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For Immediate Release
April 20, 2011

Media Contact:

Michael Buckley
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michael.buckley@jhuapl.edu

Icy Moon Zaps Saturn with Electron Beams

Hopkins APL Camera Captures Auroral Activity Between Enceladus and Saturn

This artist's concept shows a glowing patch of ultraviolet light near Saturn's north pole that occurs at the “footprint” of the magnetic connection between Saturn and its moon Enceladus. The magnetic field lines and the footprint are not visible to the naked eye, but were detected by the ultraviolet imaging spectrograph and fields and particles instruments on NASA's Cassini spacecraft. The footprint, newly discovered by Cassini, marks the presence of an electrical circuit that connects Saturn with Enceladus and accelerates electrons and ions along the magnetic field lines. In this image, the footprint is in the white box marked on Saturn, with the magnetic field lines in white and purple.

Credit: NASA/The Johns Hopkins University Applied Physics Laboratory


Enceladus 'Footprint' on Saturn

NASA's Cassini spacecraft has spotted a glowing patch of ultraviolet light near Saturn's north pole that marks the presence of an electrical circuit that connects Saturn with its moon Enceladus. This newly discovered patch occurs at the "footprint" of the magnetic connection between Saturn and Enceladus and indicates electrons and ions accelerating along magnetic field lines. White boxes indicate the location of this footprint, which scientists have long predicted but never before seen.

The patch glows because of the same phenomenon that makes Saturn's well-known north and south polar auroras glow: energetic electrons diving into the planet's atmosphere. However, the footprint is not connected to the rings of auroras around Saturn's poles.

The two images shown here were obtained by Cassini's ultraviolet imaging spectrograph on Aug. 26, 2008, separated by 80 minutes. The footprint moved according to changes in the position of Enceladus. In the image, the colors represent how bright the extreme ultraviolet emissions are. The lowest emission areas (one to two extreme ultraviolet counts per pixel) are in black/blue. The brightest emission areas (500 to 1,000 extreme ultraviolet counts per pixel) are in yellow/white.

The footprint appeared at about 65 degrees north latitude. It measured about 1,200 kilometers (750 miles) in the longitude direction and less than 400 kilometers (250 miles) in latitude, covering an area comparable to that of California or Sweden.

In the brightest image the footprint shone with an ultraviolet light intensity of about 1.6 kilorayleighs, far less than the Saturnian polar auroral rings. This is comparable to the faintest aurora visible at Earth without a telescope in the visible light spectrum. 

The sun was illuminating Saturn's north pole from the left and the footprint is on the day side of the planet. The night side of the planet was to the right of the hashed line.

Credit: NASA/JPL/University of Colorado/Central Arizona College


Download QuickTime Movie (1.62 MB)

NASA's Cassini spacecraft has spotted a glowing patch of ultraviolet light near Saturn's north pole that marks the presence of an electrical circuit that connects Saturn with its moon Enceladus. This newly discovered patch occurs at the "footprint" of the magnetic connection between Saturn and Enceladus and indicates electrons and ions accelerating along magnetic field lines. White boxes indicate the location of this footprint, which scientists have long predicted but never before seen.

The patch glows because of the same phenomenon that makes Saturn's well-known north and south polar auroras glow: energetic electrons diving into the planet's atmosphere. However, the footprint is not connected to the rings of auroras around Saturn's poles.

The three spot detections in the movie were obtained by Cassini's ultraviolet imaging spectrograph at 8 p.m. and 9:20 p.m. PST on Aug. 25, 2008, and 1:02 a.m. PST on Aug. 26 (3 a.m., 4:20 a.m. and 8:02 a.m. UTC on Aug. 26), a period of time spanning five hours and two minutes. The footprint moved according to changes in the position of Enceladus. In the image, the colors represent how bright the extreme ultraviolet emissions are. The lowest emission areas (one to two extreme ultraviolet counts per pixel) are in black/blue. The brightest emission areas (500 to 1,000 extreme ultraviolet counts per pixel) are in yellow/white.

The footprint was located about 65 degrees north latitude. It measured about 1,200 kilometers (750 miles) in the longitude direction and less than 400 kilometers (250 miles) in latitude, covering an area comparable to that of California or Sweden.

In the brightest image the footprint shone with an ultraviolet light intensity of about 1.6 kilorayleighs, far less than the Saturnian polar auroral rings. This is comparable to the faintest aurora visible at Earth without a telescope in the visible light spectrum. 

The sun was illuminating Saturn's north pole from the left and the footprint was on the day side of the planet. The night side of the planet was to the right of the hashed line.

Credit: NASA/The Johns Hopkins University Applied Physics Laboratory

Scientists working with data from NASA’s Cassini mission — now in its sixth year of operations at Saturn — have discovered an electrical current running between Saturn and its moon Enceladus that creates an observable emission on the ringed planet.

Don Mitchell, Cassini science team co-investigator from the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., first observed the current connection as a strong “bull’s-eye” emission in the middle of images snapped by the APL-built ion and neutral camera (INCA) on Cassini. “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 Mitchell, co-author of a paper on the research appearing in the April 21 issue of the journal Nature.

This planet-moon connection also happens 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 APL’s 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 APL’s Abigail Rymer, a lead author of the study and a Cassini team scientist, to find evidence of a very strong co-aligned electron beam in Cassini plasma spectrometer data.

“I immediately pulled up the electron data and, sure enough, there was a very strong electron beam propagating away from Saturn toward Enceladus,” says Rymer. “It was actually a fairly rare opportunity to capture that, since when Cassini flies close to a moon we are generally looking at the moon — not away from it.”

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, the other study lead, from Central Arizona College. “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 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 for NASA’s Science Mission Directorate, Washington, D.C. The ultraviolet imaging spectrograph team is based at the University of Colorado, Boulder. The magnetospheric imaging team is based at the Johns Hopkins University Applied Physics Lab, Laurel, Md. The Cassini plasma spectrometer team is based at the Southwest Research Institute, San Antonio.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

The Applied Physics Laboratory, a division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu