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A drawing of space near Jupiter, showing a portion of the radiation belts (in red), the Io torus (green) and the Europa torus (blue). The blue and green belts come from the atmospheres of the moons Europa and Io. The picture comes from measurements taken by the Cassini spacecraft.
Click on image for full size version (136K GIF)
Image from: NASA
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The Galileo mission discovered something amazing! Europa has its own atmosphere, although it is very, very thin. This atmosphere is created when fast moving molecules in Jupiter's magnetosphere hit the surface of Europa and knock out a water molecule. These molecules may float around Europa for awhile, but because of Europa's weak gravity, the "atmosphere" rapidly drifts away.
This picture shows what sometimes happens inside the magnetosphere of Jupiter (shape indicated by the white lines) when molecules from one of its moons float into it. They create a 'torus' (the blue and green belts that are shown in this picture).
Because Europa has an atmosphere with molecules that can escape, there is a donut-shaped belt of material inside Jupiter's magnetosphere that is called the "Europa torus". Jupiter's moon Io also has a torus. Even though Ganymede and Callisto also have thin atmospheres, they do not seem to produce torii in Jupiter's inner magnetosphere. This probably has something to do with the shape of Jupiter's magnetosphere.
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