Io is about the size of the Earth's moon but geographically very different. It is one of the solar system's most active bodies, with over 300 volcanoes and fissures. Io's proximity to Jupiter and its eccentric orbit combine to exert gravitational stresses on its molten interior, making it extremely active volcanically. Io's surface appears yellow because it is covered by sulphurous volcanic deposits and lava flows. Here, a large volcanic plume is erupting, its blue hues the result of sulphur dioxide condensing as it cools. The picture was taken by the Galileo space probe from a distance of about a million kilometers.