Cassini Captures Light Show on Jupiter's Moon, Io, During Eclipse

Lori Stiles
May 31, 2001

The Cassini spacecraft, passing through the Jupiter system on Jan. 1, 2001, en route to Saturn, recorded a sequence of images showing Jupiter's moon Io glowing in the darkness of the giant planet's shadow.

University of Arizona planetary scientist Paul Geissler presented a color version of this "movie" today at the American Geophysical Union meeting in Boston. The movie shows details of moon's visible aurorae that solve some of the puzzles presented by the Galileo spacecraft observations, Geissler said.

The new animation is available online from the Cassini Imaging Science team at the University or Arizona, Tucson, at and from NASA's Jet Propulsion Laboratory, Pasadena, Calif., at

Little was known about these dazzling light shows before the Galileo spacecraft arrived at the Jupiter system in late 1995. Galileo pictures of Io in eclipse showed a colorful display of red, greenish and blue emissions bright enough to be seen with the naked eye. These glows are due to various gases in Io's tenuous atmosphere that are excited by electrical currents, much like the Aurora Borealis on Earth.

Galileo could send back only a few snapshots because of its nonfunctional radio antenna, and these pictures were taken in only six colors, so it was impossible to be sure of the exact composition of the glowing gases. From ground-based telescopes and Hubble Space Telescope observations it was believed that the red glow was caused by neutral atomic oxygen and that sodium was the source of the greenish glow. The blue glow was unidentified, but suspected to be sulfur dioxide.

Although the Cassini spacecraft did not come as close to Io as did the Galileo spacecraft, Cassini could stare at Io for two hours at a time and record sequences of images ("movies") of entire eclipses, Geissler said. Cassini's camera is also sensitive to shorter wavelengths than is Galileo's camera, and it could record more colors using different filters. Cassini caught Io's aurorae in motion and detected emissions at previously unknown wavelengths. Both red atomic oxygen and blue molecular sulfur dioxide emissions are seen in the Cassini images, along with thermal glows from hot lava at several active volcanoes.

The motion of the aurorae suggests that the visible emissions are powered by electrical currents that connect Io to Jupiter, similar to the ultraviolet aurorae seen on Io by the Hubble Space Telescope. A volcanic plume erupting on the opposite side of Io can just be detected over the moon's north pole. This eruption left an enormous red ring around the volcano Tvashtar, seen erupting by both Galileo and Cassini, Geissler noted. These results tell us more about the patchy atmosphere of Io and the electrical currents that excite the visible emissions, he added.

*** (EDITORS: UA planetary sciences Professor Alfred McEwen, a member of the Cassini Imaging Science team as well as the Galileo Imaging Science team, is a contact on this story. He can be reached today in Tucson at 520-621-4573 or 520-320-5766.

The Jet Propulsion Laboratory Media Relations Office today is releasing news stories and image advisories the Io Color Eclipse Movie. The JPL Media Relations Office contact Guy Webster, 818- 354-6278, is at the AGU meeting is Boston.)****


Resources for the media