Io is one of Jupiter's large moons. Io is the closest large moon to Jupiter. It is a bit larger than the Earth's Moon.
Just like Earth's Moon, Io is tidally locked to its planet. This means it makes an orbit of Jupiter in the same time it takes to rotate once. This means that it always shows the same side to Jupiter.
The orbit of Io is very elliptical. Sometimes the moon is very close to Jupiter and sometimes it much further away. This large change in distance has an effect on the moon. When it is close to Jupiter it feels a much stronger gravitational pull from the planet. The changes in gravity pull at the moons surface. It is thought that this is why the moon is so active.
Io is the most active object in the Solar System. Its surface is covered with hundreds of volcanoes. There are huge numbers of lava flows across the surface. When the Galileo spacecraft passed the moon it was able to take images of this activity.
Io's surface is also dotted with more than 100 mountains. Some of these them are taller than Earth's Mount Everest.
The volcanoes on Io pump out sulphur. The plumes can reach a height of up to 500 km (310 miles) above the moon's surface. All the gas released from the volcanoes has created a thin atmosphere around Io. But it would be far too poisonous for people to breathe. It is very unlikely that life exists on Io.