The Battle of Io was a major naval battle over Io in which rogue elements of the United Nations Navy, under the command of Admiral Augusto Nguyen, fought the UNN's Jupiter Fleet, commanded by Admiral Souther, along with elements of the Martian Congressional Republic Navy.
Elements of the UN leadership on Earth had been compromised by Mao-Kwikowski Mercantile/Protogen and were secretly backing research on the weaponization of the Protomolecule on Io. Sent to Ganymede on a Mao-Kwikowski-owned yacht by Sadavir Errinwright in order to isolate her, Assistant Deputy Undersecretary Chrisjen Avasarala fled to the Rocinante to try and spread word of the conspiracy to her leadership at the UN.
A sizable Martian force was also present nearby; due to the heightened state of tension in the solar system following the Ganymede Incident, MCRN forces had been shadowing Admiral Nguyen's fleet as it approached Io to protect the research station.
Once aboard the Rocinante, Avasarala pleaded with the MCRN fleet to protect her from Nguyen's now pursuing UNN forces. Shortly thereafter, Admiral Souther's Jupiter Fleet arrived and issued a proclamation that anyone siding with Admiral Nguyen would be committing treason, and ordered all UNN vessels to stand down. Some captains under Nguyen's command obeyed, but several loyalists stood their ground. Fighting broke out on several UNN warships as their crews wrestled over which admiral to side with.
The battle was marked by chaos and confusion. Nguyen's forces opened up the fighting with a round of torpedoes aimed at the MCRN fleet. Admiral Souther quickly returned fire and formed a battle line with their hastily-made MCRN allies. Due to the sudden onslaught of fighting combined with the unknown IFF status of several of Nguyen's ships, mass confusion ensued as UNN vessels were hesitant to fire on their own side. Likewise, the MCRN was slow to return fire as they were unsure of which vessels had defected to Souther's command.
Sensing the desperation of the moment, the researchers on the moon's surface threatened to launch a barrage of stealth missiles containing Protogen Hybrid Soldiers towards Mars. After it became apparent that the MCRN would not back down, they followed through on their threat.
In the midst of the fighting, Admiral Nguyen's command ship, the UNN Agatha King, was struck by one of the missiles. The unleashed Hybrid managed to kill a majority of the crew and spread the Protomolecule throughout the vessel. By the time it was boarded by James Holden, most of the crew had been infected and become vomit zombies.