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Galileo spies Io's light show.

Eerie flashes of blue, green, and red loom on the horizon of a tiny body 600 million kilometers from Earth. Images from the Galileo spacecraft reveal that Jupiter's moon Io makes a spectacle of itself in more ways than one: The most volcanically active body in the solar system also generates the most dazzling auroras.

Like auroras on Earth, those on Io are produced when electrons, directed by a strong magnetic field, crash into gases in the moon's atmosphere. Jupiter's intense magnetic field provides the driving force, as well as maintaining a doughnut-shaped reservoir of charged particles that bathe Io.

In the Aug. 6 SCIENCE, Paul E. Geissler of the University of Arizona in Tucson and his colleagues analyze the auroral images.

A bright-blue glow, centered on the equator and extending several hundred kilometers above the moon, emanates from volcanic plumes. This glow probably arises from electrons colliding with sulfur dioxide gas spewed by the volcanoes. A weaker, red glow, which is brightest near the north pole, may stem from electrons striking oxygen atoms. A faint green glow, concentrated on the moon's night side, may be due to electrons slamming into sodium atoms.

Galileo viewed the auroras several times when Io was in Jupiter's shadow. The red and green glows dimmed during these eclipses. In the absence of sunlight, atmospheric gases freeze onto the moon's surface and electrons have fewer particles with which to collide, Geissler's team explains.

To the team's surprise, however, the blue glow intensified. The group traces this effect to the large electric current that flows from Io to Jupiter. Jupiter's magnetic field, rotating with the planet, generates the current as it sweeps past the moon.

This giant circuit normally passes through Io's atmosphere, but an eclipse may modify that route, the team theorizes. As the darkened atmosphere thins and becomes a poorer electrical conductor, the circuit may connect instead through the moon's interior, passing through volcanic plumes and enhancing their bluish glow.

The Saturn-bound Cassini craft may snap images of the auroras when it passes by Jupiter in late 2000 and early 2001.
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Title Annotation:spacecraft sends images of Io's aurora
Publication:Science News
Article Type:Brief Article
Date:Oct 2, 1999
Words:347
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