Io is the fourth largest moon in the solar system, about the same size as Earth's. But, whereas Earth's moon (like most) is a boring ball of dirt, Io is bat guano insane, with over 400 volcanoes spewing plumes of material from its molten core as high as 500 km into space, creating a thin atmosphere of sulphur which disperses, due to Io's low gravity.
The volcanoes were first noticed by a navigation engineer named Linda Morabito when she was analyzing images sent from Voyager 1. It is also covered with mountains (most tectonic and not volcanic), some higher than any on Earth. It's also highly radioactive. And as pockmarked and hard to look at as it is, it has no known impact craters. Io remains difficult to look at for dermatosiophobes like myself. If you also have this probelm, maybe it will help to compare it to a moldy fruit.
It was first discovered in 1610 by Galileo Bonaiuti de' Galilei, an astronomer curiously referred to, in most cases, by his first name (like Bjork, Sadam, Lawrence, Madonna and Prince) -- a fact which I find fascinating. It's not as if Galileo is an overly common family name. Though named "Io" by Simon Marius in 1614, the moon was usually referred to as Jupiter I until the mid-20th century. Marius claimed to have discovered Io, in fact, a week before Galilei.
The name "Io" comes from one of the priestesses of Hera. Her father was Inachus, a river god who inaguarated the worship of Hera in Argos. As a hot young priestess, Io caught Zeus' eye. Zeus came to her nightly and begged her to meet him in a meadow for a fling. She told her father about having weird dreams and he reluctantly sent her away. An implacable serial cheater, Zeus disguised himself as a cloud so that he could get it on with Io without Hera noticing. He turned her into a cow to avoid Hera's suspicion. It didn't work. Hera was no slouch, used to Zeus going to all kinds of magical lengths to cheat on her, and when she noticed the earth blanketed in a thick fog with Zeus no where to be found in Olympus, she figured it out so she asked for the cow as a gift.
Hera then tied her to a tree and charged 100-eyed Argus Panoptes with guarding her. Zeus sent Hermes to lull Argus to sleep so that he might resume his affair with the girl. Instead, Hera set her free to graze and wander, driven and plauged by a vexatious gadfly. In her travels, she wandered out of Greece and her name was later applied to the Ionian Sea. After she crossed the Bosporous, she encountered Promotheus who -- despite having his liver pecked out repeatedly by an eagle -- comforted her with the knowledge that she'd become human again someday. When she got to Egypt, she gave birth to Zeus' son Epaphus, as well as a daughter, Keroessa. There she married the Egyptian king, Telegonus. Their grandson, Danaos, eventually returned to Greece with his fifty daughters (the world famous Danaids), as recalled in Aeschylus's play The Suppliants. Io's nickname was "the horned virgin."
As a pretty cool moon, Io has not surprisingly shown up in more than one work of television and film. The space western Outland was set there. In V, the baddies blew it up. On Red Dwarf, Judas Rimmer was raised there. It was home to an Earth Alliance colony in Babylon 5 with the Sol system's jumpgate orbiting it. There was a mining colony there in Escape from Jupiter. In that curious genre of dramatized, speculative sci-fi documentaries, the BBC made Space Odyssey: Voyage to the Planets. When an astronaut lands there to collect samples, he's overcome by exhaustion and, compounded by the risk of radiation exposure, the samples are abandoned.
Io also shows up in a few cartoons. In the Transformers episode "Five Faces of Darkness," Blurr and Wheelie were stranded there following an attack by the Deceptacons. In Heroic Age, in an attempt to knock Io out of orbit, a massive space weapon destroys Jupiter in the process -- (one of my biggest phobias). In Exosquad, Io is where the Exofleet is based.
And you gamers can get your Io on in Ultima II, where the player must travel to Io to complete the game. Many game programers revolve around Io's sulfur mines. Pipeline, level 11 of Descent, and a level of Disruptor are all set in those mines. Final Doom breaks the mold and includes a resarch base. Io is also featured in ZeroZone and Battlezone I.