Io

Resembling a giant pizza covered with melted cheese and splotches of tomato, ripe olives and basil, Io is the fourth largest and most volcanically active Galilean Moon in the Solar System.

A bit larger than Earth’s moon and with over 400 active volcanoes and Volcanic plumes of sulfur and sulfur dioxide that rise over 300 kilometers (190 miles) above the surface, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Unlike most satellites in the outer Solar System, which are instead mostly composed of water ice. Io’s surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon’s silicate crust. Some of these peaks are taller than Earth’s Mount Everest. The materials produced by this volcanism provide material for Io’s thin, patchy atmosphere and Jupiter’s extensive magnetosphere.

Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede perturb Io’s orbit into an irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces. These forces cause Io’s surface to bulge in and out by as much as 100 meters (330 feet)! This tidal pumping generates a massive amount of heat within Io, keeping much of its subsurface crust in liquid lava form seeking any available escape route to the surface to relieve the pressure. 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. The composition of this material is not yet entirely clear, but theories suggest that it is largely molten sulfur and its compounds (which would account for the multiple coloring) or silicate rock.

Io’s orbit, keeping it at more or less a cozy 422,000 kilometers (262,000 miles) from Jupiter, cuts across the planet’s powerful magnetic lines of force, where Io receives about 3,600 rem of radiation per day, turning it into a electric generator that can develop 400,000 volts across itself and create an electric current of 3 million amperes. This current takes the path of least resistance along Jupiter’s magnetic field lines to the planet’s surface, creating lightning in Jupiter’s upper atmosphere.

In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io’s interior structure and surface composition. These spacecraft also revealed

Further observations have been made by Cassini–Huygens in 2000 and New Horizons in 2007, as well as from Earth-based telescopes and the Hubble Space Telescope as their technology has advanced.