- “Another whisper, sir, near Io. We have probes en route.”— Cortana
- Main article: Jovian Moons campaign
In 2160, before the formation of the UNSC, the Frieden, a resurgent fascist movement springing from anti-Koslovic sentiment in the Jovian Moons was formed to achieve peace by destroying the "oppressors on Terra Firma" and it attacked the UN Colonial Advisers on the moon, sparking the Jovian Moons campaign, the Rain Forest Wars, a series of clashes of Mars, and the Interplanetary War. The Frieden and the Koslovics were defeated, and the moons returned to the control of the UNSC. This conflict was called the Jovian Moons campaign.
The Galilean Moons
- Main article: Europa
The fourth largest, yet only fully terraformed Jovian moon. It features a frosty atmosphere. Although scientists had theorized that it might already contain oceans beneath the surface with amino acids and the building blocks of life, the first colonists found that this was not the case. Colonization occurred without the discovery of any new life forms.
The SPARTAN-II super-soldiers had a mission on Europa some time before 2552. Europa was left unscathed when the Covenant attacked the Sol system, as it had little military value to the UNSC.
In the post-war years, Europa was home to the Watershed Division, a private corporation contracted by ONI. The MJOLNIR/Prefect armor was developed by the Watershed Division and tested in Denegroth, West Conamara.
- Main article: Io
The third largest Jovian moon. This is where the Frieden first attacked the United Nations. Despite being massively volcanic and atmospherically volatile, Io supports a large colony of Humans. The fleet of the High Prophet of Regret passed near Io where they were detected by Io Station, just minutes before the Battle of Earth, allowing UNSC forces to muster a timely response.
- Main article: Ganymede
The largest Jovian Moon and the largest natural satellite in the Sol System, bigger than both Luna and Mercury. It has been partially terraformed, though supposedly retains a harsh climate. Ganymede is well-populated. The EVA Armor was developed here.
- Main article: Callisto
Callisto is the second largest Jovian Moon. In 2170 it was the site of the signing of the Callisto Treaty which officially ended the Interplanetary War. With little civilian population to speak of, Callisto's purpose is primarily militaristic in nature; the moon's several bases are capable of launching their units and ships within an hour's notice. Herzog claimed to have to deal with paperwork regarding surveillance here.
- Jove was another Latin name for the god Jupiter.