What could be "ice volcanoes" showed up today in photographs of two moons of Jupiter taken from less than 3 million miles by the approaching Voyager spacecraft.
"The idea of ice volcanoes is almost too bizarre to mention," said the U.S. Geological Survey's Dr. Laurence A. Soderblom, one of the key investigators among the Voyager scientists at the Jet Propulsion Laboratory where Voyager is directed, "but I don't know anything else right now that could explain what it is we're seeing on Callisto and Ganymede."
Callisto and Ganymede are the two largest of Jupiter's 13 known moons. Scientists today saw large bright spots on their otherwise dark surfaces. On the earth's moon such spots would be explained as craters left by impacts of meteorites in the last 10 million to 20 million years. But half the composition of Callisto and Ganymede is ice, which means that even craters a few million years old would probably be filled in by water seeping up from cracks in the surface, freezing into a permanent cover over the craters.
That leaves one explanation: volcanoes that spew ice instead of fire and ash, an explanation that scientists agree is bizarre but not impossible. Scientists said ice volcanoes were proposed as possiblities when astronomers on earth identified the two moons' surfaces as at least 50 percent water-formed ice.
"The difficulty with ice volcanoes is explaining how they'd get the ice up to the surface through a crest of rock and ice 200 kilometers thick," Socerblom said. "Also, on earth, rock gets less dense when it melts inside volcanoes. But when you melt ice it gets more dense, which makes it harder to bring to the top."
Among the ways ice could erupt from a volcano, Soderblom said, might involve mechanisms similar to those of geysers or artesian wells on Earth, where dissolved gases under pressure bring water to the surface.
"It might even imvolve a principle like a champagne bottle," Soderblom said, "where gases trapped above the liquid would pop the stuff up like what happens when you uncork the bottle."
A mosaic of photographs, taken today by Voyager, of the Great Red Spot on Jupiter appeared to explain why this permanent feature in the southern hemisphere of Jupiter rotates in a counterclockwise direction, like a hurricane on earth. The mosaic shows jet streams above and below the red spot which are moving at different speeds in opposite directions.
The jet stream just north of the red spot is carrying clouds and gas from east to west at about 105 miles per hour, while the jet stream just south of the red spot is traveling from west to east at about 120 miles per hour. This would be enough of a difference to cause the red spot, which has been described as the solar system's only permanent hurricane, to move in a counterclockwise direction.
The photographs appear to show some smaller storms above the clouds of Jupiter moving around the red spot ant disappearing into it. At least as many of the storms rotate around the red spot and slide off to the east as are swallowed up.
The storms that disappear into the red spot could explain where the red spot gets some of the energy to maintain itself. But they do little to explain where the smaller storms get the heat to cause them to form above the clouds. It also does not explain why there is only one Great Red Spot and not others.
Traveling today at a speed of more than 30,000 miles per hour, the 1,800-pound Voyager will reach the environs of Jupiter at 7:42 a.m. EST on Monday, when the spacecraft files as close as 173,000 miles from the planet. At that point, Voyager will be less than 20,000 miles below the south pole of Jupiter's moon Io, which is the closest of the four largest moons of the largest planet in the solar system. When Voyager flies by Jupiter, it will scan the orbits close to the planet in a hunt for even more moons that are too small to be observed from earth.