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Thursday, August 5, 1999 Published at 19:45 GMT 20:45 UK


Io's ghostly light show

Astronomers are getting a new insight into Io's atmosphere

By BBC News Online Science Editor Dr David Whitehouse

It may have a little name but Io, a moon of Jupiter, has the most dazzling light show in the Solar System.

Curtains of red, blue and green hang in a dark sky over an alien landscape of deep red and dirty yellow icefloes. This is the image astronomers paint of the large rocky moon in a new analysis of its atmosphere, published in Science magazine.

[ image: Io's curtains of colour]
Io's curtains of colour
The remarkable Io is an active world because of its close proximity to Jupiter. Gravitational forces raise huge tides that pump energy into the moon's interior, driving large volcanoes. Io is the most volcanically-active world in the Solar System.

But these volcanoes are like nothing seen on Earth. Io's volcanoes are composed of plumes of sulphur that spray the surrounding landscape with lurid chemical colours.

The plumes play a role in producing Io's wonderful auroral lights. Showers of electrons striking the moon's thin atmosphere give rise to a blue glow. Red light comes from oxygen in the atmosphere, and green light from sodium.

Jupiter eclipse

The team of scientists reporting in Science analysed auroral images of Io taken by the Galileo spacecraft. The pictures of colourful light emissions on the moon were taken during an eclipse by Jupiter.

"This is our first detailed look at visible aurorae on a Solar System satellite", said Paul Geissler of the University of Arizona, lead author of the report. "The pictures help us to understand Io's atmosphere and the processes that generate the emissions."

[ image: Io: The most volcanically-active world in the Solar System]
Io: The most volcanically-active world in the Solar System
Io's eerie glow dims noticeably when the moon lingers in Jupiter's shadow. The likely explanation, concludes the team, is a partial collapse of the moon's atmosphere.

Some of Io's atmosphere comes from sulphur dioxide ice on the surface that is warmed by the Sun and evaporates. This gas probably begins to recondense during eclipse.

Surprisingly, the blue glows associated with volcanic plumes appear to intensify while Io is in darkness. In recent days, astronomers monitoring Io have reported an outburst, probably from a volcanic eruption.

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