This picture shows Jupiter seen from the surface of its closest Galilean moon, Io. Because Io is so close to Jupiter, it is constantly engaged in a tug-of-war with the planet's considerable gravity. On Earth, the Moon's gravitational pull, and to a lesser extent the Sun's, are responsible for the oceans' tides. However, on Io, the Jupiter-induced tides are so strong that the little moon is literally being turned inside out — in the form of volcanoes. Io is the most volcanically active object known to exist in the entire Solar System. A slightly modified version of this image made the cover of the May 2000 issue of the US magazine Asimov's Science Fiction.