Spacecrafts Shed Light on
By RICK CALLAHAN
.c The Associated Press
Astronomers probing the vast northern lights that ring Jupiter's north pole have found a mysterious X-ray ``hot spot'' that flares up like a beacon every 45 minutes.
Scientists said it could take years to explain this pulsating region, which its discoverers speculate may be related to bursts of radio waves that emanate from the giant planet at a similar interval.
``It came as a complete surprise, but scientists live for that. Sometimes the things that are most unexpected are the most important,'' said Christopher Russell, a University of California, Los Angeles professor of geophysics. Russell was not involved in the research.
The discovery of the hot spot is one of several surprises that have emerged from a unique opportunity scientists had last year to study Jupiter and the enormous magnetic cocoon that surrounds it.
The occasion was a space science first - the first time two spacecraft had visited Jupiter, or any outer planet, at the same time.
In early January 2001, the Galileo orbiter that has been circling Jupiter since 1995 and the Cassini probe, which swung past Jupiter on its way to Saturn, passed through Jupiter's magnetosphere - a zone of magnetically charged particles trapped within its magnetic field.
Images taken during the same period by the Hubble space telescope and the Chandra X-Ray Observatory, both in orbit around the Earth, complemented the spacecraft's observations.
Seven papers that arose from that data were highlighted in the Feb. 28 issue of the journal Nature, describing various aspects of Jupiter's magnetosphere and its interaction with the planet's upper atmosphere.
The data transmitted back showed, as scientists had predicted, that Jupiter's magnetosphere changes shape as it is buffeted by interplanetary shock waves created by the solar wind - the stream of particles thrown off by the sun. Earth's magnetosphere acts similarly.
But Jupiter's is more complex and far larger. At about 100 times as wide as the planet, it's so large that if it were visible to the naked eye, it would appear larger than the full moon to an observer on Earth. And its cometlike tail extends past the orbit of Saturn.
Randy Gladstone, a planetary scientist at the Southwest Research Institute in San Antonio who was lead author on the X-ray hot spot paper, said his team is still in the early stages of trying to figure out what is causing the pulsating spot.
He said the disturbance accounts for most of the X-ray emissions that are seen in Jupiter's northern auroras. Scientists had thought those emissions were produced by sulfur and oxygen ions blasted into space by Jupiter's volcanic moon, Io, and then energized by circulating through the Jovian magnetic field.
Now, it's not clear what is behind those emissions, Gladstone said. The evidence points to an origin much farther away from Jupiter, near the edge of its magnetosphere, where it meets the solar wind.
``Something is causing these emissions - from X-rays to ultraviolet to radio (waves). There's something connecting all of these emissions to have them happen over all these wavelengths,'' Gladstone said.
John Clarke, a professor of astronomy at Boston University, and colleagues authored another paper that reported the discovery of a ghostly glowing trail etched by Io into Jupiter's ionosphere, the region high in its atmosphere where auroras form.
His team also found that two of Jupiter's three other large moons - Ganymede and Europa - etch smaller, oval ``magnetic footprints.''
Scientists already knew Io, the most volcanically active body known in the solar system, was producing a similar footprint.
But Clarke said that since Europa and Ganymede do not have volcanoes but still produce footprints in Jupiter's ionosphere, it appears some unknown mechanism is causing all three moons to leave their mark there.
Jupiter's fourth large moon, Callisto, may cast such a spot, too. But whether it does may remain unanswered for years to come because no new missions to Jupiter are planned, and the Galileo spacecraft will end with a fiery plunge into Jupiter next year.
A proposed spacecraft that would be the first to visit Pluto, the outermost planet, would swing by Jupiter on its decade-long journey, giving it a chance to study the planet. But that mission's funding remains in question.
On the Net:
X-ray spot animation: http://pluto.space.swri.edu/yosemite/jupiter/chandra-hrc.html
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Researcher Steven L. Wilson, Sr