Io is a large, rocky, active Moon orbiting the planet Jupiter. It's the innermost of Jupiter's large Moons (the Galilean Moons) at a distance of 220,000 miles (422,000 km). Io is very close in size to the Earth's Moon with a diameter of 1,942 miles (3,636 km). Io's orbiting period around Jupiter is 1.77 days. Io was discovered on January 8, 1610 by Galileo Galilei and is the most volcanically active body in the solar system. These and other features are what make Jupiter's moon Io unique in the solar system.
A little bit larger than the Earth's Moon, Io is the third largest of Jupiter's moons, and the most geologically active. Io is primarily composed of silicate rock surrounding a molten iron core. The majority of Io's surface is covered by sulfur and sulfur dioxide frost. Io's volcanic eruptions and lava flows produce a variety of color (red, white, black, green) shades on its surface. It is believed, due to the gravitational perturbations detected on the Galileo spacecraft, that Io could have an iron or iron sulfide core of 1800 km (1116 miles) diameter. A decrease on Jupiter's magnetic field suggests a magnetic field on Io, as well.
What makes Jupiter's moon Io unique in the solar system is that it presents the most active volcanic activity in the planetary system, exceeding the one shown in other planets, including Mercury, Earth, Mars and Venus. This was first discovered during the Voyager mission in 1979. Eight volcanic eruptions were seen during the first mission; however, it is known to have more than 400 hundred active volcanoes, making it the most geologically active body in the solar system.
This geologic activity is caused by the extreme temperatures generated at its core which is the result of the strong gravitational pull due to its proximity to Jupiter and its huge gravitational field. Jupiter exerts a strong gravitational pull on Io; however, Europa and Ganymede, other Galilean moons, also interact gravitationally with Io. In the same way in which the Earth's Moon exerts a gravitational pull on the oceans, causing tides, Jupiter pulls on the side facing Io, stretching Io's surface to a sort of egg-shape.
On its orbit around Jupiter, Io is further pulled by the gravitational forces of Europa and Ganymede, which causes Io's surface to bulge up and down. The bulging on both sides produces solid ground tides on its surface as high as 300 meters (186 miles). Unlike the tides on Earth, where the Moon's gravity is the result of a one way pull, on Io, the pull of gravity is produced on either side, leading to tremendous friction and melting of its interior, allowing Io to remain molten and maintain tremendous high temperatures.
This bulging and bending on Io's surface causes extreme accumulations of heat in its interior, producing the melting and boiling of the inside material. The molten material tries to find its way out from Io's interior, blowing away holes on its surface, and producing high altitude eruptions of hot gas plumes. The tidal effects on Io and the capability to produce enough heat to cause volcanic activity is what makes Io unique among other objects in the solar system.