Galileo’s telescopic observations of Jupiter in 1610 revealed the presence of four moons orbiting the planet; Io, Europa, Ganymede, and Callisto are now collectively known as the Galilean satellites. In March 1979, the Voyager 1 spacecraft discovered nine active volcanos erupting material up to 100 kilometers (60 miles) above the surface of Io, an object comparable in size to Earth’s Moon. Nearly 17 years later, the Galileo spacecraft (in orbit around Jupiter since 1995) observed some of those same volcanos still active, while others had ceased erupting and new volcanic centers had started erupting. This striking image of Io was taken on September 7, 1996, with the clouds of Jupiter as a backdrop. The bright ring around a dark spot near the center of the disk is the product of one of the active eruptions; the other dark spots may be volcanos that were not active during the spacecraft encounters. The unusual coloration of Io is largely a result of variations in the color of sulfur that is widely distributed across its surface. The power source for this tremendous volcanic activity is a flexing of the crust and mantle of Io resulting from competing gravitational pulls exerted by Jupiter, Europa, and Ganymede.
Galileo Press Release P-47971.