But by analyzing ice samples from the plumes, gathered by NASA's Cassini orbiter, the research team determined that 99 percent of the mass of the plumes is accounted for by salt-rich ices.
The simplest way to pick up the salts is for water to leach them out of rock over long periods of time. It's a process similar to the one that salts Earth's oceans.
The new results indicate that, despite its sound basis in physics, the "dry" solution to the geyser puzzle "is now off the table," Dr. Kempf says. "Nature did not choose that option."
The results conjure up an image of a sea in a vast subsurface cavern, Kempf acknowledges, although he adds that the team doesn't know whether the sea is global or local to the south pole.
According to the model proposed by the research team, water boils off the top of the subsurface sea, into the near-vacuum environment. As the water bubbles burst, the spray freezes into tiny crystals that get swept along with gases, up through fissures in the moon's ice crust.
The new results resonate with at least one scientist who help craft the "dry" explanation for the plumes, which appeared in a 2006 research paper in the journal Science. "I prefer this straightforward explanation," offers John Spencer, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "The warm temperatures implied by liquid water will provide plenty of pressure to drive the plumes," he writes in an email exchange.