Last year, when the Cassini spacecraft discovered an enormous plume erupting on Enceladus, one of Saturn's moons, scientists speculated that liquid water lay at shallow depths beneath the icy surface. Now, Susan Kieffer, a geology professor and planetary scientist at the University of Illinois at Urbana-Champaign, Alexandra Navrotsky, interdisciplinary professor at UC Davis, and colleagues have proposed an alternate model to account for this spectacular plume. The paper is published in the Dec. 15 issue of Science.

Instruments on the Cassini spacecraft revealed a gigantic plume of gas, water vapor and ice particles erupting from Enceladus' surface. Some of the ice escapes the moon's feeble grasp and replenishes a ring of ice particles around Saturn, called the "E ring."

Initial reports speculated that chambers of liquid water lay close to the moon's surface and erupted in a giant geyser. The water would be near freezing, so scientists dubbed the model "Cold Faithful," after the familiar, but hotter, Old Faithful geyser in Yellowstone National Park.

"A problem with this model," Kieffer said, "is that 10 percent of the plume consists of the gases carbon dioxide, nitrogen and methane. You might get a carbon dioxide-driven liquid geyser there, but you cannot put this much nitrogen and methane into liquid water at the low pressures found inside Enceladus."

Nitrogen and methane are nearly insoluble in liquid water, but highly soluble in frozen water -- in an ice phase called clathrate. When clathrate is exposed to a vacuum, the gas molecules burst out, ripping the ice lattice to shreds and carrying the fragments away.

The researchers propose that the gases are dissolved in a reservoir of clathrate under the water ice cap in the south polar region. The clathrate model allows an environment that would be 80 to 100 degrees Celsius colder than liquid water, with a "Frigid Faithful" plume emanating from clathrates, rather than from liquid water reservoirs.

Exposed to near-vacuum conditions by fractures at the south pole, the clathrates decompose violently, spewing out nitrogen, methane and carbon dioxide gases, and ice particles, as well as leaving fracture walls coated with water ice. Some ice particles and ice coatings evaporate to produce the water vapor observed with the other gases, Kieffer said.

The other authors of the paper are postdoctoral researcher Xinli Lu and geologists Craig Bethke and Steve Marshak at the University of Illinois, and John Spencer at the Southwest Research Institute. The work was funded by NASA.