Io

Io
Astrographical
Star, position:	Sol, Jupiter
Physical
Diameter:	3642.58921
Gravity:	0.183 g
Length of day:	1.77 (synchronous)
Atmosphere:	Sulfer dioxide (SO2) ; Sulfur monoxide (SO) ; Sodium chloride (NaCl) (pre-Traveler)
Surface temperature:	-202°F
Societal
Demonym:	Ionian
Species:	Vex ; Cabal ; Taken
Government:	Red Legion, Vex, and Taken occupation

There is more information available on this subject at Io on the English Wikipedia.

"Io was the last place visited by The Traveler, and the site of great interest to both humanity and the timeless Vex machines. The Red Legion, too, now seek to unravel the secrets and power of this place, gouging at the surface, seeking to uncover new sources of power to aid in their quest to dominate our worlds. It is up to you to protect this place, and preserve its great legacy."

— Website description

Io (also called Jupiter I) is one of the four major moons of Jupiter.

Overview

In the final days of the Golden Age the Traveler was in the midst of terraforming Io when it detected the Darkness approaching; abandoning the moon as it fled to Earth.^[2] 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. ^[2] Echo Mesa, a mountain on Io,^[3] has become a popular pilgrimage site for Guardians.^[2] The Mesa overlooks a crater where the Traveler left behind a mysterious city-like structure.

One of the legends of the Nine state they are some of the original Awoken transcended to mental form and live inside the Jupiter-Io flux tube.^[4]

History

During the the Red War, Io became a warzone for Guardian, Red Legion, Vex and Taken forces. When the Red Legion arrived in the solar system they set up a base, Terrabase Charon, on Io 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 Solar 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 active AI was still using the facility but did not identify itself. 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.