Callisto is surrounded by an extremely thin atmosphere made out of carbon dioxide and probably oxygen molecules. There is also a rather intense ionosphere. An ocean is thought to exist within Callisto. This may mean that life might exist. However, this is less likely than on nearby Europa. Various space probes from Pioneers 10–11 to Galileo and Cassini−Huygens have studied the moon. Callisto is considered the most suitable place for humans to settle for future exploration of the Jovian system.