Io is the closest of the four large moons of Jupiter.
Due to the strong gravitational pull of the parent planet, Io is one of the most volcanically active objects in the Solar System. Its appearance is due to over 400 active lava flows and volcanoes, some on which have even been seen by passing spacecraft.
The volcanoes pump out sulphur and sulphur dioxide to a height of up to 500 km (310 miles) above the moon's surface, and have given Io a thin atmosphere. However, the atmosphere would be far too poisonous for people to breath.
Close-up, we see that Io's surface is also dotted with more than 100 mountains, some of which are taller than Earth's Mount Everest.