On March 8, 1979, NASA's Voyager 1 spacecraft discovered active volcanoes on Jupiter's moon Io.

Before Voyager 1 flew by Io, scientists believed the moon to be a dead world much like Earth's moon. The mission discovered the volcanoes somewhat by accident. Three days after Voyager 1 made its closest approach to Jupiter, mission controllers took pictures of Jupiter's moons to help determine the spacecraft's exact location. This process is known as optical navigation. While looking at the photos, a NASA engineer noticed a huge cloud protruding almost 200 miles above Io's surface. Scientists determined that this plume likely erupted from a volcano. Then the team went back to earlier Voyager images of Io and found seven more plumes.