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Io (also called Jupiter I) is one of the four major moons of Jupiter.
In the final days of the Golden Age the Traveler was in the midst of terraforming Io when it detected the Darkness approaching; The Traveler began to move to Earth in a defensive posture, leaving Io half-completed and flooded with the Traveler's formatted energies. As a result of this, the Ionian climate and geography do not support traditional agriculture, meaning that some remnant of Io's original atmospheric composition prevented the full growth of plants. The moon is still charged with the Traveler's pure energy, which lingers in the atmosphere of the moon, visible from orbit.  Echo Mesa, a mountain on Io, has become a popular pilgrimage site for Guardians. The Mesa overlooks a crater where the Traveler left behind a mysterious city-like structure.
The terrain of Io is covered in fossils, many of which closely resemble enlarged versions of ancient Earth organisms such as trilobites, ammonites and dinosaurs. It is unknown whether these fossils are a by-product of the Traveler's paracausal terraforming, or whether they point to some unrecorded history of transplantation between Earth and a more habitable Io in the distant past.
During the Red War, Io became a warzone for Guardian, Red Legion, Vex and Taken forces. When the Red Legion arrived in the Sol System they set up at Terrabase Charon, on Echo Mesa and were met with resistance from the Vex and Taken forces there. Io was the final place that the Traveler influenced with Light in the Sol System, and as such is sacred ground to Warlocks. During the conflict, The Guardian travels to Io on Zavala's orders to find Ikora. After locating the Warlock Vanguard leader, Ikora sends the Guardian on several missions into a Cabal research base, ultimately revealing the Red Legion's plans for the Traveler and the Light. The Cabal were mining out the remaining Light energy from the Traveler, which attracted the forces of the Taken and forced the Cabal to evacuate. During the Guardian's time on Io, they also meet Asher Mir, an Awoken scientist fixated on the Vex, along with their activities on the planet.
A Vex structure known as the Pyramidion is found there, which was fought over by the Taken in another attempt to infect the Vex network like Oryx had with the Vault of Glass. The Pyramidion is described by Asher Mir and Ikora as a "conversion engine", responsible for converting Io into a Vex Machine World and is believed to be one of many entry points into Vex space, where one of their Axis Minds performed experiments on captured Guardians. The Pyramidion is only accessible during the Curse of Osiris story mission, Deep Storage and in the strike, The Pyramidion.
A hidden Warmind vault, JYS-2, was buried beneath Io's surface and inadvertently uncovered by one of the Cabal's drills. An unidentified active AI - implied through various details to be Rasputin - was still using the facility. Instead, it communicated via pre-Golden Age music and philosophical quotations, which proved to intrigue the Vex. This can be observed in the Adventure Arecibo.
Following the destruction of The Almighty, a Pyramid touched down on the moon, disabling Rasputin and positioning itself above the Cradle, where a Tree of Silver Wings lay inside. It is unknown if the tree was always there or if the Pyramid spawned it.
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