Io, one of the most volcanically active satellites in the solar system is in the orbit of Jupiter and was discovered by Simon Marius and Galileo Galilei in 1610. Io is about the size Mercury with an orange color. It has high reflectivity caused by the formation of a crystalline layer that originated from the many volcanoes on the surface. Observations on Earth detected sodium vapor emissions that formed a sodium vapor cloud extending 16,000 kilometers (10,000 miles) from the surface.
Io has an ionosphere detected by the passage of radio waves through its surface. The ionosphere measures to be about 700 kilometers (435 miles) at the time when Pioneer 10 made its flyby.
The surface temperature of Io is about -143° C (-230° F) with varying temperatures around the active volcanic regions. One "hot spot" on Io's surface has a temperature of about 17° C (60° F), probably a non-molten lava lake.
The volcanoes, the most interesting feature of Io, are caused by a complex gravitational interaction between Europa, Ganymede and Jupiter's tidal forces. The results can often be seen by large bulging, as great as 100 meters (330 feet) on the surface.
Some prominent surface features include the Loki Patera, P3 Prometheus, Haemus Mons, Volcano Pele and the Ra Patera. The Loki Patera is a large dark area composed of a lake of liquid and solid sulfur. The P3 Prometheus, Volcano Pele and the Ra Patera are some of the largest volcanoes on Io's surface. The Haemus Mons is a large mountain located near the terminator of Io.
Io 
