A Planetary World in the Making. Io is one of the four Galilean satellites of Jupiter i.e. the larger moons of the gas giant. Being the nearest moon to Jupiter, Io endures gravitational stresses bringing to internal heat and volcanoes. Io is mostly plains and mountains with a surface younger than a million years as the moon is constantly resurfaced. Volcanic plumes are often seen at this moon. One is conspicuous here, on the limb, rising 86 miles (140 km) over Pillan Patera as another is seen only as a shadow, right of Prometheus, the circular feature, center, near the terminator. Such an active world might be a good picture of how primitive planets are looking, with an intense geologic activity, creating an atmosphere or first oceans at the surface. Io is about the size of Earth's Moon. This picture is part of the work Galileo mission performed at Jupiter system between 1995 and 2003, as the former Galileo site was recently transformed into the "Galileo Legacy Site" providing a wealth of data and pictures. picture courtesy University of Arizona / LPL