Scientists studying images sent back by the the Voyager spacecraft last August have discovered the fastest winds in the solar system whipping around Neptune at about 1,500 miles an hour.
The scientists found the unexpected winds after they plotted fleecy white clouds shown in a series of Voyager photographs. The clouds are being pushed at nearly supersonic speed on Neptune by winds that are similar to the jet stream on Earth, Caltech physicist Edward Stone said.
Stone, the chief scientist on the Voyager project, was one of several scientists who reported new findings during the fall meeting of the American Geophysical Union here.
Neptune and its rings and satellites continue to dazzle and baffle scientists, who are still trying to unscramble the enormous amount of data sent back by the intrepid spacecraft, which is now on its way out of the solar system.
Scientists, for instance, still have no explanation for why Neptune should have such fierce winds--nearly a third faster than the 1,100-m.p.h. jet stream on Saturn.
In the months since Voyager swept past Neptune, scientists have also confirmed the most spectacular discovery growing out of the August encounter: They thought they had detected something that looked like an ice volcano on Neptune's strangest moon, Triton, but they were so unsure of what they were seeing that they described it as a "mad idea."
Now, however, they are convinced that Triton does, indeed, have many ice volcanoes, and several were caught in the act of erupting when Voyager zipped past.
Multiple images that show the features from various angles have given scientists a stereoscopic view of Triton, and they clearly show that dark streaks coming from the volcanoes are like plumes of smoke drifting through the satellite's thin atmosphere.
The fact that Voyager captured pictures of so many volcanoes led Torrence Johnson of the Jet Propulsion Laboratory to conclude that "these things (ice volcanoes) apparently have lifetimes of several years." If they were sudden, brief and isolated events, it would seem unlikely that so many would have been going off when Voyager happened by, he said.
That still leaves scientists without a full explanation for why the small moon should have ice volcanoes--something not seen anywhere else in the solar system.
Johnson speculated that the volcanoes may work something like this: Organic materials in the atmosphere are darkened through solar radiation, in a photochemical process similar to that on Earth. The materials fall to the ground, where they form a thin skin and become covered with nitrogen ice during Triton's winter snowstorms. Then, solar radiation passes through the ice and heats the trapped material, which absorbs more radiation because it is dark. That causes the trapped material to expand and eventually explode through a weakness in the ice.
That mechanism is not too dissimilar from the "heat engine type of volcanoes" on Earth, Johnson said, although no one expected to find it on the frozen surface of Triton. While scientists seem to be increasingly comfortable with the idea of Triton's ice volcanoes, they remain baffled by the thin rings that surround Neptune. The rings are irregular in that they have some sections that are so much fatter than others that scientists first thought Neptune had only partial rings, or ring arcs, when they were discovered with ground-based telescopes several years ago.
Voyager proved that the rings are complete, numbering either four or five, depending on who is doing the counting, but they are very irregular. Scientists could see no reason why the dust particles and rocks that make up the rings should not be distributed evenly around the planet.
Yet the rings clearly are "lumpy," as one scientist put it, so scientists have spent the past few months searching Voyager images for small moons near the arcs. The leading theory explaining the arcs held that small moons could "shepherd" debris into the arcs, causing more material to accumulate in those areas.
But planetary scientist Jeff Cuzzi of NASA's Ames Research Center in Mountain View, Calif., shot that theory down Tuesday on the basis of intensive examination of images sent back by Voyager.
"You have to have satellites at specific locations, but they're not there," Cuzzi said. "That (theory) didn't work."
No one has come up with an explanation yet that appears to work, he said.
"People are extremely puzzled," Cuzzi added.