Geysers not fed by misty water vapour after all.
As much as 50% of the plume shooting out of geysers on Saturn's moon Enceladus could be ice, a researcher revealed yesterday at a meeting of the American Geophysical Union in San Francisco, California.
Previously, scientists had thought that only 10–20% of the plume was made up of ice, with the rest being water vapour.
Some researchers think that the study, led by Andrew Ingersoll, a planetary scientist from California Institute of Technology in Pasadena, backs the idea that the plumes are caused by a sub-surface lake boiling off into space rather than the product of colder processes such as sublimation.
Ingersoll based his estimate on a series of photos of Enceladus taken in 2006 by the Cassini spacecraft. That was a "very special time", he says, when two important events occurred simultaneously. Enceladus was perfectly backlit by the Sun, allowing ice particles in its geyser plumes to be easily observed. And at the same time, Cassini was in Saturn's shadow. "That allowed us to look back toward the Sun without blinding the instruments," Ingersoll says.
The photos showed Enceladus at different points in its orbit in three wavelength bands — ultraviolet, visible and near-infrared. In combination, the images allowed Ingersoll's team to determine the size of the plume's ice particles as well as their concentration.
Feeding the plume
The team then calculated how fast new ice particles had to blast out of the moon's geysers for the plume to contain the amount of ice seen in the images. They found that Enceladus must be emitting at least 200 kilograms of ice per second — almost identical to the amount of water vapour other measurements had determined it to be emitting.
That 1:1 ratio between ice and water vapour is a major constraint on how the geysers must be operating, Ingersoll says. In a paper accepted by the journal Icarus, he examined a scenario in which the geysers are fed by a misty vapour that sublimates from ice in underground chambers. But that doesn't fit the new data, he says. "You [would] get 1% ice, and 99% water vapour."
One way to get more ice, Ingersoll says, is if the geyser chambers contain water. When a crack opens up, the water is exposed to the vacuum of space and starts to boil, he says. But much of the steam immediately freezes, "and you get a large fraction of solids" in the plume.
Liquid water is an exciting idea for those hoping we might one day find life on Enceladus. But not everyone believes that water is needed to produce the plumes.
Susan Kieffer, a planetary scientist from the University of Illinois at Urbana-Champaign, for instance, was lead author of the paper that found the plume contained only 10–20% ice.1 However, she has no problem with Ingersoll's finding. "Andy had access to a whole new bunch of data," she says.
But she's not about to concede that Ingersoll's finding requires water to be present. Rather, she has her own model, in which the geysers are fed by the explosive decompression of materials called clathrates, when cracks in the crust expose them to the vacuum.
Clathrates are molecular-sized cages of a compound that can contain many other molecules. The clathrates that feed Enceladus's plumes could therefore encage the numerous other gases that make up about 10% of the plume. "Clathrates are garbage bins for storing gases," Kieffer says.
When the clathrates break down, Kieffer argues, they would release not just water vapour, but also ice particles into the plume — likely enough to account for Ingersoll's data. "We can make a lot of ice in our model," she says.
Thus, of the three main theories for the formation of Enceladus's geysers — sublimation in cold, misty chambers; liquid water (that might sustain life) boiling into vacuum; and exploding clathrates — only the first seems to be ruled out by Ingersoll's find.
The remaining debate is as alive as ever. "I think it's safe to say that there's years of debate left in it," says Carolyn Porco, head of the Cassini imaging team.
Kieffer concurs. "This argument isn't going to go away as the result of one AGU meeting," she says. "It may hang around until there is another spacecraft, or until there is a definitive observation."
Kieffer, S. W. et al., Icarus, (2009) doi:10.1016/j.icarus.2009.05.011