Io
Io is the third largest of Jupiter's moons. It's mass is 8.93* 10^22 kg, and it has a radius of 1815 km. It is the innermost of the Gallilean moons.
Rather than resembling other moons in the Solar System, Io is similar to the terrestrial planets: Mercury, Venus, Earth and Mars. It is primarily made up of molten rock with an iron core, and scientists were surprised to discover there were very few craters on the surface. This indicates that the surface is very young. There are hundreds of calderas covering the surface, which are craters formed by an explosion or collapse of a volcanic vent. There are active volcanoes, which are spewing forth some form of sulfur, or sulfur dioxide. This discovery is the first real proof that other terrestrial bodies are hot and active.