Io (moon of Jupiter)

Discovered by: Galileo Galilei, 1610
Distance from the Sun: 780,000,000 km
Distance from Jupiter: 422,000 km
Radius: 1815 km (Earth's moon: 1740 km)
Mass: 9 x 1022kg
Density: 3550 kg/m3
Major atmospheric constituent: Sulfur dioxide
Surface constituents: sulfur, silicon, sodium

Looking like a giant pizza covered with melted cheese and splotches of tomato and ripe olives, Io is the most volcanically active body in the Solar System. Volcanic plumes rise 300 kilometers (190 miles) above the surface.

The energy for all this activity derives from the gravitational forces between Io, Europa, Ganymede and Jupiter. These gravitational forces cause Io's surface to bulge up and down (or in and out) 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. 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.

In contrast to most of the moons in the outer solar system, Io is thought to be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from Galileo indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.

The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. 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.

Io has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur, mountains which are apparently NOT volcanoes, extensive flows hundreds of kilometers long of some low viscosity fluid (perhaps some form of sulfur), and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for Io's diverse 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. One current idea is that Io's lavas are molten silicate rock. Recent HST observations indicate that the material may be rich in sodium. Or there may be a variety of different materials in different locations.

Some of the hottest spots on Io may reach temperatures as high as 2000 K (1723 oC) though the average is much lower, about 130 K (-143 oC). The hot spots are the principal mechanism by which Io loses its heat.

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. Sulfur dioxide is the primary constituent of a thin atmosphere on Io.