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A schematic of the environment nearest Jupiter, showing magnetically trapped particles in a portion of Jupiter's extensive radiation belts in red, the neutral gas of the Io torus in green, and the neutral gas of the Europa torus in blue. The picture is derived from data 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 an amazing thing. 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. This process is known as "sputtering". The surface may also evaporate, or "sublimate" when the Sun is shining, the way a comet does. Because there is little gravity however, a small moon cannot hold onto an atmosphere for very long.
Water molecules lost from the surface are quickly separated into the constituents oxygen and hydrogen. These molecules can also be quickly ionized by ultraviolet radiation and charged particles in the vicinity. Thus Europa has a neutral atmosphere as well as an ionosphere. These molecules may float around Europa for awhile, but because of Europa's weak gravity, the "atmosphere" rapidly drifts away.
A neutral component of oxygen near Europa was detected by the Cassini spacecraft as it flew by Jupiter in the year 2000, and is illustrated in the drawing to the left. This suggests that the components of Europa's atmosphere that float away create a torus of material inside Jupiter's magnetosphere. This belt of material is called the "Europa torus". 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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