Small moons are cold, geologically dead worlds.

Small worlds cool faster than large ones. They have less internal volume storing heat relative to the amount of surface through which that heat can escape.

That made a simple prediction attractive: small moons should lose their original heat early and become geologically dead.

Io broke the rule spectacularly.

When Voyager 1 passed Jupiter in 1979, its images showed volcanic plumes erupting from Io. It was the first active volcanism ever observed beyond Earth.

The missing heat source was not leftover heat from Io’s formation. Jupiter’s gravity continually stretches and squeezes the moon. Because Io’s orbit is slightly non-circular and is kept that way by gravitational interactions with neighboring moons, the strength and direction of the tidal distortion keep changing. That flexing generates heat inside Io.

Later missions showed that Io was not the only surprise. Europa has a young, fractured ice shell above a global ocean. Saturn’s small moon Enceladus shows cryovolcanic activity, spraying water-rich material from fractures near its south pole and also contains an internal ocean.

These examples introduced tidal heating as a major energy source for small worlds.

Size still matters, and many small moons really are cold and inactive. The correction is to the universal rule: a small moon can remain geologically active if another energy source, especially tidal flexing, keeps its interior warm.