Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede perturb Io's orbit into an irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces. Compare these tides on Io's solid surface to the tides on Earth's oceans. This tidal pumping generates a tremendous amount of heat within Io, keeping much of its subsurface crust in liquid form, seeking any available escape route to the surface to relieve the pressure. Thus, the surface of Io is constantly renewing itself, filling in any impact craters with molten lava lakes and spreading smooth new floodplains of liquid rock.
Sulfur dioxide is the primary constituent of a thin atmosphere on Io.
It has no water to speak of, 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. Io can develop 400,000 volts across itself and create an electric current of 3 million amperes. This current takes the path of least resistance along Jupiter's magnetic field lines to the planet's surface, creating lightning in Jupiter's upper atmosphere. This material becomes ionized in the magnetic field and forms a doughnut-shaped cloud of intense radiation referred to as a plasma torus.