Jupiter from Io, artwork. Io, the third largest of the moons of Jupiter, is also one of the closest (422,000 kilometres). The powerful Jovian gravity powers volcanoes on Io, forming surface hotspots so that surface temperatures vary greatly, ranging from 2000 to -140 degrees Celsius. Io has a thin atmosphere of sulphur dioxide and a gravity that is less than a fifth of that on Earth. Jupiter, a gas giant of hydrogen and helium, is the fifth planet from the Sun. Another of its moons is seen at upper right.