Io

"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. 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. 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.

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.

History
During the the Red War, Io became a warzone for Guardian, Red Legion, Vex and Taken forces. 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 like Oryx had with the Vault of Glass. The exact purpose of the Pyramidion is unknown, but it is believed to be merely one of many entry points into Vex space, where one of their Axis Minds performed experiments on captured Guardians.

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.

Locations

 * Cradle
 * Echo Mesa
 * Javelin-4
 * Lost Oasis
 * Grove of Ulan-Tan
 * Spine Burrows
 * Terrabase Charon
 * The Wraith Mines
 * The Rupture
 * Aphix Conduit
 * Pyramidion
 * Sanctum of Bones
 * The Endless Gate
 * Giant's Scar
 * Excavation Site II
 * Warmind Vault JYS-2

List of appearances

 * Destiny
 * Age of Triumph
 * Destiny 2

Trivia

 * In Destiny one Ghost states during the loading screen while on your way to meet the Queen of the Reef for the first time that the asteroid belt/the Reef was the last place the Light reached yet in Destiny 2, Io is stated to be the last place. This may be a developer oversight.