Io, moon of Jupiter, was discovered by Galileo Galilei on January 7, 1610 while he was inventing the telescope. Io is fifth in distance from Jupiter and third largest of its moons. It is one of the most volcanic bodies known in the solar system. It surface has lava flows and lava lakes. Volcanic geysers that rise over 500 kilometers occur on the surface of the moon. Some of its mountains are 52,000 feet high, much taller than Mt. Everest’s 29,028 feet. Io is one of the Galilean moons, which are the largest of Jupiter. Io is closest to Jupiter and most dense of the Galilean moons. Io is yellow, brown, and red in color. This is caused by sulfur on its surface.
The Danish astronomer Ole Romer made the first estimate of the speed of light using the Galilean moons (ones discovered by Galileo—Io, Europa, Ganymede, Callisto) and the shadow of Jupiter. The French astronomer Pierre-Simon de Laplace discovered that the orbits of Io, Europa, and Ganymede are in an approximate 1:2:4 ratio. This means that for every orbit of Jupiter by Ganymede, Europa orbits twice and Io orbits four times.
In 1979 the Voyager satellite took pictures of Io. The mean density (3.5 g/cm^3) of Io was calculated using these pictures. Based on this density, the gravity of Io, and analysis of the surface, Io is considered to have an iron sulfide core. Io is an average of 421,600 kilometers from Jupiter. Its escape velocity is 2.56 kilometers per second. Its mean orbital velocity is 17.34 kilometers per second. The mean surface temperature is -143 Celsius. This is 143 degrees below freezing, so it is very cold on Io. Its radius is 1,815 kilometers, which is a little less than one-third that of Earth’s.
Some of the volcanoes on Io are the size of California. The temperatures exceed 1527 Celsius, which is 15.27 times hotter than the boiling point of water (100 Celsius). The volcanoes are caused by the gravitational fields of Jupiter, Europa, and Ganymede. Io is the most volcanically active body in the solar system.
The satellite Galileo was launched aboard Space Shuttle Atlantis on October 18, 1989. On May 3, 1996, NASA’s satellite Galileo found that Io’s diameter is about one-half iron core. It might also have a mantle of molten rock. It also found that it might have its own magnetic field. It was correct; Io can develop 400,000 volts of electricity and an electric current of 3 million amperes.