Cassini at Jupiter: Eyeing the Io torus

Headed for a 2004 rendezvous with Saturn, the Cassini spacecraft is flying past Jupiter. As it picks up a gravitational boost from the giant planet, the NASA-European Space Agency spacecraft is taking the opportunity to explore the behemoth’s harsh environment of energetic charged particles, strong magnetic fields, and ionizing radiation.

The Io torus seen by Cassini. JPL/NASA

The craft’s ultraviolet camera has now recorded the most detailed images ever taken of a doughnut-shape ring of charged particles surrounding Jupiter. Known as the Io torus, the ring is fueled by neutral oxygen and sulfur atoms spewed by volcanoes on Io, one of the planet’s largest moons. Radiation striking the atoms turns them into ions.

The ions are constrained to move along the lines of force created by Jupiter’s powerful magnetic field. As Jupiter rotates, it drags the field and the ions in circular paths around the planet.

The ions radiate most of their light at ultraviolet wavelengths, making it a prime target for Cassini’s camera, observes John T. Clarke of the University of Michigan in Ann Arbor. The images, released last month, show that the brightness of the torus has slowly declined over the past few months. That’s most likely because the last batch of particles to have entered the torus have cooled and dissipated and have yet to be replenished by material from the next episode of volcanism on Io.

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