The volcanoes on the moon of Jupiter called Io probably have been erupting since the beginning of time and probably will continue to erupt until the end of time.

"The vulcanism we see on Io has probably been going on for the age of the solar system," Dr. Laurance A. Soderblom of the U.S. Geological Survey said in an interview at the Jet Propulsion Laboratory, where the flights by Jupiter of two Voyager spacecraft were directed. "There's no reason it can't go on that way forever."

Eight erupting volcanoes were discovered by Voyager 1 on March 8, 9 and 10 as it flew by Io. Four months later, Voyager 2 flew past Io and found seven of the eight still erupting, sending steady plumes of black ash and white sulfur dioxide gas as far as 160 miles into space.

"If the volcanoes were turning themselves on and off in a matter of minutes or even hours or days, we would have seen that in our pictures," Soderblom said. "But every time we look at them they're still going and the suggestion is that they all erupt for at least several years at a time and we just happened to catch one turning itself off to take a rest."

The two Voyager spacecraft took more than 200 pictures of Io's volcanoes, photographing each plume at least five times on each of the two flights. Both spacecraft photographed the vents of the eight volcanoes and the enormous black calderas around the volcanoes, where hot sulfur poured onto the surface, forming huge lakes of frozen sulfur as far as 500 miles from the vent holes, which themselves were 40 miles across.

The Voyager pictures showed Io's entire surface covered by the blacks, reds, yellows and oranges of different forms of sulfur that had exploded from the moon's interior. Some of the sulfur lakes on the Ionian surface were covered by what appeared to be fresh snow. In a way, it was snow - the pure white snow of frozen sulfur dioxide.

"Sulfur and frozen sulfur dioxide can explain all the rich coloring we see on the surface of Io," Soderblom said. "There are as many as 52 different crystalline forms of sulfur and they all have a different coloring."

The average plume from the eight volcanoes reached a height of more than 100 miles, suggesting the black ash clouds were rocketing out of the vent holes at speeds of more than 5,000 miles an hour, unretarded by any atmosphere. The heat and explosive decompression causing the Ionian eruptions were described as being more violent than any volcanic eruption on Earth.

"If you put Krakatoa [the largest volcano in the Hawaiian Islands] up there on Io," Soderblom said, "it would be just a minuscule little pimple."

Io, the closest of Jupiter's four large moons to the planet, is pulled by huge tidal forces emanating from Jupiter. As it sails through this tidal sea, Io also is yanked up and down like a yo-yo by another moon, Europa, as it passes in its own orbit.

"Europa comes overhead and everytime it does it lifts Io up and then drops it into Jupiter's gravity field," Soderblom explained. "It's being squeezed and worked like one of those hard rubber balls athletes use to strengthen their hands.

The result is that the rock and minerals making up Io's interior are heated by the friction from being squeezed and bent by the constant tidal motion. Add that to the radioactive decay within Io's rocky core and to the electrical bombardment its surface undergoes from Jupiter, and the result is more than enough to heat and run the engine that drives all eight Ionian volcanoes.

The way Soderblom explains it, the eight volcanoes are replenished continuously by sulfur that piles up so high on the surface the sulfur on the bottom of the pile heats to the melting point. No longer solid, the molten sulfur allows sulfur dioxide to travel through it. The liquid sulfur and dissolved sulfur dioxide gas migrate beneath the moon's surface until they find one of the eight vents and explode outward through it.

"What we see on Io is a planetary body that is fed fluids and heat constantly," Soderblom said. "Sulfur goes up, falls on the crust, goes down and comes back up again. It's a process that never has to stop."

What astonishes Soderblom the most about Io is the short time it took for its exotic and dynamic history to unfold. In four months, the most volcanic body in the solar system was revealed to science through the two Voyager flights.

"We've gone further in developing a model that says how Io works than we have for the earth over the last 100 years," Soderblom said. "That's incredible, no matter how you look at it."