"This gives nice closure to the discovery Bob Brown made in 1974 of sodium in neutral clouds of gas around Io, " said Darrell Strobel, a professor of earth and planetary sciences in the Krieger School of Arts and Sciences at Johns Hopkins and an author of a paper on the new results in the Jan. 2 issue of "Nature."

Further analysis of the results, including modeling how the salt is broken down into sodium and chlorine atoms, could help planetary scientists move closer to determining what kinds of meteoritic materials originally came together to form Io, according to Strobel.

Strobel said Brown, who later became a project scientist at the Space Telescope Science Institute, found the sodium around Io while testing out a spectrograph he had built.

"He told me some years afterwards, 'This discovery of mine is so simple. I was amazed somebody hadn't done it 30 to 40 years earlier,'" Strobel said. "Nobody was looking for it; nobody would have guessed it was there."

Astronomers winnowed the list of theoretical suspects for the source of sodium for years before determining the most likely suspect was salt, or sodium chloride. That conclusion was reached after the detection two years ago of chlorine in a doughnut-shaped, electrically charged cloud of gas around Io known as the plasma torus. Based on the new chlorine finding and the theoretical work, astronomers decided to conduct the exacting studies necessary to look for salt.

"The bottom line is that there seems to be enough salt in Io's volcanic atmosphere to supply both the amount of sodium that one sees in the neutral clouds and the chlorine in the plasma torus," said Strobel, who is also a professor of physics and astronomy at Johns Hopkins.

A slightly eccentric orbit around Jupiter and the gravitational fields of two nearby large moons, Europa and Ganymede, subject Io to a great deal of stress, flexing the moon's crust and heating its core. As a result, Io is hands-down the most volcanically active planetary body in the solar system. Roughly comparable in size to Earth's moon, Io's frequently active volcanoes would make it a hell for anyone who might want to visit, but it's a heaven for scientists eager to watch a planetary body regularly belch up tons of its innards.

"Roughly two tons of volcanic material are tossed into Io's magnetosphere every second, and then when this material is ionized [electrically charged], the inner magnetosphere starts to resemble a miniature pulsar," Strobel said.

Interactions between the clouds of electrically charged gas around Io and electrically charged particles in Jupiter's polar atmosphere speed up the rotation of the charged particles around Io but also apply an infinitesmal drag to the rotation of Jupiter, gradually slowing the speed at which the giant planet spins.

"It's a remarkable, unique system of interaction," Strobel said. "We've learned quite a bit since the days when Voyager 1 first swept by the moon in 1979 and revealed eight active volcanoes, but we don't understand it completely."

Strobel said the lead author of the new "Nature" paper, Emmanuel Lellouch of the Observatoire de Paris, had looked previously for salt in Io's atmosphere and failed to find signs of it. Co-author Nicholas Snyder of the University of Colorado at Boulder, one of the researchers who discovered chlorine in Io's plasma torus, suggested using millimeter-wavelength radio telescope at the Institut de Radio-Astronomie Millimetrique in Granada, Spain, to perform a definitive search for salt.

Observations with a millimeter-wavelength radio telescope force astronomers to focus on very tiny regions of the spectrum, making it necessary to carefully choose the frequencies they want to observe. But when the team conducted its studies in January 2002, they found the characteristic spectroscopic lines they were looking for. An examination of potential sources for the salt in the atmosphere pointed to the volcanoes as the most likely point of origin for the salt.

Other authors on the paper were Gabriel Paubert of the Institut de Radio-Astronomie Millimetrique; and Julianne Moses of NASA's Lunar and Planetary Institute. This research was supported by the NASA Planetary Atmospheres Program.