One of Jupiter's moons, used by Mao-Kwikowski Mercantile as a secret research facility for protomolecule-human hybrid experiments. After the destruction of Protogen, Jules-Pierre Mao relocated his remaining scientists to Io to continue weaponising the protomolecule, creating hybrid soldiers from kidnapped children with latent protomolecule compatibility. The facility was raided by a combined force including the Rocinante crew and Bobbie Draper's Marines, ending the hybrid programme. The battle above Io saw the launch of hybrid-carrying missiles toward Mars, which were destroyed by the combined UN and Martian fleets in a rare moment of cooperation.