Moon that has only known active volcanoes not on Earth; one of four Galilean moons of Jupiter.

• A. Europa
• B. Callisto
• C. Ganymede
• D. Io

Answer: (D)

The main idea is that strong tidal forces from Jupiter can heat a moon’s interior and drive volcanism. Io experiences intense tidal flexing because it orbits very close to Jupiter and is gravitationally tugged also by Europa and Ganymede in resonance. This constant stretching melts interior rock, fueling numerous active volcanoes that eject plumes well above the surface. That ongoing, widespread activity is why Io is known for active volcanoes not seen on Earth, among the Galilean moons.

Europa, by contrast, has a relatively smooth icy crust and is believed to hide a subsurface ocean, but there’s no confirmed ongoing volcanic activity. Callisto and Ganymede show mostly ancient or tectonically quiet surfaces rather than current volcanism. So Io stands out as the moon with active volcanism, making it the correct choice.