Io is a moon of which planet?

• A. Earth
• B. Jupiter
• C. Mars
• D. Saturn

Answer: (B)

Io orbits the gas giant Jupiter. It’s one of the Galilean moons—Io, Europa, Ganymede, and Callisto—discovered by Galileo. A standout feature is Io’s intense volcanism, driven by tidal heating from Jupiter’s strong gravity as Io interacts with other moons. Among the planets listed, Io isn’t a moon of Earth (which has its own Moon), Mars (which has Phobos and Deimos), or Saturn (which has many moons of its own). The correct association is with Jupiter.