Io is the most volcanically active body in the solar system, said CU-Boulder Associate Professor Nick Schneider and former post-doctoral researcher Michael Kueppers of CU's Laboratory for Atmospheric and Space Physics. They believe the presence of chlorine -- an ingredient of sodium chloride, or common table salt -- is related to the violent volcanic activity.

"In fact, Io seems to have a higher proportion of chlorine in its atmosphere than any other object in the solar system," said Schneider.

"The huge volcanoes on Io are similar to giant geysers, spewing material hundreds of miles into the atmosphere, said Schneider. The two researchers used a telescope at the National Science Foundation's Kitt Peak National Observatory in Arizona to make their findings, including the discovery of chlorine emissions in the doughnut-shaped ring of charged particles surrounding Jupiter known as the Io torus.

The Io torus, which is five times larger than Jupiter, glows with a power greater than all the electricity generated on Earth, he said.

Prior to the discovery of chlorine, the only elements observed escaping from Io's atmosphere were sulfur, oxygen, sodium and potassium.

The most common inorganic compounds of chlorine are sodium chloride, which is ordinary table salt, and hydrogen chloride, a colorless gas that is emitted from the volcanoes, said Schneider.

"It's not yet clear how salt would form on Io," said Schneider. "Unlike Earth, Io has no oceans that could evaporate and leave behind salt deposits. But it is possible that underground rivers or aquifers fuel Io's volcanoes and may carry dissolved salt. Salt also could be made by chemical reactions in Io's atmosphere."

A paper on the subject was presented by Schneider and Kueppers at the spring meeting of the American Geophysical Union held in Boston June 1 through June 4. Kueppers is currently at the University of Berne in Switzerland.

The recently discovered chlorine may be emitted by Io's volcanoes or may come from the breakup of salt on Io's surface by charged particles in the torus that constantly bombard the surface of the moon.

The discovery also has implications for the chemistry of Io's atmosphere, he said. On Earth, relatively small amounts of chlorine from human-made CFC's play a major role in breaking down fragile molecules like ozone in the atmosphere.

"My guess is that we won't find any ozone on Io," said Schneider, noting the proportion of chlorine in Io's atmosphere is a billion times greater than that on Earth.

"Chemical reactions may actually produce salt in the atmosphere," he said. "The study of chlorine on Io is sure to benefit from the extensive research on Earth's ozone hole, which in turn benefited from the study of chlorine in the atmospheres of other planets."