Geysers on Dione too?

Researchers at March's conference also made a case for active outgassing on another icy Saturnian moon: Dione. Jeff Moore (NASA/Ames Research Center) described evidence in Cassini images for volcanic flows on Dione's surface. Then Jared Leisner (UCLA) announced that Cassini's magnetometer had sensed a tenuous cloud of ions surrounding Dione as the spacecraft flew past it. A similar detection was the first clue that Enceladus had an atmosphere.

Leisner and UCLA colleague Krishan Khurana estimate that Dione sheds just 650 grams of material per second into space--far less than the 300 kilograms per second spewed by Enceladus, but greater than the 40 grams per second that would be expected if Dione were completely inert. Tethys, another Saturnian moon that Moore says shows volcanic flow features, has no detectable outgassing at all.

Confirming that Dione is gassy has proven difficult. Amanda Hendrix (JPL) said that images of Dione backlit by the Sun--the same viewing geometry that revealed Enceladus's geysers--came up empty. A different try will come later this year, when Cassini's spectrometer will look for signatures of a trace atmosphere as Dione occults a star.