Stargazing UK A to Z. I is for...
Io is the innermost of what is termed the Galilean Moons of Jupiter (remember Jupiter has 69 moons!). It is slightly larger that Earth’s Moon. Io is similar in composition to the terrestrial planets, being composed primarily of molten silicate rock. Io’s surface is radically different to any other body in the solar system in that there are few, if any, impact craters. This means that the surface is very young. Instead, Io has hundreds of volcanic calderas, some of which are very active. Images sent back from the Voyager missions showed eruptions in progress - with plumes as high as 300km. This was the first real proof that the interiors of other terrestrial bodies are hot and active. Unlike the other Galilean satellites, Io has little or no water and its thin atmosphere is mainly composed of sulphur dioxide and perhaps some other gases.
Io orbits Jupiter every 1.77 days compared to our Moon orbiting us every 27.3 days - pretty quick hey!
Jupiter can be enjoyed along with the four Galilean moons through a telescope or binoculars. It is a very rewarding thing to see with your own eyes.