No fewer than six active volcanoes on Jupiter's moon Io have been photographed in the last four days spewing clouds of black ash, dust and gas into space 400 miles above the surface.

The Voyager spacecraft that flew by Jupiter and Io eight days ago continues to photograph the moon. And it has seen debris being pumped from the six volcanoes at speeds of almost 40 miles a minute, undeterred by any atmosphere around the moon and helped along by a gravity one-sixth that of Earth's. All six volcanoes have erupted at least twice since last Friday. One photograph shows three of the six volcanoes going at once.

"Volcanically, Io is much more active than the Earth," Dr. Laurence A. Soderblom of the U.S. Geological Survey said yesterday at the Jet Propulsion Laboratory in Pasadena, Calif., where Voyager is directed. "Io is easily the most active thing in the solar system."

One of the six volcanic plumes was seen soaring to an altitude of 300 miles, the others reached heights of 200 miles. The largest plume is spread out to where it appears to be as wide as it is high.

"Two of the plumes look like whale spouts against the light-colored background of the planet," Soderblom said. "The biggest plume looks like a giant black tarantula."

An ultraviolet instrument aboard Voyager has seen gas clouds above the ash-and-dust plumes that reach heights of more than 400 miles. An infrared instrument on the spacecraft has taken the temperature of at least one of the plumes and found it to be almost 200 degrees Fahrenheit, the temperature of scalding water.

Besides the six active ash plumes, Voyager has photographed dozens of black calderas across the surface of Io that suggest volcanoes momentarily inactive or that have become extinct only recently. One caldera lies in a heart-shaped bed of lava 1,000 miles across.

The infrared instrument on Voyager has measured the temperature around two calqeras at least 40 degrees Fahrenheit, which is about 300 degrees warmer than the temperature of the surface where no volcano exists.

"These look like lava lakes," Soderblom said, "that have not cooled down yet."

What Dr. Bradford Smith of the University of Arizona calls "the most exciting finding yet of the Voyager mission" is sure to trigger a scientific debate about what makes Io a unique heavenly body. Oddly, one thing is sure about Io: It has no detectable water to help carry volcanic ash to the surface.

"Io is very, very dry," Soderblom said. "There is no ice there and no water there at all."

A growing number of Voyager scientists, Soderblom included, now think that Io is a volcanic island in orbit around Jupiter because it is so close to Jupiter and because its orbit is perturbed by the wanderings of the larger moon Europa as they both circle the giant planet.

What Europa does, postulates Dr. Stanford J. Peale of the University of California at Santa Barbara, is to "jostle" Io up and down in a way that causes the immense gravitational field of Jupiter to heat the entire moon, inside and out.Most of the heat is distributed to the interior by the tidal pushes and pulls of both Io and Europa on each other, providing a mechanism continually to heat the interior of Io.

"This little oscillation Europa sets up on Io would be enough to have continuous volcanism independent of any radioactive rock at the interior of Io," Soderblom said. "We now have enough continuous heat inside Io to keep the volcanoes going and to keep the lava flowing all through time."

The countless volcanic eruptions on Io explain its vivid coloring, which runs from bright red to orange to yellow to brown to black. Scientists believe Io gets its color from sulfur compunds expelled through volcanic vents.

"It's very primitive volcanic juice that produces this brilliant array of colors," Soderblom said. "And over time we've just gotten rid of all the water on Io, which would also tend to produce this unrelieved pastel of color."

The mountains of lava on Io also explain the nonexistence of craters on Io. Soderblom said the lava has filled in every crater ever left on Io by a meteorite impact.