|Discovered by||S. Marius
|Mean radius||421,600 km|
|Revolution period||1d 18h 27.6m|
|Inclination||0.040°||Is a satellite of||Jupiter|
|Equatorial diameter||3632 km|
|Surface area||41,000,000 km2|
|Mean density||3.55 g/cm3|
|Surface gravity||1.81 m/s2|
|Rotation period||1d 18h 27.6m|
|Atmospheric pressure||trace kPa|
The energy for all this activity probably derives from tidal interactions between Io, Europa, Ganymede, and Jupiter. The three moons are locked into Laplace-resonant orbits such that Io orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though Io always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters and generates heat through internal friction.
Io also cuts across Jupiter's magnetic field lines, generating an electric current. Though not a large source of energy compared to the tidal heating, this current may carry more than 1 trillion watts with a potential of 400,000 volts. It also strips ionized atoms away from Io at the rate of a thousand kilograms per second which form a torus of intense radiation around Jupiter that glows brightly in the ultraviolet. Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere, their outward pressure inflating it from within. Recent data from the Galileo probe indicate that Io may have its own magnetic field.
The location of Io with respect to the Earth and Jupiter has a strong influence on the Jovian radio emissions as seen from the earth. When Io is visible, radio signals from Jupiter increases considerably.
Some of Io's volcanic plumes have been measured rising over 300 km above the surface before falling back, the material being ejected from the surface at approximately one kilometer per second. The volcanic eruptions change rapidly; in just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.
Unlike most of the moons in the outer solar system, Io may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from the Galileo probe indicates that Io has a core of iron (perhaps mixed with iron sulfide[?]) with a radius of at least 900 km.
Io's surface is almost completely lacking in craters[?], meaning it must be very young. In addition to volcanoes, the surface includes non-volcanic mountains, numerous lakes of molten sulfur, calderas up to several kilometers deep, and extensive flows hundreds of kilometers long of low viscosity fluid (possibly some form of molten sulfur or silicate). Sulfur and its compounds take on a wide range of colors which are responsible for Io's variegated appearance.
Analysis of the Voyager images led scientists to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur. However, subsequent ground-based infrared studies indicate that they are too hot for liquid sulfur; some of the hottest spots on Io may reach temperatures as high as 2000 K (though the average is much lower, about 130 K). One current idea is that Io's lavas are molten silicate rock. Recent Hubble space telescope observations indicate that the material may be rich in sodium. There may be a variety of different materials in different locations.
Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.
Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not hot enough to do so farther out.