Scientists discover key to Io light show
The Galileo spacecraft captured the diffuse optical emissions from Io while the moon was eclipsed by Jupiter
August 5, 1999
Web posted at: 5:38 p.m. EDT (2138 GMT)
By Robin Lloyd
CNN Interactive Senior Writer
(CNN) -- A pearly electric light show at Jupiter's moon Io, so vibrant it could short circuit several planetariums, ebbs and flows as a result of the jovian satellite's tricky atmosphere, a scientist said Thursday.
Paul Geissler of the University of Arizona and his colleagues studied images of the rocky, volcanic moon -- about the size of Earth's moon -- taken by NASA's Galileo orbiter when Io was eclipsed by Jupiter. The spacecraft returned images of auroras, phenomena that had distracted scientists in previous attempts to study the surface of Io.
"They are things you could actually see if you were hanging on to Galileo and were looking over its shoulder," Geissler says.
The data give scientists insight into Io's complex atmosphere, which is influenced by its wealth of active volcanoes and proximity to Jupiter -- Io is the innermost of Jupiter's four largest moons.
By viewing images through filters that picked up different light wavelengths and colors, Geissler's team singled out glowing blue gas plumes at Io's equator, a fainter red glow along the moon's northern edge and a green glow against the night side of Io.
The results were published in the latest issue of Science magazine.
Auroras, like the aurora borealis occasionally visible in Canada, occur when electrons strike atmospheric gas molecules, causing them to glow.
An enhanced color image of Io taken by Voyager
At Io, the blue light is the result of supersonic gas plumes associated with the moon's many volcanoes, reaching up as high as 800 km (500 miles) above Io's surface, Geissler said. Jupiter's magnetic field sweeps like a magnet through Io's atmosphere, stripping away electrons.
"What you're looking at with the blue is the terminals of this huge electrical generator that has a potential across it of 400,000 volts and a power dissipation of a trillion watts, more than the power output of all the electric generation stations in the United States," Geissler said.
The red component of Io's auroras, thought to indicate the presence of oxygen, is sensitive to a donut-shaped cloud of charged particles that surrounds Jupiter at an angle and tugs waves of light back and forth between Io's north and south poles, Geissler said.
The Io aurora's green light, observed only once and thought to indicate the presence of sodium, showed up only on Io's back side where the atmosphere dissipates, Geissler said.
Scientists first discovered the auroras last October but now understand why they fluctuate and darken when Io goes into Jupiter's shadow.
The tenuous atmosphere of Io, made up of gases that upwell off of Io's frosty surface when it is hit by sunshine, actually collapses in the ecliptic darkness, Geissler says.
Geissler and his colleagues relied mainly on data collected by NASA's Galileo spacecraft on May 31, 1998.
The spacecraft, which has been orbiting Jupiter and studying the planet and its moons since 1995, is being prepared for its closest encounters ever with Io (as close as 300 km) in October and November, passing repeatedly by Jupiter's moon Callisto to bring its orbit closer to Io.
Scientists are anxious about those events, as the intense radiation that jets between Io and Jupiter could damage the spacecraft. Also, they hope to make up for a lost opportunity in December 1995 when the spacecraft passed directly by Io but failed to collect data because of a broken tape recorder.
"I'll be fair and say that somewhere out in jovian magnetosphere there's a charged particle with our name on it and that's ultimately going to be the demise of Galileo," Geissler said.
But Galileo is an "old-timer," built of big, sturdy electronics in the 1970s, so Geissler is optimistic.
"It's certainly an important thing to get back to Io," he said. "We've made so many discoveries about Io from so far away."
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