Closeup of Io
This incredibly detailed view of Jupiter's moon Io was taken by NASA's Galileo spacecraft. The clouds of Jupiter can be seen in the background. Io is the most volcanic object in the Solar System, and is the only object besides Earth known to have active volcanoes. The amazing colors are caused by molten sulfur and sulfut deposits. An active volcano known as Prometheus can be seen just to the right of the center. Galileo was only 564,000 miles (908,000 kilometers) when this image was taken.
NASA / JPL / Ames Research Center