So any volcanoes have been erupting or so long on Io, a moon of Jupiter, that they have created a thin atomsphere of sulfur dioxide gas around it.

"It looks to the instruments on the Voyager spacecraft like no more than a faint haze," Voyager 2 project scientist Edward C. Stone said at a press conference today at the Jet Propulsion Laboratory. "But there's no question there is a tenuous atmosphere around Io of sulfur dioxide."

The discovery of an atmosphere is interesting to scientists because it would not be expected on a body as small as Io. The gravitational pull of small bodies, such as moons, is too weak to keep gaseous molecules from escaping into space. Unless there is a mechanism for replenshing it, the atmosphere disappears.

Now two million miles from jupiter, Voyager 2 will train its instruments on Io for 10 hours on Monday, when the spacecraft is 448,000 miles from it. During that 10-hour "Io watch," the cameras on Voyager 2 will scan the moon to see if any of its eight known volcanoes are erupting and to see if it can find any others. The eight volcanoes were found last March by Voyager 1.

"Half of Io was obscured the last time we flew by," Dr. Bradford Smith of the University of Arizona said here today, "so there could be twice the number of volcanoes on Io than we've already seen."

On Thursday night, the rapidly approaching Voyager 2 photographed the plume of an eruption on Io that came from one of the eight volcanoes photographed for the first time by Voyager 1. The plume was more than 124 miles high, suggesting that it may have been erupting continuously for the last four months.

"Io's volcanic activity is such that it appears to be feeding material to the entire magnetosphere of Jupiter," Stone said. "We have found sodium, oxygen and sulfur that presumably come from Io as far as two million miles away and traveling at speeds of more than 10 percent the velocity of light."

The photographs taken by Voyager 1 have shown plumes from the volcanoes reaching heights of 220 miles. Photographs also show that the volcanic ash, driven that high, rains right back down on the surface, covering whatever craters have been punched in the moon's surface during the past three billion years.

"The question is, how does the sulfur and sodium we see at two million miles from Io get there?" Stone asked. "Is it sent there through some high ejection mechanism from the volcanoes or is there another mechanism that helps to carry it there?"

Besides a photographic "Io watch," Voyager 2 will get far better close-up pictures of the large moons, Ganymede and Europa, as it flies by Jupiter on Monday. Voyager 2 will go as close as 38,000 miles from Ganymede and 127,000 miles from Europa, where it will get pictures 10 times better than those taken by Voyager 1.

Europa may be almost as interesting as Io. Criss-crossed by dozens of long deep cracks that scientists believe are canyons, Europa may be suffering continuous quakes and movements in its crust that could be sending clouds of water ice above the moon and creating a thin atmosphere there also.

Through we may not see any of these events in progress," Smith said, "we may see dozens of things on the surface of Europa that will tell us this moon is a very busy [place]."