In 1979, the NASA probe Voyager 1 passed the innermost of the Galilean moons, Io, and spotted something remarkable: hot lava volcanoes. It’s the only body in the Solar System other than our own world to have these fiery features and, in terms of volcanism, it makes Earth look lazy. As the Solar System’s most volcanically active body, Io is host to over 400 active volcanoes spewing 100 times more lava each year than all the volcanoes on Earth. These are more or less evenly spread across Io’s surface, supplied with lava from an ocean of magma 30-50 kilometres (20-30 miles) beneath its crust.
Unlike Earth, whose volcanism results from massive internal pressure, Io’s geological activity comes from the influence of Jupiter’s enormous gravity, which squeezes and stretches the moon in its eccentric orbit, generating friction and heat, keeping the rock inside Io molten.