“Steaming gun” identified

A water jet on the small moon Enceladus feeds material into Saturn's E ring.Bas den Hond
By | Published: December 8, 2005
False-color plumes
This false-color view shows the extent of Enceladus’ fountain-like plumes, which emanate from the moon’s warm south polar region.
December 8, 2005
The plume of water vapor and ice spewed out by Saturn’s moon Enceladus turns out to be the source of the planet’s tenuous E ring. Spectra from the spray match the material in the E ring. This prompted researchers from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, to call the plume the “steaming gun” that confirms the E-ring-formation hypothesis. They made the announcement this week at the fall meeting of the American Geophysical Union in San Francisco.

The team that manages the Cassini spacecraft’s magnetometer, which detected changes in Saturn’s magnetic field near Enceladus’ south pole, initially discovered the plume.

The moon has no strong magnetic field of its own, so the only explanation for the changes could be that Saturn’s field lines cross a cloud of charged particles. That would mean Enceladus has an atmosphere, which is not very likely because of its size (500 miles [805 kilometers] in diameter) and low gravity.

The plume was imaged for the first time in July, during a particularly close flyby that was requested because of the intriguing magnetometer observations. Candice Hansen, a member of Cassini’s Ultraviolet Imaging Spectrograph team, says, “In situ instruments on board Cassini, the Ion Neutral Matter Spectrometer, and the Cassini Dust Analyzer actually detected the particles.”

Images from the Visible and Infrared Mapping Spectrometer camera aboard Cassini show a spray of ice particles extending hundreds of miles above the moon. Says Hansen, “If it had been known beforehand that the plume would extend to that height, I’m not sure permission for a flyby this close would have been given!”

According to Hansen, water is ejected from Enceladus at an estimated 800 pounds per second (360 kilograms per second). That isn’t a lot by planetary standards, but scientists have estimated that just 2 lbs/sec (1 kg/sec) would be enough to sustain the E ring.

The water is ejected from Enceladus as vapor, which then condenses into ice particles. Researchers think the vapor’s source is below the surface. The energy source heating the water is still a mystery. It doesn’t seem like Enceladus has enough radioactive material in its rocks to provide the required heat; nor does it display any motions in its orbit that would stress and flex the rocks, heating them.

According to Bonnie Buratti of JPL, the ice that Enceladus spreads around accounts for not only all of the E ring, but also for a dusting of ice on Saturn’s moon Dione, and for the amount of dissociated water that drifts around the E ring’s orbit in the form of hydrogen and OH molecules.

Bas den Hond writes about science from Linschoten in the Netherlands.