s ion and neutral mass spectrometer group.

That instrument found that the plume is mostly water vapor, with methane, carbon monoxide, carbon dioxide, and a mix of simple and complex carbon molecules. The cometlike chemistry suggests that the moon actually may have been a pristine object, captured in Saturn’s orbit sometime in the past, says Waite.

But Chyba is skeptical. Saturn’s many-mooned system has “well-behaved, orderly objects,” he says. There’s no evidence for the arrival of an outside object that would have to shed a lot of energy—perhaps by smashing into another object—to remain in the system.

Whether Cassini tasted life is still an open question, the researchers say. But the mixture of water and organics—precursors to the building blocks of life—is “suggestive,” Waite says. Chyba agrees. “The baby steps toward the origins of life are there,” he says.