Cassini "Sees" Invisible Gas Doughnut Around Jupiter

The doughnut, called the Io torus, draws its raw material from gases spewed into space by volcanoes on Io, one of Jupiter's large moons.

The torus was detected in the 1970s, but almost all of its light is invisible to the human eye. It is big enough that if Earth were in the middle, the orbit of Earth's Moon would fit inside the hole of the doughnut.

The first movie clip of the gyrating Io torus in extreme ultraviolet light is available online from NASA's Jet Propulsion Laboratory, Pasadena, Calif., at http://www.jpl.nasa.gov/video/cassini.

"We're visualizing the torus and seeing it evolve and change in a level of detail that people haven't seen before," said Dr. Larry Esposito, a planetary scientist at the University of Colorado, Boulder, and principal investigator for Cassini's ultraviolet imaging spectrograph instrument.

He and University of Colorado's Ian Stewart and other colleagues are examining what the ultraviolet information can add to understanding about the composition and temperature of the torus and the processes that produce it.

The source of specific wavelengths of ultraviolet glow coming from the torus can be identified as positively charged ions of oxygen and sulfur, Esposito said.

Cassini, built, operated and managed by JPL, is monitoring the concentrations, temperatures and distribution of the ions over a six-month period to check and build upon current explanations for the torus. Gases from Io's volcanoes contain oxygen and sulfur. The torus apparently gets its ingredients and shape when some of the neutral oxygen and sulfur atoms around Io become ionized by exposure to radiation from the Sun or from a radiation belt that surrounds Jupiter. As ions, they are stuck to lines of magnetic force in Jupiter's strong magnetic field. That field twirls along with the planet's rotation, dragging the ions in circular paths around Jupiter, so a shape that begins as a sphere around Io becomes a torus around Jupiter.

The ultraviolet instrument on Cassini has checked the Io torus almost daily since Oct. 1, 2000. So far, the monitoring shows a gradual decline in overall brightness. The torus material apparently dissipates and cools over time, to be replenished and re-energized by the next episode of volcanic activity from Io.

"We might be seeing the tail end of one of those episodes," Esposito said. "We're hoping Io will give us a new injection of material so we can track the effects."

Cassini passed its closest to Jupiter on Dec. 30, gaining a gravitational boost needed for reaching its main destination, Saturn. It will continue studying the environment around Jupiter until March 22.

Esposito said he looks forward to investigating clues of a related phenomenon at Saturn involving gases from the dense atmosphere of Saturn's largest moon, Titan. Cassini will reach Saturn in July 2004. Information about the dynamics of the Io torus and its possible cousin at Saturn could deepen understanding of other phenomena linked to magnetic fields, said Espositio. These phenomena include powerful magnetic storms that can disrupt communications on Earth, and the shaping of nascent solar systems called planetary accretion discs that can exist within the magnetic-field influence of newly forming stars.

Cassini is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages Cassini for NASA's Office of Space Science, Washington, D.C. Further information about the mission is available at http://www.jpl.nasa.gov/cassini.