Note: A little larger than Earth's moon, Io is the third largest of Jupiter's moons, and the most volcanically active body in the solar system. The large moons Europa and Ganymede perturb Io's orbit into an irregularly elliptical one and with its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces which cause its surface to bulge up and down by as much as 100 meters (330 feet). This tidal pumping generates a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form. Sulfur dioxide is the primary constituent of Io's atmosphere and it has no water unlike the other, colder Galilean moons. Data from the Galileo spacecraft indicates that an iron core may form Io's center, thus giving Io its own magnetic field.