Jupiter has 63 known moons, and the planet's faint rings are produced when debris from space collides with the four moons closest to the planet, researchers said. Although the rings are normally within the orbits of those moons, protrusions of dust sometimes extend beyond the orbit of Thebe -- the most-distant of the four closest moons.

The cause of the protrusions has been a long-standing mystery.

Now, Professor Douglas Hamilton of the University of Maryland and Harald Kruger of the Max-Planck Institute for Solar System Research in Germany say they've solved the mystery. Hamilton and Kruger analyzed data on dust grain sizes, speeds and orbital orientations taken by the spacecraft Galileo as it traversed Jupiter's rings in 2002 and 2003.

As they orbit Jupiter, "dust grains in the rings alternately discharge and charge when they pass through the planet's shadow," said Hamilton. The charged dust particles interact with Jupiter's powerful magnetic field, and "are pushed beyond the expected ring outer boundary," he said.

The study appeared in a recent issue of the journal Nature.