Jupiter from Io. Computer artwork of a view across a pool of lava on the surface of Io, towards Jupiter. Io is the third largest of the moons of Jupiter, and also one of the closest (422,000 kilometres). The powerful Jovian gravity powers volcanoes, forming surface hotspots so that surface temperatures vary greatly, ranging from 2000 to -140 degrees Celsius. There are places on Io where molten lava has been observed actively welling up from beneath the surface of this otherwise frozen world.