The Galileo mission discovered an amazing thing: Callisto seems to have its own atmosphere, although it is very, very thin, and may come and go with time. The atmosphere is created when molecules from the magnetosphere, moving very fast, hit the surface and knock out a water molecule.
One of the instruments on the spacecraft measured the presence of an Ionosphere, a part of the atmosphere that is filled with ions. But it wasn’t always there.
Even though Ganymede and Callisto have thin atmospheres, there does not seem to be a Ganymede or Callisto torus, as there is at Europa and Io.
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