The Ulysses spacecraft discovered evidence that volcanic activity on one of Jupiter's moons has slowed since the Voyager probes passed in 1979, scientists said Tuesday.
Ulysses also determined that the size of the front end of Jupiter's gigantic magnetic field varies radically over time, said researchers for the National Aeronautics and Space Administration and the European Space Agency.
The spacecraft found that Jupiter's magnetic field now stretches almost 5-million miles toward the sun, twice as large as it was during the Voyager flybys but about the same size as it was when Pioneer 10 flew near Jupiter in 1973.
"It's much bigger than we expected," said Andre Balogh of London's Imperial College.
Ulysses, jointly operated by NASA and ESA, flew within 235,000 miles of Jupiter on Saturday, using the giant planet's gravity to dive south in order to study the sun's south polar regions in 1994 and its northern polar latitudes in 1995.
The discoveries at Jupiter mean "some of the textbooks will have to be rewritten," Klaus-Peter Wenzel, ESA's chief scientist for the $750-million mission, said during a news conference at Jet Propulsion Laboratory.
Voyagers 1 and 2 photographed eight or nine volcanoes on the moon Io spewing sulfur up to 175 miles skyward when they flew past Jupiter and its moons 12 years ago.
They also measured the amount of material in Jupiter's "torus," a 135,000-mile-thick, doughnut-shaped ring of sodium, sulfur and oxygen particles hurled into orbit around the planet by Io's volcanoes.
When Ulysses zoomed past Jupiter, two of its instruments found 30 percent to 50 percent less material in the torus, researchers said.
Ulysses also found the torus no longer forms a complete ring, "but seems to be broken up into fragments or patches," said Ed Smith, NASA's Ulysses project scientist. "The most obvious explanation is that Io is not nearly as active volcanically as it was during the Voyager flybys."
That could mean that fewer volcanoes are erupting on Io or that the eruptions may not be as strong.