The mysterious Great Red Spot seen above the southern clouds of Jupiter for the last 300 years has now been carefully observed by the Voyager spacecraft -- and it is a bigger puzzle than ever.
"The currents that flow all around it suggest they are removing energy from the red spot rather than replacing energy," Dr. Andrew Ingersoll of the California Institute of Technology said today at the Jet Propulsion Laboratory, where the Voyager flight is directed. "This is exactly the opposite of what should be happening to explain a hurricane three times the size of the Earth that may be a permanent feature on Jupiter."
Just as difficult to explain is why the Great Red Spot appears permanently rooted in Jupiter's south tropical zone. While the red spot often changes its shape, size and color, it never drifts very far from the same place above the clouds of Jupiter.
"Again, what we see from Voyager indicates little upwelling of material from below the red spot that would say it is being fed from roots it has deep below the clouds," Ingersoll said. "To be honest, we don't know why it doesn't move and why it doesn't lose enough energy to break up and disappear."
Even the spot's color is still unexplained, Ingersoll said, though infrared measurements by Voyager may soon shed some light on it. As of now, Ingersoll said, the red color could come from "all the obvious candidates" from phosphorus to hydrogen cyanide.
While the observations of the red spot left scientists disappointed, Voyager's pictures of the rest of the planet and its four largest moons still provided a wealth of information for the 106 scientists who make up the Voyager team.
A photograph of the moon Io today revealed the unmistakable outlines of a volcano. Radiating out 100 miles in almost every direction from the volcano's caldera were lava flows that geologists estimated to be about one mile deep.
Dr. Laurence A. Goderblom of the U.S. Geological Survey said the lava appeared to have filled in much of the caldera, which itself appeared to be flat. Soderblom said the red and yellow lava in the flow looked featureless, "as if it had been painted on" the surface of Io.
Soderblom said that hot water was probably the vehicle that carried sulfur and ash exploding from the volcano to the surface. He said that the fumarole appearance meant that the yellow and red color in the lava probably came more from the steam and hot water percolating through the flow rather than the sulfur in the flow itself.
If the volcano that produced the lava flow is extinct, Soderblom went on, it has not been extinct for very long. He estimated its age as anywhere from "right now" to 10 million to 20 million years.
"If the lava flow was old we should see [meteorite]impact craters in the lava," Soderblom said. "Voyager's picture of this volcano has five times higher resolution than any other pictures we have of Io and we don't see a single crater in the picture. That means that what we're seeing has to be very young."
In still another surprise, a photograph taken by Voyager of the night side of Jupiter when the spacecraft flew behind the planet on its way out toward Saturn shows the clear outlines of what may be lightning strikes.