The active volcanoes that pepper the surface of the jovian moon Io spew minuscule dust particles that stream out as far as 180 million miles (290 million kilometers) from the Jupiter system, scientists using data collected by NASA's Galileo spacecraft report.

Scientists have long fingered Io and its plenitude of volcanoes as the prime suspect in the hunt for the source of the dust stream, first detected by the Ulysses spacecraft in 1992. Other possible culprits included Jupiter's main ring and the comet Shoemaker-Levy 9, fragments of which impacted the planet in 1994.

An analysis of the number of collisions these submicron-sized particles made with Galileo's dust detector showed the peaks that coincided with the periods of Io's orbit (42 hours) and Jupiter's rotation (10 hours).

Jupiter's moon Io.

By poring through several years' worth of Galileo data, the dust scientists found that Jupiter's magnetic field strongly influenced the motion of the particles -- no larger that bits of smoke -- in a way that pointed back to Io as their source.

"Now, for the first time, we have direct evidence that Io is the dominant source of the jovian dust streams," said Amara Graps of the Max Planck Institute of Nuclear Physics in Heidelberg, Germany. A paper describing the findings appears in the May 4 issue of the journal Nature.

Study of the magnetically controlled dust can shed light on charging processes within Jupiter's magnetosphere, said Eberhard Gruen, also of the Max Planck Institute, who built the dust detectors on Galileo, Ulysses and Cassini.

In situ measurements of the dust can also help scientists keep tabs on Io's volcanic plumes, which can spew columns of debris that are easily spotted by Galileo and from Earth-based telescopes. The plumes are also thought to be the source of the reddish sulfur that coats Amalthea, one of Jupiter's tiny inner moons.

In December 2000, scientists hope that when Cassini swings by Jupiter to gain momentum for its journey to Saturn, the spacecraft will join Galileo in making joint dust measurements.

Io is the most volcanically active body in the solar system, with literally dozens of active vents. The moon, slightly larger than Earth's own natural satellite, is one of 16 known to orbit Jupiter.

Because of its proximity to Io, Jupiter's gravitational pull flexes the moon's crust with tidal regularity. That effect, coupled with a wobble introduced by neighboring moons Ganymede and Europa, generates heat through friction, just like a piece of wire repeatedly bent back and forth, which contributes to the volcanism.

In addition to Graps and Gruen, other authors on the Nature paper are: Mihaly Horanyi, Harald Krueger, Andreas Heck, Sven Lammers and Hakan Svedhem.