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Probably the Voyager image that stunned scientists the most was that of an active volcano on Jupiter's moon Io. The Pioneer probes had detected volcanoes on Io but none were believe to be alive.
Here's a closer look at an inactive volcano on Io. The moon has about 100 times the volcanic activity as Earth.
Scientists made another major discovery on Io, called the Io torus, a ring of ionized sulfur and oxygen which comes from the moon and adds to Jupiter's magnetic field.
This was the closest view of the old and massive storm that gives Jupiter its trademark red spot. The Great Red Spot has been raging on Jupiter for at least 400 years when humans first observed the giant planet. Three Earths can fit into the storm, which is similar to a hurricane. The Voyagers discovered many hurricane-like storm systems rampaging in Jupiter's atmosphere.