Io (contd)
There are very few, if any, impact craters on Io. Therefore, the surface is very young.
Instead of craters, Voyager 1 found hundreds of volcanic calderas. Some of the volcanoes are active!. This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other "terrestrial" bodies are actually hot and active. The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. The volcanic eruptions change rapidly. In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.