Last year, Cassini spacecraft found one of the most beautiful phenomena is our solar system: a gigantic plume of gas, water vapor and ice particles erupting from the surface of Enceladus, one of Saturn's moons. "With a diameter of only 300 miles (500 km), Enceladus is a tiny moon; it would fit easily between Los Angeles and San Francisco," said Susan Kieffer, a geology professor and planetary scientist at the University of Illinois at Urbana-Champaign. "This tiny satellite should be cold and inactive, like our own moon. But it isn't."

Enceladus' surface is covered by ice with traces of carbon dioxide. "Part of this surface does appear old and cratered like Earth's moon," Kieffer said. "The south polar region, however, is geologically active, with many surface features, indicating current activity."

Some of the ice escapes the moon's feeble grasp and joins a ring of ice particles around Saturn, called the "E ring". Initial reports speculated that chambers of liquid water lay at shallow depths beneath the icy surface of the moon and erupted in a giant geyser. The water would be almost freezing, so scientists named the model "Cold Faithful," after the hotter Old Faithful geyser in Yellowstone National Park.

Now the researchers have proposed an alternate model to explain the plume. "A problem with this model is that 10 % of the plume consists of the gases carbon dioxide, nitrogen and methane. You might get a carbon dioxide-driven liquid geyser there, but you can't put this much nitrogen and methane into liquid water at the low pressures found inside Enceladus" said Kieffer.

Nitrogen and methane are practically insoluble in liquid water, but highly soluble in an ice phase called clathrate, that forms at -80 to -100 degrees Celsius. When clathrate is exposed to a vacuum, the gases burst out, ripping the ice lattice to shreds carried away.

Scientists now think the plume's gases are dissolved in a reservoir of clathrate under the ice cap in the south pole of Enceladus. "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," said Kieffer.

"Some ice particles and ice coatings evaporate to produce the water vapor observed with the other gases," she said. Active tectonic processes at the south pole of Enceladus provoke continuous formation of cracks in the ice, through which many separate vents create a plume.

Even if the total "ammunition" is not higher to that of Old Faithful, the plume is huge because it is erupting at very low gravity into the near vacuum of space. "We propose that cracks in Enceladus' ice cap may be opening and closing continuously, producing the spectacular plume we see reaching high above Enceladus' surface," Kieffer said. "Even if conditions are as cold as our model suggests, there is no problem launching ice particles into Saturn's E-ring."

Image credit: NASA