This photo of a volcanic eruption on Jupiter's satellite Io (dark fountain-like feature near the limb) was taken March 4, 1979, about 12 hours before Voyager 1's closest approach to Jupiter. This and the accompanying photo present the evidence for the first active volcanic eruption ever observed on another body in the solar system. This photo taken from a distance of 310,000 miles (499,000 kilometers), shows a plume-like structure rising more than 60 miles (100 kilometers) above the surface, a cloud of material being produced by an active eruption. At least four eruptions have been identified on Voyager 1 pictures and many more may yet be discovered on closer analysis. On a nearly airless body like Io, particulate material thrown out of a volcano follows a ballistic trajectory, accounting for the dome-like shape of the top of the cloud, formed as particles reach the top of their flight path and begin to fall back. Spherical expansion of outflowing gas forms an even larger cloud surrounding the dust. Several regions have been identified by the infrared instrument on Voyager 1 as being several hundred degrees Fahrenheit warmer than surrounding terrain, and correlated with the eruptions. The fact that several eruptions appear to be going on simultaneously makes Io the most active surface in the solar system and suggests to scientists that Io is undergoing continuous volcanism, revising downward the age of Io's surface once again. JPL manages and controls the Voyager Project for NASA's Office of Space Science.