The Galileo mission discovered that Ganymede not only has a magnetosphere
, but its own atmosphere as well, although it is very, very thin. The atmosphere is created when molecules from the magnetosphere, moving very fast, hit the surface and knock out a water molecule. Because there is a magnetosphere, the atmosphere of Ganymede does not simply ‘float away’ like the atmosphere of Europa does.
One of the instruments on the spacecraft measured a stream of hydrogen molecules flowing away from the moon over its north pole. Another instrument measured the presence of an Ionosphere, a part of the atmosphere that is filled with ions. If molecules are leaving Ganymede all the time, that means they have to be replaced all the time, and that means there must be an active process that is creating the atmosphere. This image from the Hubble Space Telescope shows ozone on Ganymede. Ozone is an important constituent of the Earth’s atmosphere.
Even though Ganymede and Callisto have thin atmospheres, there does not seem to be a Ganymede or Callisto torus, the way there is at Europa and Io.
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