Saturn's Icy Moon Blows Bubbles
Artist's impression of the Cassini spacecraft making a close pass by Saturn's inner moon Enceladus to study plumes from geysers that erupt from giant fissures in the moon's southern polar region.
Credit: Copyright 2008 Karl Kofoed / NASA

As Saturn's icy moon Enceladus circles around the ringed giant, it plows through the charged plasma surrounding the planet, leaving a complex pattern of ripples and bubbles in its wake.

Enceladus sits deep within Saturn's protective magnetic casing, called a magnetosphere, which is filled with electrically charged particles (plasma) that originate from both the planet its many moons.

NASA's Cassini spacecraft has conducted nine flybys of the mysterious sixth-largest moon since 2005.

The closest of these flybys brought the probe to within just 15.5 miles (25 kilometers) of Enceladus's surface, which many scientists believe conceals a saline ocean. Heated vents at the south pole of the moon release a plume of material, consisting mainly of icy grains and water vapor, into space.

Cassini measurements show that both the moon and its plume are continuously soaking up the plasma, which rushes past at around 67,000 mph (108,000 kilometers per hour), leaving a cavity downstream. In addition, the most energetic particles which zoom up and down Saturn's magnetic field lines are swept up, leaving a much larger void in the high energy plasma. Material from Enceladus, both dust and gas, is also being charged and forming new plasma.

Cassini scientists have also discovered mysterious spiky features in the plasma wake of Enceladus that present a complex picture of readjustment downstream from the icy moon.

"Eventually the plasma closes the gap downstream from Enceladus but our observations show that this isn't happening in a smooth, orderly fashion," said Sheila Kanani of University College London, who helped discover the odd features. "We are seeing spiky features in the plasma that last between a few tens of seconds and a minute or two. We think that these might represent bubbles of low energy particles formed as the plasma fills the gap from different directions."

There are no conventional images of the bubbles, however.

Since Cassini arrived at Saturn, it has been building up a picture of the vital and unexpected role that Enceladus plays in Saturn's magnetosphere. Enceladus may play a role similar to Jupiter's moon Io, which pumps plasma into Jupiter's environment. A picture of plasma adjustments in the wake of Enceladus could provide clues to how plasma gets transported around the Saturnian environment.

"Enceladus is the source of most of the plasma in Saturn's magnetosphere, with ionized water and oxygen originating from the vents forming a big torus of plasma that surrounds Saturn. We may see these spiky features in the wake of Saturn's other moons as they interact with the plasma but, to date, we have only studied Enceladus in sufficient detail," Kanani said.