Io is the fith of Jupiter's known satellites, and is a little bigger then earth's moon. Io is the innermost satellite of the galileo satellites.

Io was discovered by Galileo and Marius in 1610.

In contrast to most of the moons in the outer solar system, Io and Europa may 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.

Io's surface is radically different from any other body in the solar system. The satellite has very few impact craters, indicating that it's surface is still very young.
In stead of craters, Io has hundreds of volcanoes. This is a proof that the interial of the satellites is actually hot and active.

Io has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur (below right), mountains which are apparently not volcanoes extensive flows hundreds of kilometers long of some low viscosity fluid, and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for Io's varigated appearance.

The hottest spots on Io can reach a temperature up to 1500 K, the average temperature is much lower, somewhere around 130 K

Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.