Months later on January 17, 1997, Galileo made its closest flyby of Io, the most volcanically active moon in our solar system.

Almost no craters are visible on its surface because Io is a hotbed of volcanic activity, flowing with eruptions of sulphur compounds. The same forces that keep Europa's interior liquid keep Io's geology active: tidal heating from the ever changing distances and angles between Io and Jupiter.

Volcanic cones cover five percent of Io's surface. The Voyager 1 space probe found eight of them to be active during its brief observations alone. Click here to see Io through the eyes of Voyager 1, as the spacecraft passed by it in March 1979.