Saturn-Moon connection: It’s Electric



A northern lights-like aurora on Saturn is created by an electrical current between the ringed planet and one of its innermost moons.

The connection with Enceladus was first observed as a strong “bull’s eye” emission in the middle of images taken by NASA’s Cassini spacecraft.

“The ion beam seen by the camera appears at exceptionally high energy, between about 30,000 and 80,000 electron volts, surprising for an interaction with such a small moon,” says Don Mitchell, Cassini science team co-investigator from Johns Hopkins University.

The research is reported in the journal Nature.

Largely covered with ice, Enceladus is Saturn’s sixth-largest moon. Similar planet-moon connections happen at Jupiter: Io, Europa, and Ganymede all produce visible auroral footprints.

“Accurately identifying the magnetospheric location of a source of auroral emission is very exciting,” says Chris Paranicas, a Cassini scientist not directly involved with the study.

“At Jupiter, the identification of the satellite footprints in the auroral region allowed scientists to connect the polar region with the equatorial one magnetically. This paper will give us a great reference point for future studies of Saturn’s aurora.”

The ion beam set the stage for Abigail Rymer, a lead author of the study, to find evidence of another phenomenon, a very strong co-aligned electron beam in plasma spectrometer data from Cassini. The probe is now in its sixth year of operations at Saturn.

“I immediately pulled up the electron data and, sure enough, there was a very strong electron beam propagating away from Saturn toward Enceladus,” Rymer says.

The electrons Rymer discovered were of sufficient energy that they could stimulate an observable auroral output on the planet, a glowing spot formed the same way as the Earth’s northern lights, with electrons precipitating into the ionosphere.

At Earth, however, the electrons come from interplanetary space; at Saturn they represent an enormous current system looping through Enceladus all the way back to Saturn, more than 150,000 miles away.

Two weeks after the initial observations, with Cassini flying at higher latitudes, the ultraviolet imaging spectrograph captured three images of Saturn’s ionosphere that included a visible glowing spot at exactly the expected location.

“We searched for an auroral footprint on Saturn by using Cassini’s ultraviolet spectrograph to make images,” explains Wayne Pryor of Central Arizona College, the other lead author. “It turns out that ultraviolet light from the Enceladus footprint is not always visible; in fact, of 282 images that could include the signal, only seven provide convincing evidence for a bright spot.”

That the footprint appears to “flicker” suggests variable outgassing from Enceladus, Rymer says, but the Cassini team is not yet convinced that the plume activity on Enceladus is variable.

“Scientists have been wondering whether the venting rate is variable and these new data suggest that it is,” Rymer says.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the mission.