Jupiter and Io
This image of Jupiter and its moon Io was taken by the Cassini spacecraft. Io is the closest of Jupiter's large moons. In fact, it is so close to the giant planet that tidal gravitational forces distort the shape of the moon, generating tremendous amounts of heat. This heat melts the inside of the moon and creates volcanic activity. The Voyager spacecraft was the first to spot active volcanoes on the surface of Io. The small moon is so active that its surface features change drastically over short periods of time.
NASA / JPL / University of Arizona