Astronomers say they have found what may be the largest visible structure in the solar system: a vast cloud of sodium streaming about 20 million miles away from Jupiter.

The astronomers believe the sodium is being ejected from volcanos on Jupiter's moon Io. Collisions between the sodium gas and electrically charged particles trapped in Jupiter's strong magnetic field create a narrow jet of superfast sodium, which gushes away from the giant planet.Scientists have known for years that Jupiter possessed sodium clouds. What was surprising was the size. At 20 million miles, the sodium cloud stretches a distance equal to more than 200 times the diameter of Jupiter, the largest planet of the solar system.

"As we moved the telescope further and further from Jupiter, we kept detecting sodium - fainter but still there," said Jeffrey Baumgardner of Boston University, who found the sodium fog with colleagues Michael Mendillo and Brian Flynn.