Jupiter's Storms and Rings
Using data from the Galileo Probe's plunge into the top cloud layers of Jupiter, Galileo has discovered that Jupiter has thunderstorms many times larger than Earth's. These storms result from the vertical circulation of water in the top layers, leaving large areas where air descends and becomes dry like the Sahara desert, and other areas where water rises to form the thunderstorms. Galileo has also found that Jupiter's rings are made of small dust grains blasted off the surface of Jupiter's four innermost satellites by the impacts of meteoroids.

Hot Active Volcanoes on Io
Now considered the solar system's most active body, Io's volcanoes were first discovered by Voyager 1 in 1979 and result from 100 meter (328 ft) tides in its solid surface. By taking Io's temperature with Galileo's instruments, scientists now know that some of Io's volcanoes are hotter than Earth's. From this, scientists surmise that lava made of silicate material rich in magnesium erupts from below Io's surface.

A Possible Ocean on Europa
Possessing more water than the total amount found on earth, Europa appears to have had a salty ocean beneath its icy cracked and frozen surface. Galileo images show ice "rafts" the size of cities that appear to have broken off and drifted apart, a frozen "puddle" smooths over older cracks, warmer material bubbles up from below to blister the surface, evaporative-type salts are exposed. A remarkable lack of craters show the surface to be relatively young.Europa has a thin oxygen atmosphere and an ionosphere.

Ganymede's Own Magnetic Field
Internal tidal friction again causes surprising effects on the solar system's largest moon. Galileo revealed that Ganymede has its own magnetic field. Perhaps from a slightly different orbit in its past, enough heat from tidal friction caused the separation of material inside Ganymede and this stirring of a molten core or iron sulfide is believed to generate Ganymede's magnetic field.

Does An Ocean Hide Beneath Callisto's Surface?
There is evidence to support the existence of a subsurface ocean on Callisto. The ocean would have to be deep enough inside the moon that it does not affect the heavily cratered surface on top. Instead the ocean could be showing itself indirectly, through the magnetic field it generates. This could come from electric flow in a salty ocean generated by Jupiter's strong magnetic field passing through it.